Exam 2 Practice Problems_1

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21.1 Resistors in Series and Parallel Note: Data taken from figures can be assumed to be accurate to three significant digits. 1. (a) What is the resistance of ten 275-Ω resistors connected in series? (b) In p a r a ll e l ? 2. (a) What is the resistance of a 1.00 × 10 2 resistor connected in series? (b) In p a r a ll e l ? −Ω , a 2.50-kΩ , and a 4.00-kΩ 3. What are the largest and smallest resistances you can obtain by connecting a 36.0-Ω , a 50.0-Ω , and a 700-Ω resistor together? 4. An 1800-W toaster, a 1400-W electric frying pan, and a 75-W lamp are plugged into the same outlet in a 15-A, 120-V circuit. (The three devices are in parallel when plugged into the same socket.). (a) What current is drawn by each device? (b) Will this combination blow the 15-A fuse? 5. Your car’s 30.0-W headlight and 2.40-kW starter are ordinarily connected in parallel in a 12.0-V system. What power would one headlight and the starter consume if connected in series to a 12.0-V battery? (Neglect any other resistance in the circuit and any change in resistance in the two devices.) 6. (a) Given a 48.0-V battery and 24.0-Ω and 96.0-Ω resistors, find the current and power for each when connected in series. (b) Repeat when the resistances are in parallel. 7. Referring to the example combining series and parallel circuits and Fi gure 21.6 , calculate 3 in the following two different ways: (a) from the known
values of and 2 ; (b) using Ohm’s law for 3 . In both parts explicitly show how you follow the steps in the Problem-Solvin g Strategies for Series and Parallel Resistors . 8. Referring to Fi gure 21.6 : (a) Calculate 3 and note how it compares with 3 found in the first two example problems in this module. (b) Find the total
power supplied by the source and compare it with the sum of the powers dissipated by the resistors. 9. Refer to Fi gure 21.7 and the discussion of lights dimming when a heavy appliance comes on. (a) Given the voltage source is 120 V, the wire resistance is 0.400 Ω , and the bulb is nominally 75.0 W, what power will the bulb dissipate if a total of 15.0 A passes through the wires when the motor comes on? Assume negligible change in bulb resistance. (b) What power is consumed by the motor? 10. A 240-kV power transmission line carrying 5.00 × 10 2 A is hung from grounded metal towers by ceramic insulators, each having a 1.00×10 9 −Ω resistance. Fi gure 21.51 . (a) What is the resistance to ground of 100 of these insulators? (b) Calculate the power dissipated by 100 of them. (c) What fraction of the power carried by the line is this? Explicitly show how you follow the steps in the Problem-Solvin g Strategies for Series and Parallel Resistors . Figure 21.51 High- voltage (240-kV) transmission line carrying 5.00 × 10 2 A is hung from a grounded metal transmission tower. The row of ceramic insulators provide
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1.00 × 10 9 Ω of resistance each. 11. Show that if two resistors 1 and 2 are combined and one is much greater than the other ( 1 >> 2 ): (a) Their series resistance is very nearly equal to the greater resistance 1 . (b) Their parallel resistance is very nearly equal to smaller resistance 2 . 12. Unreasonable Results Two resistors, one having a resistance of 145 Ω , are connected in parallel to produce a total resistance of 150 Ω . (a) What is the value of the second resistance? (b) What is unreasonable about this r e su l t ? (c) Which assumptions are unreasonable or inconsistent? 13. Unreasonable Results Two resistors, one having a resistance of 900 kΩ , are connected in series to produce a total resistance of 0.500 MΩ . (a) What is the value of the second resistance? (b) What is unreasonable about this r e su l t ? (c) Which assumptions are unreasonable or inconsistent? 21.2 Electromotive Force: Terminal Volta ge 14. Standard automobile batteries have six lead-acid cells in series, creating a total emf of 12.0 V. What is the emf of an individual lead-acid c e ll ? 15. Carbon-zinc dry cells (sometimes referred to as non-alkaline cells) have an emf of 1.54 V, and they are produced as single cells or in various combinations to form other voltages. (a) How many 1.54-V cells are needed to make the common 9-V battery used in many small electronic devices? (b) What is the actual emf of the
approximately 9-V battery? (c) Discuss how internal resistance in the series connection of cells will affect the terminal voltage of this approximately 9-V battery. 16. What is the output voltage of a 3.0000-V lithium cell in a digital wristwatch that draws 0.300 mA, if the cell’s internal resistance is 2.00 Ω ?
2 bulb 17. (a) What is the terminal voltage of a large 1.54-V carbon-zinc dry cell used in a physics lab to supply 2.00 A to a circuit, if the cell’s internal resistance is 0.100 Ω ? (b) How much electrical power does the cell produce? (c) What power goes to its l o a d ? 18. What is the internal resistance of an automobile battery that has an emf of 12.0 V and a terminal voltage of 15.0 V while a current of 8.00 A is charging it? 19. (a) Find the terminal voltage of a 12.0-V motorcycle battery having a 0.600-Ω internal resistance, if it is being charged by a current of 10.0 A. (b) What is the output voltage of the battery charger? 20. A car battery with a 12-V emf and an internal resistance of 0.050 Ω is being charged with a current of 60 A. Note that in this process the battery is being charged. (a) What is the potential difference across its terminals? (b) At what rate is thermal energy being dissipated in the battery? (c) At what rate is electric energy being converted to chemical energy? (d) What are the answers to (a) and (b) when the battery is used to supply 60 A to the starter motor? 21. The hot resistance of a flashlight bulb is 2.30 Ω , and it is run by a 1.58-V alkaline cell having a 0.100-Ω internal resistance. (a) What current flows? (b) Calculate the power supplied to the bulb using same as calculated using ? 2 bulb . (c) Is this power the 22. The label on a portable radio recommends the use of rechargeable nickel- cadmium cells (nicads), although they have a 1.25-V emf while alkaline cells have a 1.58-V emf. The radio has a 3.20-Ω resistance. (a) Draw a circuit diagram of the radio and its batteries. Now, calculate the power delivered to the radio. (b) When
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using Nicad cells each having an internal resistance of 0.0400 Ω . (c) When using alkaline cells each having an internal resistance of 0.200 Ω . (d) Does this difference seem significant, considering that the radio’s effective resistance is lowered when its volume is turned up? 23. An automobile starter motor has an equivalent resistance of 0.0500 Ω and is supplied by a 12.0-V battery with a 0.0100-Ω internal resistance. (a) What
is the current to the motor? (b) What voltage is applied to it? (c) What power is supplied to the motor? (d) Repeat these calculations for when the battery connections are corroded and add 0.0900 Ω to the circuit. (Significant problems are caused by even small amounts of unwanted resistance in low- voltage, high-current applications.) 24. A child’s electronic toy is supplied by three 1.58-V alkaline cells having internal resistances of 0.0200 Ω in series with a 1.53- V carbon-zinc dry cell having a 0.100-Ω internal resistance. The load resistance is 10.0 Ω . (a) Draw a circuit diagram of the toy and its batteries. (b) What current flows? (c) How much power is supplied to the l o a d ? (d) What is the internal resistance of the dry cell if it goes bad, resulting in only 0.500 W being supplied to the l o a d ? 25. (a) What is the internal resistance of a voltage source if its terminal voltage drops by 2.00 V when the current supplied increases by 5.00 A? (b) Can the emf of the voltage source be found with the information su pp l i e d ? 26. A person with body resistance between his hands of 10.0 kΩ accidentally grasps the terminals of a 20.0-kV power supply. (Do NOT do this!) (a) Draw a circuit diagram to represent the situation. (b) If the internal resistance of the power supply is 2000 Ω , what is the current through his body? (c) What is the power dissipated in his body? (d) If the power supply is to be made safe by increasing its internal resistance, what should the internal resistance be for the maximum current in this situation to be 1.00 mA or less? (e) Will this modification compromise the effectiveness of the power supply for driving low-resistance devices? Explain your reasoning. 27. Electric fish generate current with biological cells called electroplaques, which are physiological emf devices. The electroplaques in the South American eel are arranged in 140 rows, each row stretching horizontally along
the body and each containing 5000 electroplaques. Each electroplaque has an emf of 0.15 V and internal resistance of 0.25 Ω . If the water surrounding the fish has resistance of 800 Ω , how much current can the eel produce in water from near its head to near its t a i l ?
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28. Integrated Concepts A 12.0-V emf automobile battery has a terminal voltage of 16.0 V when being charged by a current of 10.0 A. (a) What is the battery’s internal resistance? (b) What power is dissipated inside the battery? (c) At what rate (in ºC/min ) will its temperature increase if its mass is 20.0 kg and it has a specific heat of 0.300 kcal/kg ºC , assuming no heat escapes? 29. Unreasonable Results A 1.58-V alkaline cell with a 0.200-Ω internal resistance is supplying 8.50 A to a load. (a) What is its terminal vo l t a g e ? (b) What is the value of the load resistance? (c) What is unreasonable about these results? (d) Which assumptions are unreasonable or inconsistent? 30. Unreasonable Results (a) What is the internal resistance of a 1.54-V dry cell that supplies 1.00 W of power to a 15.0-Ω b u l b ? (b) What is unreasonable about this r e su l t ? (c) Which assumptions are unreasonable or inconsistent? 21.3 Kirchhoff’s Rules 31. Apply the loop rule to loop abcdefgha in Fi gure 21.25 . 32. Apply the loop rule to loop aedcba in Fi gure 21.25 . 33. Verify the second equation in Exam ple 21.5 by substituting the values found for the currents 1 and 2 . 34. Verify the third equation in Exam ple 21.5 by substituting the values found for the currents 1 and 3 .
35. Apply the junction rule at point a in Fi gure 21.52 .
Figure 21.52 36. Apply the loop rule to loop abcdefghija in Fi gure 21.52 . 37. Apply the loop rule to loop akledcba in Fi gure 21.52 . 38. Find the currents flowing in the circuit in Fi gure 21.52 . Explicitly show how you follow the steps in the Problem-Solvin g Strategies for Series and Parallel Resistors . 39. Solve Exam ple 21.5 , but use loop abcdefgha instead of loop akledcba. Explicitly show how you follow the steps in the Problem-Solvin g Strategies for Series and Parallel Resistors . 40. Find the currents flowing in the circuit in Fi gure 21.47 . 41. Unreasonable Results Consider the circuit in Fi gure 21.53 , and suppose that the emfs are unknown and the currents are given to be 1 = 5.00 A , 2 = 3.0 A , and
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3 = –2.00 A . (a) Could you find the emfs? (b) What is wrong with the assumptions?
Figure 21.53 21.4 DC Voltmeters and Ammeters 42. What is the sensitivity of the galvanometer (that is, what current gives a full-scale deflection) inside a voltmeter that has a 1.00-MΩ resistance on its 30.0-V sc a l e ? 43. What is the sensitivity of the galvanometer (that is, what current gives a full-scale deflection) inside a voltmeter that has a 25.0-kΩ resistance on its 100-V sc a l e ? 44. Find the resistance that must be placed in series with a 25.0- Ω galvanometer having a 50.0−μA sensitivity (the same as the one discussed in the text) to allow it to be used as a voltmeter with a 0.100-V full-scale reading. 45. Find the resistance that must be placed in series with a 25.0- Ω galvanometer having a 50.0-μA sensitivity (the same as the one discussed in the text) to allow it to be used as a voltmeter with a 3000-V full-scale reading. Include a circuit diagram with your solution. 46. Find the resistance that must be placed in parallel with a 25.0-Ω galvanometer having a 50.0-μA sensitivity (the same as the one discussed in the text) to allow it to be used as an ammeter
with a 10.0-A full-scale reading. Include a circuit diagram with your solution.
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47. Find the resistance that must be placed in parallel with a 25.0-Ω galvanometer having a 50.0-μA sensitivity (the same as the one discussed in the text) to allow it to be used as an ammeter with a 300-mA full-scale reading. 48. Find the resistance that must be placed in series with a 10.0- Ω galvanometer having a 100-μA sensitivity to allow it to be used as a voltmeter with: (a) a 300-V full-scale reading, and (b) a 0.300-V full-scale reading. 49. Find the resistance that must be placed in parallel with a 10.0-Ω galvanometer having a 100-μA sensitivity to allow it to be used as an ammeter with: (a) a 20.0-A full-scale reading, and (b) a 100-mA full-scale reading. 50. Suppose you measure the terminal voltage of a 1.585-V alkaline cell having an internal resistance of 0.100 Ω by placing a 1.00-kΩ voltmeter across its terminals. (See Fi gure 21.54 .) (a) What current flows? (b) Find the terminal voltage. (c) To see how close the measured terminal voltage is to the emf, calculate their ratio. Figure 21.54 51. Suppose you measure the terminal voltage of a 3.200-V lithium cell having an internal resistance of 5.00 Ω by placing a 1.00-kΩ voltmeter across its terminals. (a) What current flows? (b) Find the terminal voltage. (c) To see how close the measured terminal voltage is to the emf, calculate their ratio.
52. A certain ammeter has a resistance of 5.00× 10 −5 Ω on its 3.00-A scale and contains a 10.0-Ω galvanometer. What is the sensitivity of the g a l va n o m e t e r ?
53. A 1.00-MΩ voltmeter is placed in parallel with a 75.0-kΩ resistor in a circuit. (a) Draw a circuit diagram of the connection. (b) What is the resistance of the combination? (c) If the voltage across the combination is kept the same as it was across the 75.0-kΩ resistor alone, what is the percent increase in current? (d) If the current through the combination is kept the same as it was through the 75.0-kΩ resistor alone, what is the percentage decrease in vo l t a g e ? (e) Are the changes found in parts (c) and (d) significant? Discuss. 54. A 0.0200-Ω ammeter is placed in series with a 10.00-Ω resistor in a circuit. (a) Draw a circuit diagram of the connection. (b) Calculate the resistance of the combination. (c) If the voltage is kept the same across the combination as it was through the 10.00-Ω resistor alone, what is the percent decrease in current? (d) If the current is kept the same through the combination as it was through the 10.00-Ω resistor alone, what is the percent increase in vo l t a g e ? (e) Are the changes found in parts (c) and (d) significant? Discuss. 55. Unreasonable Results Suppose you have a 40.0-Ω galvanometer with a 25.0-μA sensitivity. (a) What resistance would you put in series with it to allow it to be used as a voltmeter that has a full-scale deflection for 0.500 mV? (b) What is unreasonable about this r e su l t ? (c) Which assumptions are r e sp o n si b l e ? 56. Unreasonable Results (a) What resistance would you put in parallel with a 40.0-Ω galvanometer having a 25.0-μA sensitivity to allow it to be used as an ammeter that has a full-scale deflection for 10.0-μA ? (b) What is unreasonable about this r e su l t ? (c) Which assumptions are r e sp o n si b l e ?
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21.5 Null Measurements 57. What is the emf x of a cell being measured in a potentiometer, if the standard cell’s emf is 12.0 V and the potentiometer balances for x = 5.000 Ω and s = 2.500 Ω ? 58. Calculate the emf x of a dry cell for which a potentiometer is balanced when x = 1.200 Ω , while an alkaline standard cell with an emf of 1.600 V requires s = 1.247 Ω to balance the potentiometer. 59. When an unknown resistance x is placed in a Wheatstone bridge, it is possible to balance the bridge by adjusting 3 to be 2500 Ω . What is x if 2 = 0.625 ? 1 60. To what value must you adjust 3 to balance a Wheatstone bridge, if the unknown resistance x is 100 Ω , 1 is 50.0 Ω , and 2 is 175 Ω ? 61. (a) What is the unknown emf x in a potentiometer that balances when x is 10.0 Ω , and balances when s is 15.0 Ω for a standard 3.000-V emf? (b) The same emf x is placed in the same potentiometer, which now balances when s is 15.0 Ω for a standard emf of 3.100 V. At what resistance x will the potentiometer b a l a n c e ?
62. Suppose you want to measure resistances in the range from 10.0 Ω to 10.0 kΩ using a Wheatstone bridge that has 2 1 = 2.000 . Over what range should 3 be a d j ust a b l e ? 21.6 DC Circuits Containin g Resistors and Ca pacitors 63. The timing device in an automobile’s intermittent wiper system is based on an time constant and utilizes a 0.500-μF capacitor and a variable resistor. Over what range must be made to vary to achieve time constants from 2.00 to 15.0 s? 64. A heart pacemaker fires 72 times a minute, each time a 25.0- nF capacitor is charged (by a battery in series with a resistor) to 0.632 of its full voltage.
What is the value of the resistance? 65. The duration of a photographic flash is related to an time constant, which is 0.100 μs for a certain camera. (a) If the resistance of the flash lamp is 0.0400 Ω during discharge, what is the size of the capacitor supplying its energy? (b) What is the time constant for charging the capacitor, if the charging resistance is 800 kΩ ? 66. A 2.00- and a 7.50-μF capacitor can be connected in series or parallel, as can a 25.0- and a 100-kΩ resistor. Calculate the four-time constants possible from connecting the resulting capacitance and resistance in series. 67. After two time constants, what percentage of the final voltage, emf, is on an initially uncharged capacitor , charged through a resistance ? 68. A 500-Ω resistor, an uncharged 1.50-μF capacitor, and a 6.16-V emf are connected in series. (a) What is the initial current? (b) What is the time constant? (c) What is the current after one time constant? (d) What is the voltage on the capacitor after one time constant? 69. A heart defibrillator being used on a patient has an time constant of 10.0 ms due to the resistance of the patient and the capacitance of the defibrillator. (a) If the defibrillator has an 8.00-μF capacitance, what is the resistance of the path through the patient? (You may neglect the capacitance of the patient and the resistance of the defibrillator.) (b) If the initial voltage is 12.0 kV, how long does it take to decline to 6.00 × 10 2 V ? 70. An ECG monitor must have an time constant less than 1.00 × 10 2 μs
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to be able to measure variations in voltage over small time intervals. (a) If the resistance of the circuit (due mostly to that of the patient’s chest) is 1.00 kΩ , what is the maximum capacitance of the circuit? (b) Would it be difficult in practice to limit the capacitance to less than the value found in ( a ) ? 71. Fi gure 21.55 shows how a bleeder resistor is used to discharge a capacitor after an electronic device is shut off, allowing a person to work on the electronics with less risk of shock. (a) What is the time constant? (b) How
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long will it take to reduce the voltage on the capacitor to 0 . 250 % ( 5 % of 5%) of its full value once discharge begins? (c) If the capacitor is charged to a voltage 0.865 0 0 through a 100-Ω resistance, calculate the time it takes to rise to (This is about two time constants.) Figure 21.55 72. Using the exact exponential treatment, find how much time is required to discharge a 250-μF capacitor through a 500-Ω resistor down to 1 . 00 % of its original voltage. 73. Using the exact exponential treatment, find how much time is required to charge an initially uncharged 100-pF capacitor through a 75.0-MΩ resistor to 90 . 0 % of its final voltage. 74. Integrated Concepts If you wish to take a picture of a bullet traveling at 500 m/s, then a very brief flash of light produced by an discharge through a flash tube can limit blurring. Assuming 1.00 mm of motion during one constant is acceptable, and given that the flash is driven by a 600-μF capacitor, what is the resistance in the flash tube? 75. Integrated Concepts
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A flashing lamp in a Christmas earring is based on an discharge of a capacitor through its resistance. The effective duration of the flash is 0.250 s, during which it produces an average 0.500 W from an average 3.00 V. (a) What
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energy does it dissipate? (b) How much charge moves through the l am p ? (c) Find the capacitance. (d) What is the resistance of the l am p ? 76. Integrated Concepts A 160-μF capacitor charged to 450 V is discharged through a 31.2- resistor. (a) Find the time constant. (b) Calculate the temperature increase of the resistor, given that its mass is 2.50 g and its specific heat is kJ , kg ºC noting that most of the thermal energy is retained in the short time of the discharge. (c) Calculate the new resistance, assuming it is pure carbon. (d) Does this change in resistance seem significant? 77. Unreasonable Results (a) Calculate the capacitance needed to get an time constant of 1.00 × 10 3 s with a 0.100-Ω resistor. (b) What is unreasonable about this r e su l t ? (c) Which assumptions are r e sp o n si b l e ? 78. Construct Your Own Problem Consider a camera’s flash unit. Construct a problem in which you calculate the size of the capacitor that stores energy for the flash lamp. Among the things to be considered are the voltage applied to the capacitor, the energy needed in the flash and the associated charge needed on the capacitor, the resistance of the flash lamp during discharge, and the desired time constant. 79. Construct Your Own Problem 1.6 7
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Consider a rechargeable lithium cell that is to be used to power a camcorder. Construct a problem in which you calculate the internal resistance of the cell during normal operation. Also, calculate the minimum voltage output of a battery charger to be used to recharge your lithium cell. Among the things to
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be considered are the emf and useful terminal voltage of a lithium cell and the current it should be able to supply to a camcorder.
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22.4 Ma gnetic Field Strength: Force on a Moving Char ge in a Magnetic Field 1. What is the direction of the magnetic force on a positive charge that moves as shown in each of the six cases shown in Fi gure 22.50 ? Figure 22.50 2. Repeat [link] for a negative charge. 3. What is the direction of the velocity of a negative charge that experiences the magnetic force shown in each of the three cases in Fi gure 22.51 , assuming it moves perpendicular to ?
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Figure 22.51 4. Repeat [link] for a positive charge. 5. What is the direction of the magnetic field that produces the magnetic force on a positive charge as shown in each of the three cases in the figure below, assuming is perpendicular to ? Figure 22.52 6. Repeat [link] for a negative charge. 7. What is the maximum magnitude of the force on an aluminum rod with a 0.100- C μ charge that you pass between the poles of a 1.50-T permanent magnet at a speed of 5.00 m/s? In what direction is the force?
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8. (a) Aircraft sometimes acquire small static charges. Suppose a supersonic jet has a 0.500- C μ charge and flies due west at a speed of 660 m/s over the Earth’s magnetic south pole (near Earth's geographic north p o l e ) , where the 8.00 × 10 −5 -T magnetic field points straight down. What are the direction and the magnitude of the magnetic force on the plane? (b) Discuss whether the value obtained in part (a) implies this is a significant or negligible e ect. ff 9. (a) A cosmic ray proton moving toward the Earth at 5.00 × 10 7 m/s experiences a magnetic force of 1.70 × 10 −16 N . What is the strength of the magnetic field if there is a 45º angle between it and the proton’s velocity? (b) Is the value obtained in part (a) consistent with the known strength of the Earth’s magnetic field on its surface? Discuss. 10. An electron moving at 4.00 × 10 3 m/s in a 1.25-T magnetic field experiences a magnetic force of 1.40 × 10 −16 N . What angle does the velocity of the electron make with the magnetic field? There are two answers. 11. (a) A physicist performing a sensitive measurement wants to limit the magnetic force on a moving charge in her equipment to less than 1.00 × 10 −12 N . What is the greatest the charge can be if it moves at a maximum speed of 30.0 m/s in the Earth’s field? (b) Discuss whether it would be di cult to limit the charge to less than the value found in (a) by comparing it with typical static electricity and noting that static is often absent. 22.5 Force on a Movin g Charge in a Magnetic Field: Exam ples and Applications If you need additional support for these problems, see More A pplications of Ma gnetism . 12. A cosmic ray electron moves at 7.50 × 10 6 m/s perpendicular to the Earth’s magnetic field at an altitude where field strength is 1.00 × 10 −5 T . What is the radius of the circular path the electron follows?
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13. A proton moves at 7.50 × 10 7 m/s perpendicular to a magnetic field. The field causes the proton to travel in a circular path of radius 0.800 m. What is
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the field strength? 14. (a) Viewers of Star Trek hear of an antimatter drive on the Starship Enterprise . One possibility for such a futuristic energy source is to store antimatter charged particles in a vacuum chamber, circulating in a magnetic field, and then extract them as needed. Antimatter annihilates with normal matter, producing pure energy. What strength magnetic field is needed to hold antiprotons, moving at 5.00 × 10 7 m/s in a circular path 2.00 m in radius? Antiprotons have the same mass as protons but the opposite (negative) charge. (b) Is this field strength obtainable with today’s technology or is it a futuristic possibility? 15. (a) An oxygen-16 ion with a mass of 2.66 × 10 −26 kg travels at 5.00 × 10 6 m/s perpendicular to a 1.20-T magnetic field, which makes it move in a circular arc with a 0.231-m radius. What positive charge is on the ion? (b) What is the ratio of this charge to the charge of an electron? (c) Discuss why the ratio found in (b) should be an integer. 16. What radius circular path does an electron travel if it moves at the same speed and in the same magnetic field as the proton in [link] ? 17. A velocity selector in a mass spectrometer uses a 0.100-T magnetic field. (a) What electric field strength is needed to select a speed of 4.00 × 10 6 m/s ? (b) What is the voltage between the plates if they are separated by 1.00 cm? 18. An electron in a TV CRT moves with a speed of 6.00 × 10 7 m/s , in a direction perpendicular to the Earth’s field, which has a strength of 5.00 × 10 −5 T . (a) What strength electric field must be applied perpendicular to the Earth’s field to make the electron moves in a straight line? (b) If this is done between plates separated by 1.00 cm, what is the voltage applied? (Note that TVs are usually surrounded by a ferromagnetic material to shield against external magnetic fields and avoid the need for such a c o rr e c t i o n . )
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19. (a) At what speed will a proton move in a circular path of the same radius as the electron in [link] ? (b) What would the radius of the path be if the
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proton had the same speed as the electron? (c) What would the radius be if the proton had the same kinetic energy as the electron? (d) The same momentum? 20. A mass spectrometer is being used to separate common oxygen-16 from the much rarer oxygen-18, taken from a sample of old glacial ice. (The relative abundance of these oxygen isotopes is related to climatic temperature at the time the ice was d e p o si t e d . ) The ratio of the masses of these two ions is 16 to 18, the mass of oxygen-16 is 2.66 × 10 −26 kg, and they are singly charged and travel at 5.00 × 10 6 m/s in a 1.20-T magnetic field. What is the separation between their paths when they hit a target after traversing a semicircle? 21. (a) Triply charged uranium-235 and uranium-238 ions are being separated in a mass spectrometer. (The much rarer uranium-235 is used as reactor f u e l . ) The masses of the ions are 3.90 × 10 −25 kg and 3.95 × 10 −25 kg , respectively, and they travel at 3.00 × 10 5 m/s in a 0.250-T field. What is the separation between their paths when they hit a target after traversing a semicircle? (b) Discuss whether this distance between their paths seems to be big enough to be practical in the separation of uranium-235 from uranium- 238. 22.6 The Hall Effect 22. A large water main is 2.50 m in diameter and the average water velocity is 6.00 m/s. Find the Hall voltage produced if the pipe runs perpendicular to the Earth’s 5.00 × 10 −5 -T field. 23. What Hall voltage is produced by a 0.200-T field applied across a 2.60- cm-diameter aorta when blood velocity is 60.0 cm/s? 24. (a) What is the speed of a supersonic aircraft with a 17.0-m wingspan, if it experiences a 1.60-V Hall voltage between its wing tips when in level flight over the north magnetic pole, where the Earth’s field strength is 8.00 × 10 −5 voltage.
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T? (b) Explain why very little current flows as a result of this Hall 25. A nonmechanical water meter could utilize the Hall e ect ff by applying a magnetic field across a metal pipe and measuring the Hall voltage produced. What is the average fluid velocity in a 3.00-cm-diameter pipe, if a 0.500-T field across it creates a 60.0-mV Hall voltage? 26. Calculate the Hall voltage induced on a patient’s heart while being scanned by an MRI unit. Approximate the conducting path on the heart wall by a wire 7.50 cm long that moves at 10.0 cm/s perpendicular to a 1.50-T magnetic field. 27. A Hall probe calibrated to read 1.00 V μ when placed in a 2.00-T field is placed in a 0.150-T field. What is its output voltage? 28. Using information in Exam ple 20.6 , what would the Hall voltage be if a 2.00-T field is applied across a 10-gauge copper wire ( 2 . 588 mm in diameter) carrying a 20.0-A current? 29. Show that the Hall voltage across wires made of the same material, carrying identical currents, and subjected to the same magnetic field is inversely proportional to their diameters. (Hint: Consider how drift velocity depends on wire d i a m e t e r . ) 30. A patient with a pacemaker is mistakenly being scanned for an MRI image. A 10.0-cm-long section of pacemaker wire moves at a speed of 10.0 cm/s perpendicular to the MRI unit’s magnetic field and a 20.0-mV Hall voltage is induced. What is the magnetic field strength? 22.7 Ma gnetic Force on a Current- Carrying Conductor 31. What is the direction of the magnetic force on the current in each of the six cases in Fi gure 22.53 ?
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Figure 22.53 32. What is the direction of a current that experiences the magnetic force shown in each of the three cases in Fi gure 22.54 , assuming the current runs perpendicular to ? Figure 22.54 33. What is the direction of the magnetic field that produces the magnetic force shown on the currents in each of the three cases in Fi gure 22.55 ,
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assuming is perpendicular to ? Figure 22.55 34. (a) What is the force per meter on a lightning bolt at the equator that carries 20,000 A perpendicular to the Earth’s 3.00 × 10 −5 -T field? (b) What is the direction of the force if the current is straight up and the Earth’s field direction is due north, parallel to the ground? 35. (a) A DC power line for a light-rail system carries 1000 A at an angle of 30.0º to the Earth’s 5.00 × 10 −5 -T field. What is the force on a 100-m section of this line? (b) Discuss practical concerns this presents, if any. 36. What force is exerted on the water in an MHD drive utilizing a 25.0- cm- diameter tube, if 100-A current is passed across the tube that is perpendicular to a 2.00-T magnetic field? (The relatively small size of this force indicates the need for very large currents and magnetic fields to make practical MHD drives.) 37. A wire carrying a 30.0-A current passes between the poles of a strong magnet that is perpendicular to its field and experiences a 2.16-N force on the 4.00 cm of wire in the field. What is the average field strength? 38. (a) A 0.750-m-long section of cable carrying current to a car starter motor makes an angle of 60º with the Earth’s 5.50 × 10 −5 T field. What is the
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current when the wire experiences a force of 7.00 × 10 −3 N ? (b) If you run
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the wire between the poles of a strong horseshoe magnet, subjecting 5.00 cm of it to a 1.75-T field, what force is exerted on this segment of wire? 39. (a) What is the angle between a wire carrying an 8.00-A current and the 1.20-T field it is in if 50.0 cm of the wire experiences a magnetic force of 2.40 N? (b) What is the force on the wire if it is rotated to make an angle of 90º with the field? 40. The force on the rectangular loop of wire in the magnetic field in Fi gure 22.56 can be used to measure field strength. The field is uniform, and the plane of the loop is perpendicular to the field. (a) What is the direction of the magnetic force on the loop? J u st i f y the claim that the forces on the sides of the loop are equal and opposite, independent of how much of the loop is in the field and do not a ect ff the net force on the loop. (b) If a current of 5.00 A is used, what is the force per tesla on the 20.0-cm-wide loop? Figure 22.56 A rectangular loop of wire carrying a current is perpendicular to a magnetic field. The field is uniform in the region shown and is zero outside that region. 22.8 Torque on a Current Loop: Motors and Meters
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41. (a) By how many percent is the torque of a motor decreased if its permanent magnets lose 5.0% of their strength? (b) How many percent would the current need to be increased to return the torque to original values? 42. (a) What is the maximum torque on a 150-turn square loop of wire 18.0 cm on a side that carries a 50.0-A current in a 1.60-T field? (b) What is the torque when is 10.9º? 43. Find the current through a loop needed to create a maximum torque of 9.00 N m. The loop has 50 square turns that are 15.0 cm on a side and is in a uniform 0.800-T magnetic field. 44. Calculate the magnetic field strength needed on a 200-turn square loop 20.0 cm on a side to create a maximum torque of 300 N m if the loop is carrying 25.0 A. 45. Since the equation for torque on a current-carrying loop is = sin , the units of N m must equal units of A m 2 T . Verify this. 46. (a) At what angle is the torque on a current loop 90.0% of maximum? (b) 50.0% of maximum? (c) 10.0% of maximum? 47. A proton has a magnetic field due to its spin on its axis. The field is similar to that created by a circular current loop 0.650 × 10 −15 m in radius with a current of 1.05 × 10 4 A (no k i dd i n g ) . Find the maximum torque on a proton in a 2.50-T field. (This is a significant torque on a small p a r t i c l e . ) 48. (a) A 200-turn circular loop of radius 50.0 cm is vertical, with its axis on an east-west line. A current of 100 A circulates clockwise in the loop when viewed from the east. The Earth’s field here is due north, parallel to the ground, with a strength of 3.00 × 10 −5 T . What are the direction and magnitude of the torque on the loop? (b) Does this device have any practical applications as a motor?
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49. Repeat [link] , but with the loop lying flat on the ground with its current circulating counterclockwise (when viewed from above) in a location where the Earth’s field is north, but at an angle 45.0º below the horizontal and with a strength of 6.00 × 10 −5 T . 22.10 Ma gnetic Force between Two Parallel Conductors 50. (a) The hot and neutral wires supplying DC power to a light-rail commuter train carry 800 A and are separated by 75.0 cm. What is the magnitude and direction of the force between 50.0 m of these wires? (b) Discuss the practical consequences of this force, if any. 51. The force per meter between the two wires of a jumper cable being used to start a stalled car is 0.225 N / m . (a) What is the current in the wires, given they are separated by 2.00 cm? (b) Is the force attractive or repulsive? 52. A 2.50-m segment of wire supplying current to the motor of a submerged submarine carries 1000 A and feels a 4.00-N repulsive force from a parallel wire 5.00 cm away. What is the direction and magnitude of the current in the other wire? 53. The wire carrying 400 A to the motor of a commuter train feels an attractive force of 4.00 × 10 −3 N/m due to a parallel wire carrying 5.00 A to a headlight. (a) How far apart are the wires? (b) Are the currents in the same direction? 54. An AC appliance cord has its hot and neutral wires separated by 3.00 mm and carries a 5.00-A current. (a) What is the average force per meter between the wires in the cord? (b) What is the maximum force per meter between the wires? (c) Are the forces attractive or repulsive? (d) Do appliance cords need any special design features to compensate for these forces? 55. Fi gure 22.57 shows a long straight wire near a rectangular current loop. What is the direction and magnitude of the total force on the loop?
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Figure 22.57 56. Find the direction and magnitude of the force that each wire experiences in Fi gure 22.58 (a) by, using vector addition. Figure 22.58 57. Find the direction and magnitude of the force that each wire experiences in Fi gure 22.58 ( b ) , using vector addition. 22.11 More A pplications of Magnetism 58. Indicate whether the magnetic field created in each of the three situations shown in Fi gure 22.59 is into or out of the page on the left and right of the current.
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Figure 22.59 59. What are the directions of the fields in the center of the loop and coils shown in Fi gure 22.60 ? Figure 22.60 60. What are the directions of the currents in the loop and coils shown in Fi gure 22.61 ? Figure 22.61 61. To see why an MRI utilizes iron to increase the magnetic field created by a coil, calculate the current needed in a 400-loop-per-meter circular coil 0.660 m in radius to create a 1.20-T field (typical of an MRI instrument) at its center
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with no iron present. The magnetic field of a proton is approximately like that of a circular current loop 0.650 × 10 −15 m in radius carrying 1.05 × 10 4 A . What is the field at the center of such a loop? 62. Inside a motor, 30.0 A passes through a 250-turn circular loop that is 10.0 cm in radius. What is the magnetic field strength created at its center? 63. Nonnuclear submarines use batteries for power when submerged. (a) Find the magnetic field 50.0 cm from a straight wire carrying 1200 A from the batteries to the drive mechanism of a submarine. (b) What is the field if the wires to and from the drive mechanism are side by side? (c) Discuss the e ects this could have for a compass on the submarine that is not shielded. ff 64. How strong is the magnetic field inside a solenoid with 10,000 turns per meter that carries 20.0 A? 65. What current is needed in the solenoid described in [link] to produce a magnetic field 10 4 times the Earth’s magnetic field of 5.00 × 10 −5 T ? 66. How far from the starter cable of a car, carrying 150 A, must you be to experience a field less than the Earth’s (5.00 × 10 −5 T) ? Assume a long straight wire carries the current. (In practice, the body of your car shields the dashboard compass.) 67. Measurements a ect the system being measured, such as the current ff loop in Fi gure 22.56 . (a) Estimate the field the loop creates by calculating the field at the center of a circular loop 20.0 cm in diameter carrying 5.00 A. (b) What is the smallest field strength this loop can be used to measure, if its field must alter the measured field by less than 0.0100%? 68. Fi gure 22.62 shows a long straight wire just touching a loop carrying a current 1. Both lie in the same plane. (a) What direction must the current 2 in the straight wire have to create a field at the center of the loop in the direction opposite to that created by the loop? (b) What is the ratio of 1 / 2
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that gives zero field strength at the center of the loop? (c) What is the direction of the field directly above the loop under this circumstance? Figure 22.62 69. Find the magnitude and direction of the magnetic field at the point equidistant from the wires in Fi gure 22.58 ( a ) , using the rules of vector addition to sum the contributions from each wire. 70. Find the magnitude and direction of the magnetic field at the point equidistant from the wires in Fi gure 22.58 ( b ) , using the rules of vector addition to sum the contributions from each wire. 71. What current is needed in the top wire in Fi gure 22.58 (a) to produce a field of zero at the point equidistant from the wires, if the currents in the bottom two wires are both 10.0 A into the page? 72. Calculate the size of the magnetic field 20 m below a high voltage power line. The line carries 450 MW at a voltage of 300,000 V. 73. Integrated Concepts (a)A pendulum is set up so that its bob (a thin copper disk) swings between the poles of a permanent magnet as shown in Fi gure 22.63 . What is the magnitude and direction of the magnetic force on the bob at the lowest point in its path, if it has a positive 0.250 C μ charge and is released from a height of 30.0 cm above its lowest point? The magnetic field strength is 1.50 T. (b) What is the acceleration of the bob at the bottom of its swing if its mass is
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30.0 grams and it is hung from a flexible string? Be certain to include a free- body diagram as part of your analysis. Figure 22.63 74. Integrated Concepts (a) What voltage will accelerate electrons to a speed of 6.00 × 10 6 m/s ? (b) Find the radius of curvature of the path of a proton accelerated through this potential in a 0.500-T field and compare this with the radius of curvature of an electron accelerated through the same potential. 75. Integrated Concepts Find the radius of curvature of the path of a 25.0-MeV proton moving perpendicularly to the 1.20-T field of a cyclotron. 76. Integrated Concepts To construct a nonmechanical water meter, a 0.500-T magnetic field is placed across the supply water pipe to a home and the Hall voltage is recorded. (a) Find the flow rate in liters per second through a 3.00-cm-diameter pipe if the
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Hall voltage is 60.0 mV. (b) What would the Hall voltage be for the same flow rate through a 10.0-cm-diameter pipe with the same field applied? 77. Integrated Concepts (a)Using the values given for an MHD drive in [link] , and assuming the force is uniformly applied to the fluid, calculate the pressure created in N/m 2 . (b) Is this a significant fraction of an atmosphere? 78. Integrated Concepts (a)Calculate the maximum torque on a 50-turn, 1.50 cm radius circular current loop carrying 50 A μ in a 0.500-T field. (b) If this coil is to be used in a galvanometer that reads 50 A μ full scale, what force constant spring must be used, if it is attached 1.00 cm from the axis of rotation and is stretched by the 60º arc moved? 79. Integrated Concepts A current balance used to define the ampere is designed so that the current through it is constant, as is the distance between wires. Even so, if the wires change length with temperature, the force between them will change. What percent change in force per degree will occur if the wires are copper? 80. Integrated Concepts (a)Show that the period of the circular orbit of a charged particle moving perpendicularly to a uniform magnetic field is = 2 /( ) . (b) What is the frequency ? (c) What is the angular velocity ? Note that these results are independent of the velocity and radius of the orbit and, hence, of the energy of the particle. ( Fi gure 22 . 6 4 . )
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Figure 22.64 Cyclotrons accelerate charged particles orbiting in a magnetic field by placing an AC voltage on the metal Dees, between which the particles move, so that energy is added twice each orbit. The frequency is constant, since it is independent of the particle energy—the radius of the orbit simply increases with energy until the particles approach the edge and are extracted for various experiments and applications. 81. Integrated Concepts A cyclotron accelerates charged particles as shown in Fi gure 22.64 . Using the results of the previous problem, calculate the frequency of the accelerating voltage needed for a proton in a 1.20-T field. 82. Integrated Concepts (a)A 0.140-kg baseball, pitched at 40.0 m/s horizontally and perpendicular to the Earth’s horizontal 5.00 × 10 −5 T field, has a 100-nC charge on it. What distance is it deflected from its path by the magnetic force, after traveling 30.0 m horizontally? (b) Would you suggest this as a secret technique for a pitcher to throw curve balls?
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83. Integrated Concepts (a)What is the direction of the force on a wire carrying a current due east in a location where the Earth’s field is due north? Both are parallel to the ground. (b) Calculate the force per meter if the wire carries 20.0 A and the field strength is 3.00 × 10 −5 T . (c) What diameter copper wire would have its weight supported by this force? (d) Calculate the resistance per meter and the voltage per meter needed. 84. Integrated Concepts One long straight wire is to be held directly above another by repulsion between their currents. The lower wire carries 100 A and the wire 7.50 cm above it is 10-gauge ( 2 . 588 mm diameter) copper wire. (a) What current must flow in the upper wire, neglecting the Earth’s field? (b) What is the smallest current if the Earth’s 3.00 × 10 −5 T field is parallel to the ground and is not neglected? (c) Is the supported wire in a stable or unstable equilibrium if displaced vertically? If displaced horizontally? 85. Unreasonable Results (a)Find the charge on a baseball, thrown at 35.0 m/s perpendicular to the Earth’s 5.00 × 10 −5 T field, that experiences a 1.00-N magnetic force. (b) What is unreasonable about this result? (c) Which assumption or premise is responsible? 86. Unreasonable Results A charged particle having mass 6.64 × 10 −27 kg (that of a helium atom) moving at 8.70 × 10 5 m/s perpendicular to a 1.50-T magnetic field travels in
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a circular path of radius 16.0 mm. (a) What is the charge of the particle? (b) What is unreasonable about this result? (c) Which assumptions are responsible?
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87. Unreasonable Results An inventor wants to generate 120-V power by moving a 1.00-m-long wire perpendicular to the Earth’s 5.00 × 10 −5 T field. (a) Find the speed with which the wire must move. (b) What is unreasonable about this result? (c) Which assumption is responsible? 88. Unreasonable Results Frustrated by the small Hall voltage obtained in blood flow measurements, a medical physicist decides to increase the applied magnetic field strength to get a 0.500-V output for blood moving at 30.0 cm/s in a 1.50-cm-diameter vessel. (a) What magnetic field strength is needed? (b) What is unreasonable about this result? (c) Which premise is responsible? 89. Unreasonable Results A surveyor 100 m from a long straight 200-kV DC power line suspects that its magnetic field may equal that of the Earth and a ect compass readings. (a) ff Calculate the current in the wire needed to create a 5.00 × 10 −5 T field at this distance. (b) What is unreasonable about this result? (c) Which assumption or premise is responsible? 90. Construct Your Own Problem Consider a mass separator that applies a magnetic field perpendicular to the velocity of ions and separates the ions based on the radius of curvature of their paths in the field. Construct a problem in which you calculate the magnetic field strength needed to separate two ions that di er ff in mass, but not charge, and have the same initial velocity. Among the things to consider are the types of ions, the velocities they can be given before entering the magnetic field, and a reasonable value for the radius of curvature of the paths
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they follow. In addition, calculate the separation distance between the ions at the point where they are detected. 91. Construct Your Own Problem Consider using the torque on a current-carrying coil in a magnetic field to detect relatively small magnetic fields (less than the field of the Earth, for e x a m p l e ) . Construct a problem in which you calculate the maximum torque on a current-carrying loop in a magnetic field. Among the things to be considered are the size of the coil, the number of loops it has, the current you pass through the coil, and the size of the field you wish to detect. Discuss whether the torque produced is large enough to be e ectively ff measured. Your instructor may also wish for you to consider the e ects, ff if any, of the field produced by the coil on the surroundings that could a ect ff detection of the small field.
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23.1 Induced Emf and Ma gnetic Flux 1. What is the value of the magnetic flux at coil 2 in Fi gure 23.56 due to coil 1? Figure 23.56 (a) The planes of the two coils are perpendicular. (b) The wire is perpendicular to the plane of the coil. 2. What is the value of the magnetic flux through the coil in Fi gure 23.56 (b) due to the wire? 23.2 Farada y’s Law of Induction: Lenz’s Law 3. Referring to Fi gure 23.57 ( a ) , what is the direction of the current induced in coil 2: (a) If the current in coil 1 increases? (b) If the current in coil 1 decreases? (c) If the current in coil 1 is constant? Explicitly show how you follow the steps in the Problem-Solvin g Strategy for Lenz's Law .
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Figure 23.57 (a) The coils lie in the same plane. (b) The wire is in the plane of the coil 4. Referring to Fi gure 23.57 ( b ) , what is the direction of the current induced in the coil: (a) If the current in the wire increases? (b) If the current in the wire decreases? (c) If the current in the wire suddenly changes direction? Explicitly show how you follow the steps in the Problem-Solvin g Strategy for Lenz’s Law . 5. Referring to Fi gure 23.58 , what are the directions of the currents in coils 1, 2, and 3 (assume that the coils are lying in the plane of the circuit): (a) When the switch is first closed? (b) When the switch has been closed for a long time? (c) Just after the switch is opened? Figure 23.58 6. Repeat the previous problem with the battery reversed. 7. Verify that the units of Δ / Δ 1 T m 2 /s = 1 V . are volts. That is, show that 8. Suppose a 50-turn coil lies in the plane of the page in a uniform magnetic field that is directed into the page. The coil originally has an area of 0.250 m 2 . It is stretched to have no area in 0.100 s. What is the direction and magnitude of the induced emf if the uniform magnetic field has a strength of 1.50 T?
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9. (a) An MRI technician moves his hand from a region of very low magnetic field strength into an MRI scanner’s 2.00 T field with his fingers pointing in the direction of the field. Find the average emf induced in his wedding ring, given its diameter is 2.20 cm and assuming it takes 0.250 s to move it into the field. (b) Discuss whether this current would significantly change the temperature of the ring. 10. Integrated Concepts Referring to the situation in the previous problem: (a) What current is induced in the ring if its resistance is 0.0100 Ω ? (b) What average power is dissipated? (c) What magnetic field is induced at the center of the ring? (d) What is the direction of the induced magnetic field relative to the MRI’s field? 11. An emf is induced by rotating a 1000-turn, 20.0 cm diameter coil in the Earth’s 5.00 × 10 −5 T magnetic field. What average emf is induced, given the plane of the coil is originally perpendicular to the Earth’s field and is rotated to be parallel to the field in 10.0 ms? 12. A 0.250 m radius, 500-turn coil is rotated one-fourth of a revolution in 4.17 ms, originally having its plane perpendicular to a uniform magnetic field. (This is 60 rev/s.) Find the magnetic field strength needed to induce an average emf of 10,000 V. 13. Integrated Concepts Approximately how does the emf induced in the loop in Fi gure 23 . 57 ( b ) depend on the distance of the center of the loop from the wire? 14. Integrated Concepts (a)A lightning bolt produces a rapidly varying magnetic field. If the bolt strikes the earth vertically and acts like a current in a long straight wire, it will induce a voltage in a loop aligned like that in Fi gure 23 . 5 7 ( b ) . What voltage is induced in
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a 1.00 m diameter loop 50.0 m from a 2.00 × 10 6 A lightning strike, if the current falls to zero in 25.0 s μ ? (b) Discuss circumstances under which such a voltage would produce noticeable consequences. 23.3 Motional Emf 15. Use Faraday’s law, Lenz’s law, and RHR-1 to show that the magnetic force on the current in the moving rod in Fi gure 23.11 is in the opposite direction of its velocity. 16. If a current flows in the Satellite Tether shown in Fi gure 23.12 , use Faraday’s law, Lenz’s law, and RHR-1 to show that there is a magnetic force on the tether in the direction opposite to its velocity. 17. (a) A jet airplane with a 75.0 m wingspan is flying at 280 m/s. What emf is induced between wing tips if the vertical component of the Earth’s field is 3.00 × 10 −5 T ? (b) Is an emf of this magnitude likely to have any consequences? Explain. 18. (a) A nonferrous screwdriver is being used in a 2.00 T magnetic field. What maximum emf can be induced along its 12.0 cm length when it moves at 6.00 m/s? (b) Is it likely that this emf will have any consequences or even be noticed? 19. At what speed must the sliding rod in Fi gure 23.11 move to produce an emf of 1.00 V in a 1.50 T field, given the rod’s length is 30.0 cm? 20. The 12.0 cm long rod in Fi gure 23.11 moves at 4.00 m/s. What is the strength of the magnetic field if a 95.0 V emf is induced? 21. Prove that when , , and are not mutually perpendicular, motional emf is given by emf = sin . If is perpendicular to , then is the angle between and . If is perpendicular to , then is the angle between and .
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22. In the August 1992 space shuttle flight, only 250 m of the conducting tether considered in Exam ple 23.2 could be let out. A 40.0 V motional emf was generated in the Earth’s 5.00 × 10 −5 T field, while moving at 7.80 × 10 3 m/s . What was the angle between the shuttle’s velocity and the Earth’s field, assuming the conductor was perpendicular to the field? 23. Integrated Concepts Derive an expression for the current in a system like that in Fi gure 23.11 , under the following conditions. The resistance between the rails is , the rails and the moving rod are identical in cross section and have the same resistivity . The distance between the rails is l, and the rod moves at constant speed perpendicular to the uniform field . At time zero, the moving rod is next to the resistance . 24. Integrated Concepts The Tethered Satellite in Fi gure 23.12 has a mass of 525 kg and is at the end of a 20.0 km long, 2.50 mm diameter cable with the tensile strength of steel. (a) How much does the cable stretch if a 100 N force is exerted to pull the satellite in? (Assume the satellite and shuttle are at the same altitude above the E a r t h . ) (b) What is the e ective ff force constant of the cable? (c) How much energy is stored in it when stretched by the 100 N force? 25. Integrated Concepts The Tethered Satellite discussed in this module is producing 5.00 kV, and a current of 10.0 A flows. (a) What magnetic drag force does this produce if the system is moving at 7.80 km/s? (b) How much kinetic energy is removed from the system in 1.00 h, neglecting any change in altitude or velocity during that time? (c) What is the change in velocity if the mass of the system is 100,000 kg? (d) Discuss the long term consequences ( sa y , a week-long mission) on the
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space shuttle’s orbit, noting what e ect a decrease in velocity has and ff assessing the magnitude of the e ect. ff 23.4 Edd y Currents and Magnetic Damping 26. Make a drawing similar to Fi gure 23.14 , but with the pendulum moving in the opposite direction. Then use Faraday’s law, Lenz’s law, and RHR-1 to show that magnetic force opposes motion. 27. Figure 23.59 A coil is moved into and out of a region of uniform magnetic field. A coil is moved through a magnetic field as shown in Fi gure 23.59 . The field is uniform inside the rectangle and zero outside. What is the direction of the induced current and what is the direction of the magnetic force on the coil at each position shown? 23.5 Electric Generators 28. Calculate the peak voltage of a generator that rotates its 200-turn, 0.100 m diameter coil at 3600 rpm in a 0.800 T field. 29. At what angular velocity in rpm will the peak voltage of a generator be 480 V, if its 500-turn, 8.00 cm diameter coil rotates in a 0.250 T field? 30. What is the peak emf generated by rotating a 1000-turn, 20.0 cm diameter coil in the Earth’s 5.00 × 10 −5 T magnetic field, given the plane of the coil is originally perpendicular to the Earth’s field and is rotated to be parallel to the field in 10.0 ms?
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31. What is the peak emf generated by a 0.250 m radius, 500-turn coil is rotated one-fourth of a revolution in 4.17 ms, originally having its plane perpendicular to a uniform magnetic field. (This is 60 rev/s.) 32. (a) A bicycle generator rotates at 1875 rad/s, producing an 18.0 V peak emf. It has a 1.00 by 3.00 cm rectangular coil in a 0.640 T field. How many turns are in the coil? (b) Is this number of turns of wire practical for a 1.00 by 3.00 cm coil? 33. Integrated Concepts This problem refers to the bicycle generator considered in the previous problem. It is driven by a 1.60 cm diameter wheel that rolls on the outside rim of the bicycle tire. (a) What is the velocity of the bicycle if the generator’s angular velocity is 1875 rad/s? (b) What is the maximum emf of the generator when the bicycle moves at 10.0 m/s, noting that it was 18.0 V under the original conditions? (c) If the sophisticated generator can vary its own magnetic field, what field strength will it need at 5.00 m/s to produce a 9.00 V maximum emf? 34. (a) A car generator turns at 400 rpm when the engine is idling. Its 300-turn, 5.00 by 8.00 cm rectangular coil rotates in an adjustable magnetic field so that it can produce su cient voltage even at low rpms. What is the field strength needed to produce a 24.0 V peak emf? (b) Discuss how this required field strength compares to those available in permanent and electromagnets. 35. Show that if a coil rotates at an angular velocity , the period of its AC output is 2 π / ω . 36. A 75-turn, 10.0 cm diameter coil rotates at an angular velocity of 8.00 rad/s in a 1.25 T field, starting with the plane of the coil parallel to the field. (a) What is the peak emf? (b) At what time is the peak emf first reached? (c) At what time is the emf first at its most negative? (d) What is the period of the AC voltage output?
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37. (a) If the emf of a coil rotating in a magnetic field is zero at = 0 , and increases to its first peak at = 0.100 ms , what is the angular velocity of the coil? (b) At what time will its next maximum occur? (c) What is the period of the output? (d) When is the output first one-fourth of its maximum? (e) When is it next one-fourth of its maximum? 38. Unreasonable Results A 500-turn coil with a 0.250 m 2 area is spun in the Earth’s 5.00 × 10 −5 T field, producing a 12.0 kV maximum emf. (a) At what angular velocity must the coil be spun? (b) What is unreasonable about this result? (c) Which assumption or premise is responsible? 23.6 Back Emf 39. Suppose a motor connected to a 120 V source draws 10.0 A when it first starts. (a) What is its resistance? (b) What current does it draw at its normal operating speed when it develops a 100 V back emf? 40. A motor operating on 240 V electricity has a 180 V back emf at operating speed and draws a 12.0 A current. (a) What is its resistance? (b) What current does it draw when it is first started? 41. What is the back emf of a 120 V motor that draws 8.00 A at its normal speed and 20.0 A when first starting? 42. The motor in a toy car operates on 6.00 V, developing a 4.50 V back emf at normal speed. If it draws 3.00 A at normal speed, what current does it draw when starting? 43. Integrated Concepts The motor in a toy car is powered by four batteries in series, which produce a total emf of 6.00 V. The motor draws 3.00 A and develops a 4.50 V back emf
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at normal speed. Each battery has a 0.100 Ω internal resistance. What is the resistance of the motor? 23.7 Transformers 44. A plug-in transformer, like that in Fi gure 23.26 , supplies 9.00 V to a video game system. (a) How many turns are in its secondary coil, if its input voltage is 120 V and the primary coil has 400 turns? (b) What is its input current when its output is 1.30 A? 45. An American traveler in New Zealand carries a transformer to convert New Zealand’s standard 240 V to 120 V so that she can use some small appliances on her trip. (a) What is the ratio of turns in the primary and secondary coils of her transformer? (b) What is the ratio of input to output current? (c) How could a New Zealander traveling in the United States use this same transformer to power her 240 V appliances from 120 V? 46. A cassette recorder uses a plug-in transformer to convert 120 V to 12.0 V, with a maximum current output of 200 mA. (a) What is the current input? (b) What is the power input? (c) Is this amount of power reasonable for a small appliance? 47. (a) What is the voltage output of a transformer used for rechargeable flashlight batteries, if its primary has 500 turns, its secondary 4 turns, and the input voltage is 120 V? (b) What input current is required to produce a 4.00 A output? (c) What is the power input? 48. (a) The plug-in transformer for a laptop computer puts out 7.50 V and can supply a maximum current of 2.00 A. What is the maximum input current if the input voltage is 240 V? Assume 100% e ciency. (b) If the actual e ciency is less than 100%, would the input current need to be greater or smaller? Explain. 49. A multipurpose transformer has a secondary coil with several points at which a voltage can be extracted, giving outputs of 5.60, 12.0, and 480 V. (a)
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The input voltage is 240 V to a primary coil of 280 turns. What are the numbers of turns in the parts of the secondary used to produce the output voltages? (b) If the maximum input current is 5.00 A, what are the maximum output currents (each used alone)? 50. A large power plant generates electricity at 12.0 kV. Its old transformer once converted the voltage to 335 kV. The secondary of this transformer is being replaced so that its output can be 750 kV for more e cient cross- country transmission on upgraded transmission lines. (a) What is the ratio of turns in the new secondary compared with the old secondary? (b) What is the ratio of new current output to old output (at 335 kV) for the same power? (c) If the upgraded transmission lines have the same resistance, what is the ratio of new line power loss to old? 51. If the power output in the previous problem is 1000 MW and line resistance is 2.00 Ω , what were the old and new line losses? 52. Unreasonable Results The 335 kV AC electricity from a power transmission line is fed into the primary coil of a transformer. The ratio of the number of turns in the secondary to the number in the primary is s / p = 1000 . (a) What voltage is induced in the secondary? (b) What is unreasonable about this result? (c) Which assumption or premise is responsible? 53. Construct Your Own Problem Consider a double transformer to be used to create very large voltages. The device consists of two stages. The first is a transformer that produces a much larger output voltage than its input. The output of the first transformer is used as input to a second transformer that further increases the voltage. Construct a problem in which you calculate the output voltage of the final stage based on the input voltage of the first stage and the number of turns or loops in both parts of both transformers (four coils in a ll ) . Also calculate the maximum
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output current of the final stage based on the input current. Discuss the possibility of power losses in the devices and the e ect ff on the output current and power. 23.8 Electrical Safet y: Systems and Devices 54. Integrated Concepts A short circuit to the grounded metal case of an appliance occurs as shown in Fi gure 23.60 . The person touching the case is wet and only has a 3.00 k Ω resistance to e a r t h / g r o u n d . (a) What is the voltage on the case if 5.00 mA flows through the person? (b) What is the current in the short circuit if the resistance of the earth/ground wire is 0.200 Ω ? (c) Will this trigger the 20.0 A circuit breaker supplying the appliance? Figure 23.60 A person can be shocked even when the case of an appliance is grounded. The large short circuit current produces a voltage on the case of the appliance, since the resistance of the earth/ground wire is not zero.
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23.9 Inductance 55. Two coils are placed close together in a physics lab to demonstrate Faraday’s law of induction. A current of 5.00 A in one is switched o ff in 1.00 ms, inducing a 9.00 V emf in the other. What is their mutual inductance? 56. If two coils placed next to one another have a mutual inductance of 5.00 mH, what voltage is induced in one when the 2.00 A current in the other is switched o in 30.0 ms? ff 57. The 4.00 A current through a 7.50 mH inductor is switched o ff in 8.33 ms. What is the emf induced opposing this? 58. A device is turned on and 3.00 A flows through it 0.100 ms later. What is the self-inductance of the device if an induced 150 V emf opposes this? 59. Starting with emf 2 (V s)/A = s Ω ⋅ . = − Δ 1 , show that the units of inductance are 60. Camera flashes charge a capacitor to high voltage by switching the current through an inductor on and o ff rapidly. In what time must the 0.100 A current through a 2.00 mH inductor be switched on or o ff to induce a 500 V emf? 61. A large research solenoid has a self-inductance of 25.0 H. (a) What induced emf opposes shutting it o when 100 A of current through it is ff switched o ff in 80.0 ms? (b) How much energy is stored in the inductor at full current? (c) At what rate in watts must energy be dissipated to switch the current o in 80.0 ms? (d) In view of the answer to the last part, is it surprising ff that shutting it down this quickly is di cult? 62. (a) Calculate the self-inductance of a 50.0 cm long, 10.0 cm diameter solenoid having 1000 loops. (b) How much energy is stored in this inductor when 20.0 A of current flows through it? (c) How fast can it be turned o ff if the induced emf cannot exceed 3.00 V? Δ
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63. A precision laboratory resistor is made of a coil of wire 1.50 cm in diameter and 4.00 cm long, and it has 500 turns. (a) What is its self-inductance? (b) What average emf is induced if the 12.0 A current through it is turned on in 5.00 ms (one-fourth of a cycle for 50 Hz AC)? (c) What is its inductance if it is shortened to half its length and counter-wound (two layers of 250 turns in opposite directions)? 64. The heating coils in a hair dryer are 0.800 cm in diameter, have a combined length of 1.00 m, and a total of 400 turns. (a) What is their total self- inductance assuming they act like a single solenoid? (b) How much energy is stored in them when 6.00 A flows? (c) What average emf opposes shutting them o if this is done in 5.00 ms (one-fourth of a cycle for 50 Hz AC)? ff 65. When the 20.0 A current through an inductor is turned o ff in 1.50 ms, an 800 V emf is induced, opposing the change. What is the value of the self- inductance? 66. How fast can the 150 A current through a 0.250 H inductor be shut o ff if the induced emf cannot exceed 75.0 V? 67. Integrated Concepts A very large, superconducting solenoid such as one used in MRI scans, stores 1.00 MJ of energy in its magnetic field when 100 A flows. (a) Find its self- inductance. (b) If the coils “go normal,” they gain resistance and start to dissipate thermal energy. What temperature increase is produced if all the stored energy goes into heating the 1000 kg magnet, given its average specific heat is 200 J / kg·º C ? 68. Unreasonable Results A 25.0 H inductor has 100 A of current turned o ff in 1.00 ms. (a) What voltage is induced to oppose this? (b) What is unreasonable about this result? (c) Which assumption or premise is responsible?
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23.10 RL Circuits 69. If you want a characteristic RL time constant of 1.00 s, and you have a 500 Ω resistor, what value of self-inductance is needed? 70. Your RL circuit has a characteristic time constant of 20.0 ns, and a resistance of 5.00 M Ω . (a) What is the inductance of the circuit? (b) What resistance would give you a 1.00 ns time constant, perhaps needed for quick response in an oscilloscope? 71. A large superconducting magnet, used for magnetic resonance imaging, has a 50.0 H inductance. If you want current through it to be adjustable with a 1.00 s characteristic time constant, what is the minimum resistance of system? 72. Verify that after a time of 10.0 ms, the current for the situation considered in Exam ple 23.9 will be 0.183 A as stated. 73. Suppose you have a supply of inductors ranging from 1.00 nH to 10.0 H, and resistors ranging from 0.100 Ω to 1.00 M Ω . What is the range of characteristic RL time constants you can produce by connecting a single resistor to a single inductor? 74. (a) What is the characteristic time constant of a 25.0 mH inductor that has a resistance of 4.00 Ω ? (b) If it is connected to a 12.0 V battery, what is the current after 12.5 ms? 75. What percentage of the final current 0 flows through an inductor in series with a resistor , three time constants after the circuit is completed? 76. The 5.00 A current through a 1.50 H inductor is dissipated by a 2.00 Ω resistor in a circuit like that in Fi gure 23.44 with the switch in position 2. (a) What is the initial energy in the inductor? (b) How long will it take the current to decline to 5.00% of its initial value? (c) Calculate the average power dissipated, and compare it with the initial power dissipated by the resistor.
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77. (a) Use the exact exponential treatment to find how much time is required to bring the current through an 80.0 mH inductor in series with a 15.0 Ω resistor to 99.0% of its final value, starting from zero. (b) Compare your answer to the approximate treatment using integral numbers of . (c) Discuss how significant the di erence is. ff 78. (a) Using the exact exponential treatment, find the time required for the current through a 2.00 H inductor in series with a 0.500 Ω resistor to be reduced to 0.100% of its original value. (b) Compare your answer to the approximate treatment using integral numbers of . (c) Discuss how significant the di erence is. ff 23.11 Reactance , Inductive and Ca pacitive 79. At what frequency will a 30.0 mH inductor have a reactance of 100 Ω ? 80. What value of inductance should be used if a 20.0 k Ω reactance is needed at a frequency of 500 Hz? 81. What capacitance should be used to produce a 2.00 M Ω reactance at 60.0 Hz? 82. At what frequency will an 80.0 mF capacitor have a reactance of 0.250 Ω ? 83. (a) Find the current through a 0.500 H inductor connected to a 60.0 Hz, 480 V AC source. (b) What would the current be at 100 kHz? 84. (a) What current flows when a 60.0 Hz, 480 V AC source is connected to a 0.250 F μ capacitor? (b) What would the current be at 25.0 kHz? 85. A 20.0 kHz, 16.0 V source connected to an inductor produces a 2.00 A current. What is the inductance? 86. A 20.0 Hz, 16.0 V source produces a 2.00 mA current when connected to a capacitor. What is the capacitance?
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87. (a) An inductor designed to filter high-frequency noise from power supplied to a personal computer is placed in series with the computer. What minimum inductance should it have to produce a 2.00 k Ω reactance for 15.0 kHz noise? (b) What is its reactance at 60.0 Hz? 88. The capacitor in Fi gure 23.55 (a) is designed to filter low-frequency signals, impeding their transmission between circuits. (a) What capacitance is needed to produce a 100 k Ω reactance at a frequency of 120 Hz? (b) What would its reactance be at 1.00 MHz? (c) Discuss the implications of your answers to (a) and ( b ) . 89. The capacitor in Fi gure 23.55 (b) will filter high-frequency signals by shorting them to e a r t h / g r o u n d . (a) What capacitance is needed to produce a reactance of 10.0 m Ω for a 5.00 kHz signal? (b) What would its reactance be at 3.00 Hz? (c) Discuss the implications of your answers to (a) and ( b ) . 90. Unreasonable Results In a recording of voltages due to brain activity (an EE G ) , a 10.0 mV signal with a 0.500 Hz frequency is applied to a capacitor, producing a current of 100 mA. Resistance is negligible. (a) What is the capacitance? (b) What is unreasonable about this result? (c) Which assumption or premise is responsible? 91. Construct Your Own Problem Consider the use of an inductor in series with a computer operating on 60 Hz electricity. Construct a problem in which you calculate the relative reduction in voltage of incoming high frequency noise compared to 60 Hz voltage. Among the things to consider are the acceptable series reactance of the inductor for 60 Hz power and the likely frequencies of noise coming through the power lines. 23.12 RLC Series AC Circuits
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92. An RL circuit consists of a 40.0 Ω resistor and a 3.00 mH inductor. (a) Find its impedance at 60.0 Hz and 10.0 kHz. (b) Compare these values of with those found in Exam ple 23.12 in which there was also a capacitor. 93. An RC circuit consists of a 40.0 Ω resistor and a 5.00 F μ capacitor. (a) Find its impedance at 60.0 Hz and 10.0 kHz. (b) Compare these values of with those found in Exam ple 23.12 , in which there was also an inductor. 94. An LC circuit consists of a 3.00 mH inductor and a 5.00 capacitor. (a) Find its impedance at 60.0 Hz and 10.0 kHz. (b) Compare these values of with those found in Exam ple 23.12 in which there was also a resistor. 95. What is the resonant frequency of a 0.500 mH inductor connected to a 40.0 F μ capacitor? 96. To receive AM radio, you want an RLC circuit that can be made to resonate at any frequency between 500 and 1650 kHz. This is accomplished with a fixed 1.00 H μ inductor connected to a variable capacitor. What range of capacitance is needed? 97. Suppose you have a supply of inductors ranging from 1.00 nH to 10.0 H, and capacitors ranging from 1.00 pF to 0.100 F. What is the range of resonant frequencies that can be achieved from combinations of a single inductor and a single capacitor? 98. What capacitance do you need to produce a resonant frequency of 1.00 GHz, when using an 8.00 nH inductor? 99. What inductance do you need to produce a resonant frequency of 60.0 Hz, when using a 2.00 F μ capacitor? 100. The lowest frequency in the FM radio band is 88.0 MHz. (a) What inductance is needed to produce this resonant frequency if it is connected to a 2.50 pF capacitor? (b) The capacitor is variable, to allow the resonant
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frequency to be adjusted to as high as 108 MHz. What must the capacitance be at this frequency? 101. An RLC series circuit has a 2.50 Ω resistor, a 100 H μ inductor, and an 80.0 F μ c a p a c i t o r . ( a ) Find the circuit’s impedance at 120 Hz. (b) Find the circuit’s impedance at 5.00 kHz. (c) If the voltage source has rms = 5.60 V , what is rms at each frequency? (d) What is the resonant frequency of the circuit? (e) What is rms at resonance? 102. An RLC series circuit has a 1.00 k Ω resistor, a 150 H μ inductor, and a 25.0 nF capacitor. (a) Find the circuit’s impedance at 500 Hz. (b) Find the circuit’s impedance at 7.50 kHz. (c) If the voltage source has rms = 408 V , what is rms at each frequency? (d) What is the resonant frequency of the circuit? (e) What is rms at resonance? 103. An RLC series circuit has a 2.50 Ω resistor, a 100 H μ inductor, and an 80.0 F μ capacitor. (a) Find the power factor at = 120 Hz . (b) What is the phase angle at 120 Hz? (c) What is the average power at 120 Hz? (d) Find the average power at the circuit’s resonant frequency. 104. An RLC series circuit has a 1.00 k Ω resistor, a 150 H μ inductor, and a 25.0 nF capacitor. (a) Find the power factor at = 7.50 Hz . (b) What is the phase angle at this frequency? (c) What is the average power at this frequency? (d) Find the average power at the circuit’s resonant frequency. 105. An RLC series circuit has a 200 Ω resistor and a 25.0 mH inductor. At 8000 Hz, the phase angle is 45.0º . (a) What is the impedance? (b) Find the circuit’s capacitance. (c) If power supplied? rms = 408 V is applied, what is the average 106. Referring to Exam ple 23.14 , find the average power at 10.0 kHz.
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0 0 24.1 Maxwell’s Equations: Electromagnetic Waves Predicted and Observed 1. Verify that the correct value for the speed of light is obtained when numerical values for the permeability and permittivity of free space ( 0 and 0 ) are entered into the equation = 1 . 2. Show that, when SI units for 0 and 0 are entered, the units given by the right-hand side of the equation in the problem above are m/s. 24.2 Production of Electroma gnetic Waves 3. What is the maximum electric field strength in an electromagnetic wave that has a maximum magnetic field strength of 5.00 × 10 −4 Earth’s)? T (about 10 times the 4. The maximum magnetic field strength of an electromagnetic field is 5 × 10 −6 T . Calculate the maximum electric field strength if the wave is traveling in a medium in which the speed of the wave is 0.75 . 5. Verify the units obtained for magnetic field strength in Exam ple 24.1 (using the equation = ) are in fact teslas ( T ) . 24.3 The Electroma gnetic Spectrum 6. (a) Two microwave frequencies are authorized for use in microwave ovens: 900 and 2560 MHz. Calculate the wavelength of each. (b) Which frequency would produce smaller hot spots in foods due to interference e ects? ff 7. (a) Calculate the range of wavelengths for AM radio given its frequency range is 540 to 1600 kHz. (b) Do the same for the FM frequency range of 88.0 to 108 MHz. 8. A radio station utilizes frequencies between commercial AM and FM. What is the frequency of a 11.12-m-wavelength channel?
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9. Find the frequency range of visible light, given that it encompasses wavelengths from 380 to 760 nm. 10. Combing your hair leads to excess electrons on the comb. How fast would you have to move the comb up and down to produce red light? 11. Electromagnetic radiation having a 15.0 − m wavelength is classified as infrared radiation. What is its frequency? 12. Approximately what is the smallest detail observable with a microscope that uses ultraviolet light of frequency 1.20 × 10 15 Hz ? 13. A radar used to detect the presence of aircraft receives a pulse that has reflected o ff an object 6×10 −5 s after it was transmitted. What is the distance from the radar station to the reflecting object? 14. Some radar systems detect the size and shape of objects such as aircraft and geological terrain. Approximately what is the smallest observable detail utilizing 500-MHz radar? 15. Determine the amount of time it takes for X-rays of frequency 3×10 18 Hz to travel (a) 1 mm and (b) 1 cm. 16. If you wish to detect details of the size of atoms (about 10 −10 m ) with electromagnetic radiation, it must have a wavelength of about this size. (a) What is its frequency? (b) What type of electromagnetic radiation might this be? 17. If the Sun suddenly turned o , ff we would not know it until its light stopped coming. How long would that be, given that the Sun is 1.50×10 11 m away? 18. Distances in space are often quoted in units of light years, the distance light travels in one year. (a) How many meters is a light year? (b) How many meters is it to Andromeda, the nearest large galaxy, given that it is 2.00×10 6
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light years away? (c) The most distant galaxy yet discovered is 12.0×10 9 years away. How far is this in meters? light 19. A certain 50.0-Hz AC power line radiates an electromagnetic wave having a maximum electric field strength of 13.0 k V / m . (a) What is the wavelength of this very low frequency electromagnetic wave? (b) What is its maximum magnetic field strength? 20. During normal beating, the heart creates a maximum 4.00-mV potential across 0.300 m of a person’s chest, creating a 1.00-Hz electromagnetic wave. (a)What is the maximum electric field strength created? (b) What is the corresponding maximum magnetic field strength in the electromagnetic wave? (c) What is the wavelength of the electromagnetic wave? 21. (a) The ideal size (most e cient) for a broadcast antenna with one end on the ground is one-fourth the wavelength ( /4 ) of the electromagnetic radiation being sent out. If a new radio station has such an antenna that is 50.0 m high, what frequency does it broadcast most e ciently? Is this in the AM or FM band? (b) Discuss the analogy of the fundamental resonant mode of an air column closed at one end to the resonance of currents on an antenna that is one-fourth their wavelength. 22. (a) What is the wavelength of 100-MHz radio waves used in an MRI unit? (b) If the frequencies are swept over a ±1.00 range centered on 100 MHz, what is the range of wavelengths broadcast? 23. (a) What is the frequency of the 193-nm ultraviolet radiation used in laser eye surgery? (b) Assuming the accuracy with which this EM radiation can ablate the cornea is directly proportional to wavelength, how much more accurate can this UV be than the shortest visible wavelength of light? 24. TV-reception antennas for VHF are constructed with cross wires supported at their centers, as shown in Fi gure 24.27 . The ideal length for the cross wires is one-half the wavelength to be received, with the more expensive antennas having one for each channel. Suppose you measure the lengths of
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the wires for particular channels and find them to be 1.94 and 0.753 m long, respectively. What are the frequencies for these channels? Figure 24.27 A television reception antenna has cross wires of various lengths to most e ciently receive di erent ff wavelengths. 25. Conversations with astronauts on lunar walks had an echo that was used to estimate the distance to the Moon. The sound spoken by the person on Earth was transformed into a radio signal sent to the Moon, and transformed back into sound on a speaker inside the astronaut’s space suit. This sound was picked up by the microphone in the space suit (intended for the astronaut’s voice) and sent back to Earth as a radio echo of sorts. If the round-trip time was 2.60 s, what was the approximate distance to the Moon, neglecting any delays in the electronics? 26. Lunar astronauts placed a reflector on the Moon’s surface, o ff which a laser beam is periodically reflected. The distance to the Moon is calculated from the round-trip time. (a) To what accuracy in meters can the distance to the Moon be determined, if this time can be measured to 0.100 ns? (b) What percent accuracy is this, given the average distance to the Moon is 3.84×10 8 m ?
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27. Radar is used to determine distances to various objects by measuring the round-trip time for an echo from the object. (a) How far away is the planet Venus if the echo time is 1000 s? (b) What is the echo time for a car 75.0 m from a Highway Police radar unit? (c) How accurately (in nanoseconds) must you be able to measure the echo time to an airplane 12.0 km away to determine its distance within 10.0 m? 28. Integrated Concepts (a)Calculate the ratio of the highest to lowest frequencies of electromagnetic waves the eye can see, given the wavelength range of visible light is from 380 to 760 nm. (b) Compare this with the ratio of highest to lowest frequencies the ear can hear. 29. Integrated Concepts (a)Calculate the rate in watts at which heat transfer through radiation occurs (almost entirely in the infrared) from 1.0 m 2 of the Earth’s surface at night. Assume the emissivity is 0.90, the temperature of the Earth is 15ºC , and that of outer space is 2.7 K. (b) Compare the intensity of this radiation with that coming to the Earth from the Sun during the day, which averages about 800 W/m 2 , only half of which is absorbed. (c) What is the maximum magnetic field strength in the outgoing radiation, assuming it is a continuous wave? 24.4 Ener gy in Electromagnetic Waves 30. What is the intensity of an electromagnetic wave with a peak electric field strength of 125 V/m? 31. Find the intensity of an electromagnetic wave having a peak magnetic field strength of 4.00×10 −9 T .
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32. Assume the helium-neon lasers commonly used in student physics laboratories have power outputs of 0.250 mW. (a) If such a laser beam is
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projected onto a circular spot 1.00 mm in diameter, what is its intensity? (b) Find the peak magnetic field strength. (c) Find the peak electric field strength. 33. An AM radio transmitter broadcasts 50.0 kW of power uniformly in all directions. (a) Assuming all of the radio waves that strike the ground are completely absorbed, and that there is no absorption by the atmosphere or other objects, what is the intensity 30.0 km away? (Hint: Half the power will be spread over the area of a h e m i sp h e r e . ) (b) What is the maximum electric field strength at this distance? 34. Suppose the maximum safe intensity of microwaves for human exposure is taken to be 1.00 W/m 2 . (a) If a radar unit leaks 10.0 W of microwaves (other than those sent by its antenna) uniformly in all directions, how far away must you be to be exposed to an intensity considered to be safe? Assume that the power spreads uniformly over the area of a sphere with no complications from absorption or reflection. (b) What is the maximum electric field strength at the safe intensity? (Note that early radar units leaked more than modern ones do. This caused identifiable health problems, such as cataracts, for people who worked near t h e m . ) 35. A 2.50-m-diameter university communications satellite dish receives TV signals that have a maximum electric field strength (for one channel) of 7.50 V/m . (See Fi gure 24.28 . ) (a) What is the intensity of this wave? (b) What is the power received by the antenna? (c) If the orbiting satellite broadcasts uniformly over an area of 1.50×10 13 m 2 A m e r i c a ) , how much power does it radiate? (a large fraction of North
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2 2 Figure 24.28 Satellite dishes receive TV signals sent from orbit. Although the signals are quite weak, the receiver can detect them by being tuned to resonate at their frequency. 36. Lasers can be constructed that produce an extremely high intensity electromagnetic wave for a brief time—called pulsed lasers. They are used to ignite nuclear fusion, for example. Such a laser may produce an electromagnetic wave with a maximum electric field strength of 1.00×10 11 V / m for a time of 1.00 ns. (a) What is the maximum magnetic field strength in the wave? (b) What is the intensity of the beam? (c) What energy does it deliver on a 1.00-mm 2 area? 37. Show that for a continuous sinusoidal electromagnetic wave, the peak intensity is twice the average intensity ( 0 = 2 ave ) , using either the fact that 0 = rms , or 0 = rms , where rms means average (actually root mean square, a type of a ve r a g e ) . 38. Suppose a source of electromagnetic waves radiates uniformly in all directions in empty space where there are no absorption or interference e ects. ff (a) Show that the intensity is inversely proportional to 2 , the distance
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from the source squared. (b) Show that the magnitudes of the electric and magnetic fields are inversely proportional to . 39. Integrated Concepts An circuit with a 5.00-pF capacitor oscillates in such a manner as to radiate at a wavelength of 3.30 m. (a) What is the resonant frequency? (b) What inductance is in series with the capacitor? 40. Integrated Concepts What capacitance is needed in series with an 800 − H inductor to form a circuit that radiates a wavelength of 196 m? 41. Integrated Concepts Police radar determines the speed of motor vehicles using the same Doppler- shift technique employed for ultrasound in medical diagnostics. Beats are produced by mixing the double Doppler-shifted echo with the original frequency. If 1.50×10 9 -Hz microwaves are used and a beat frequency of 150 Hz is produced, what is the speed of the vehicle? (Assume the same Doppler- shift formulas are valid with the speed of sound replaced by the speed of l i g h t . ) 42. Integrated Concepts Assume the mostly infrared radiation from a heat lamp acts like a continuous wave with wavelength 1.50m . (a) If the lamp’s 200-W output is focused on a person’s shoulder, over a circular area 25.0 cm in diameter, what is the intensity in W/m 2 ? (b) What is the peak electric field strength? (c) Find the peak magnetic field strength. (d) How long will it take to increase the temperature of the 4.00-kg shoulder by 2.00º C , assuming no other heat transfer and given that its specific heat is 3.47× 10 3 J / kg ºC ?
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43. Integrated Concepts On its highest power setting, a microwave oven increases the temperature of 0.400 kg of spaghetti by 45.0ºC in 120 s. (a) What was the rate of power absorption by the spaghetti, given that its specific heat is 3.76×10 3 J / kg ºC ? (b) Find the average intensity of the microwaves, given that they are absorbed over a circular area 20.0 cm in diameter. (c) What is the peak electric field strength of the microwave? (d) What is its peak magnetic field strength? 44. Integrated Concepts Electromagnetic radiation from a 5.00-mW laser is concentrated on a 1.00-mm 2 area. (a) What is the intensity in W/m 2 ? (b) Suppose a 2.00-nC static charge is in the beam. What is the maximum electric force it experiences? (c) If the static charge moves at 400 m/s, what maximum magnetic force can it feel? 45. Integrated Concepts A 200-turn flat coil of wire 30.0 cm in diameter acts as an antenna for FM radio at a frequency of 100 MHz. The magnetic field of the incoming electromagnetic wave is perpendicular to the coil and has a maximum strength of 1.00× 10 −12 T . (a) What power is incident on the coil? (b) What average emf is induced in the coil over one-fourth of a cycle? (c) If the radio receiver has an inductance of 2.50 H , what capacitance must it have to resonate at 100 MHz? 46. Integrated Concepts If electric and magnetic field strengths vary sinusoidally in time, being zero at = 0 , then = 0 sin 2 π and = 0 sin 2 π . Let = 1.00 GHz here. (a) When are the field strengths first zero? (b) When do they reach their
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most negative value? (c) How much time is needed for them to complete one cycle? 47. Unreasonable Results A researcher measures the wavelength of a 1.20-GHz electromagnetic wave to be 0.500 m. (a) Calculate the speed at which this wave propagates. (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent? 48. Unreasonable Results The peak magnetic field strength in a residential microwave oven is 9.20× 10 −5 T . (a) What is the intensity of the microwave? (b) What is unreasonable about this result? (c) What is wrong about the premise? 49. Unreasonable Results An circuit containing a 2.00-H inductor oscillates at such a frequency that it radiates at a 1.00-m wavelength. (a) What is the capacitance of the circuit? (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent? 50. Unreasonable Results An circuit containing a 1.00-pF capacitor oscillates at such a frequency that it radiates at a 300-nm wavelength. (a) What is the inductance of the circuit? (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent? 51. Create Your Own Problem Consider electromagnetic fields produced by high voltage power lines. Construct a problem in which you calculate the intensity of this electromagnetic radiation in W/m 2 based on the measured magnetic field
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strength of the radiation in a home near the power lines. Assume these magnetic field strengths are known to average less than a T . The intensity is small enough that it is di cult to imagine mechanisms for biological damage due to it. Discuss how much energy may be radiating from a section of power line several hundred meters long and compare this to the power likely to be carried by the lines. An idea of how much power this is can be obtained by calculating the approximate current responsible for tens of meters. T fields at distances of 52. Create Your Own Problem Consider the most recent generation of residential satellite dishes that are a little less than half a meter in diameter. Construct a problem in which you calculate the power received by the dish and the maximum electric field strength of the microwave signals for a single channel received by the dish. Among the things to be considered are the power broadcast by the satellite and the area over which the power is spread, as well as the area of the receiving dish.
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25.1 The Ra y Aspect of Light 1. Suppose a man stands in front of a mirror as shown in Fi gure 25.50 . His eyes are 1.65 m above the floor, and the top of his head is 0.13 m higher. Find the height above the floor of the top and bottom of the smallest mirror in which he can see both the top of his head and his feet. How is this distance related to the man’s height? Figure 25.50 A full- length mirror is one in which you can see all of yourself. It need not be as big as you, and its size is independent of your distance from it. 25.2 The Law of Reflection 2. Show that when light reflects from two mirrors that meet each other at a right angle, the outgoing ray is parallel to the incoming ray, as illustrated in the following figure.
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Figure 25.51 A corner reflector sends the reflected ray back in a direction parallel to the incident ray, independent of incoming direction. 3. Light shows staged with lasers use moving mirrors to swing beams and create colorful effects. Show that a light ray reflected from a mirror changes direction by 2 when the mirror is rotated by an angle . 4. A flat mirror is neither converging nor diverging. To prove this, consider two rays originating from the same point and diverging at an angle . Show that after striking a plane mirror, the angle between their directions remains . Figure 25.52 A flat mirror neither converges nor diverges light
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rays. Two rays continue to diverge at the same angle after reflection.
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25.3 The Law of Refraction 5. What is the speed of light in water? In g l yc e r i n e ? 6. What is the speed of light in air? In crown glass? 7. Calculate the index of refraction for a medium in which the speed of light is 2.012 × 10 8 25.1 . m/s , and identify the most likely substance based on Table 8. In what substance in Table 25.1 is the speed of light 2.290 × 10 8 m/s ? 9. There was a major collision of an asteroid with the Moon in medieval times. It was described by monks at Canterbury Cathedral in England as a red glow on and around the Moon. How long after the asteroid hit the Moon, which is 3.84 × 10 5 km away, would the light first arrive on Earth? 10. A scuba diver training in a pool looks at his instructor as shown in Fi gure 25.53 . What angle does the ray from the instructor’s face make with the perpendicular to the water at the point where the ray enters? The angle between the ray in the water and the perpendicular to the water is 25.0º .
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Figure 25.53 A scuba diver in a pool and his trainer look at each other. 11. Components of some computers communicate with each other through optical fibers having an index of refraction = 1.55 . What time in nanoseconds is required for a signal to travel 0.200 m through such a fiber? 12. (a) Given that the angle between the ray in the water and the perpendicular to the water is 25.0º , and using information in Fi gure 25.53 , find the height of the instructor’s head above the water, noting that you will first have to calculate the angle of refraction. (b) Find the apparent depth of the diver’s head below water as seen by the instructor. Assume the diver and the d i ve r ' s image are the same horizontal distance from the normal. 13. Suppose you have an unknown clear substance immersed in water, and you wish to identify it by finding its index of refraction. You arrange to have a beam of light enter it at an angle of 45.0º ,
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and you observe the angle of refraction to be 40.3º . What is the index of refraction of the substance and its likely identity?
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14. On the Moon’s surface, lunar astronauts placed a corner reflector, off which a laser beam is periodically reflected. The distance to the Moon is calculated from the round-trip time. What percent correction is needed to account for the delay in time due to the slowing of light in Earth’s atmosphere? Assume the distance to the Moon is precisely 3.84 × 10 8 m , and Earth’s atmosphere (which varies in density with altitude) is equivalent to a layer 30.0 km thick with a constant index of refraction = 1.000293 . 15. Suppose Fi gure 25.54 represents a ray of light going from air through crown glass into water, such as going into a fish tank. Calculate the amount the ray is displaced by the glass ( Δ and the glass is 1.00 cm thick. ), given that the incident angle is 40.0º 16. Fi gure 25.54 shows a ray of light passing from one medium into a second and then a third. Show that 3 is the same as it would be if the second medium were not present (provided total internal reflection does not occur).
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Figure 25.54 A ray of light passes from one medium to a third by traveling through a second. The final direction is the same as if the second medium were not present, but the ray is displaced by Δ exaggerated). (shown
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17. Unreasonable Results Suppose light travels from water to another substance, with an angle of incidence of 10.0º and an angle of refraction of 14.9º . (a) What is the index of refraction of the other substance? (b) What is unreasonable about this r e su l t ? (c) Which assumptions are unreasonable or inconsistent? 18. Construct Your Own Problem Consider sunlight entering the Earth’s atmosphere at sunrise and sunset—that is, at a 90º incident angle. Taking the boundary between nearly empty space and the atmosphere to be sudden, calculate the angle of refraction for sunlight. This lengthens the time the Sun appears to be above the horizon, both at sunrise and sunset. Now construct a problem in which you determine the angle of refraction for different models of the atmosphere, such as various layers of varying density. Your instructor may wish to guide you on the level of complexity to consider and on how the index of refraction varies with air density. 19. Unreasonable Results Light traveling from water to a gemstone strikes the surface at an angle of 80.0º and has an angle of refraction of 15.2º . (a) What is the speed of light in the gemstone? (b) What is unreasonable about this r e su l t ? (c) Which assumptions are unreasonable or inconsistent? 25.4 Total Internal Reflection 20. Verify that the critical angle for light going from water to air is 48.6º , as discussed at the end of Exam ple 25.4 , regarding the
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critical angle for light traveling in a polystyrene (a type of plastic) pipe surrounded by air. 21. (a) At the end of Exam ple 25.4 , it was stated that the critical angle for light going from diamond to air is 24.4º . Verify this. (b) What is the critical
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angle for light going from zircon to air? 22. An optical fiber uses flint glass clad with crown glass. What is the critical a n g l e ? 23. At what minimum angle will you get total internal reflection of light traveling in water and reflected from ice? 24. Suppose you are using total internal reflection to make an efficient corner reflector. If there is air outside and the incident angle is 45.0º , what must be the minimum index of refraction of the material from which the reflector is made? 25. You can determine the index of refraction of a substance by determining its critical angle. (a) What is the index of refraction of a substance that has a critical angle of 68.4º when submerged in water? What is the substance, based on Table 25.1 ? (b) What would the critical angle be for this substance in air? 26. A ray of light, emitted beneath the surface of an unknown liquid with air above it, undergoes total internal reflection as shown in Fi gure 25.55 . What is the index of refraction for the liquid and its likely identification? Figure 25.55 A light ray inside a liquid strikes the surface at the critical angle and undergoes total internal reflection.
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27. A light ray entering an optical fiber surrounded by air is first refracted and then reflected as shown in Fi gure 25.56 . Show that if the fiber is made from crown glass, any incident ray will be totally internally reflected. Figure 25.56 A light ray enters the end of a fiber, the surface of which is perpendicular to its sides. Examine the conditions under which it may be totally internally reflected. 25.5 Dis persion: The Rainbow and Prisms 28. (a) What is the ratio of the speed of red light to violet light in diamond, based on Table 25.2 ? (b) What is this ratio in po l yst yr e n e ? (c) Which is more dispersive? 29. A beam of white light goes from air into water at an incident angle of 75.0º . At what angles are the red (660 nm) and violet (410 nm) parts of the light refracted? 30. By how much do the critical angles for red (660 nm) and violet (410 nm) light differ in a diamond surrounded by air? 31. (a) A narrow beam of light containing yellow (580 nm) and green (550 nm) wavelengths goes from polystyrene to air, striking the surface at a 30.0º incident angle. What is the angle between the colors when they emerge? (b) How far would they have to travel to be separated by 1.00 mm?
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32. A parallel beam of light containing orange (610 nm) and violet (410 nm) wavelengths goes from fused quartz to water, striking the surface between
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them at a 60.0º incident angle. What is the angle between the two colors in water? 33. A ray of 610 nm light goes from air into fused quartz at an incident angle of 55.0º . At what incident angle must 470 nm light enter flint glass to have the same angle of refraction? 34. A narrow beam of light containing red (660 nm) and blue (470 nm) wavelengths travels from air through a 1.00 cm thick flat piece of crown glass and back to air again. The beam strikes at a 30.0º incident angle. (a) At what angles do the two colors emerge? (b) By what distance are the red and blue separated when they emerge? 35. A narrow beam of white light enters a prism made of crown glass at a 45.0º incident angle, as shown in Fi gure 25.57 . At what angles, R and V , do the red (660 nm) and violet (410 nm) components of the light emerge from the prism? Figure 25.57 This prism will disperse the white light into a rainbow of colors. The incident angle is 45.0º , and the angles at which the red and violet light emerge are R and V . 25.6 Ima ge Formation b y Lenses
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36. What is the power in diopters of a camera lens that has a 50.0 mm focal l e n g t h ?
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37. Your camera’s zoom lens has an adjustable focal length ranging from 80.0 to 200 mm. What is its range of powers? 38. What is the focal length of 1.75 D reading glasses found on the rack in a pharmacy? 39. You note that your prescription for new eyeglasses is –4.50 D. What will their focal length be? 40. How far from the lens must the film in a camera be, if the lens has a 35.0 mm focal length and is being used to photograph a flower 75.0 cm away? Explicitly show how you follow the steps in the Problem-Solving Strategy for lenses. 41. A certain slide projector has a 100 mm focal length lens. (a) How far away is the screen, if a slide is placed 103 mm from the lens and produces a sharp image? (b) If the slide is 24.0 by 36.0 mm, what are the dimensions of the image? Explicitly show how you follow the steps in the Problem-Solving Strategy for lenses. 42. A doctor examines a mole with a 15.0 cm focal length magnifying glass held 13.5 cm from the mole (a) Where is the image? (b) What is its magnification? (c) How big is the image of a 5.00 mm diameter m o l e ? 43. How far from a piece of paper must you hold your father’s 2.25 D reading glasses to try to burn a hole in the paper with su n l i g h t ? 44. A camera with a 50.0 mm focal length lens is being used to photograph a person standing 3.00 m away. (a) How far from the lens must the film be? (b) If the film is 36.0 mm high, what fraction of a 1.75 m tall person will fit on it? (c) Discuss how reasonable this seems, based on your experience in taking or posing for photographs. 45. A camera lens used for taking close-up photographs has a focal length of
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22.0 mm. The farthest it can be placed from the film is 33.0 mm. (a) What is
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the closest object that can be photographed? (b) What is the magnification of this closest ob j e c t ? 46. Suppose your 50.0 mm focal length camera lens is 51.0 mm away from the film in the camera. (a) How far away is an object that is in focus? (b) What is the height of the object if its image is 2.00 cm high? 47. (a) What is the focal length of a magnifying glass that produces a magnification of 3.00 when held 5.00 cm from an object, such as a rare coin? (b) Calculate the power of the magnifier in diopters. (c) Discuss how this power compares to those for store-bought reading glasses (typically 1.0 to 4.0 D). Is the magnifier’s power greater, and should it be? 48. What magnification will be produced by a lens of power –4.00 D (such as might be used to correct myopia) if an object is held 25.0 cm away? 49. In Exam ple 25.7 , the magnification of a book held 7.50 cm from a 10.0 cm focal length lens was found to be 4.00. (a) Find the magnification for the book when it is held 8.50 cm from the magnifier. (b) Do the same for when it is held 9.50 cm from the magnifier. (c) Comment on the trend in m as the object distance increases as in these two calculations. 50. Suppose a 200 mm focal length telephoto lens is being used to photograph mountains 10.0 km away. (a) Where is the image? (b) What is the height of the image of a 1000 m high cliff on one of the mountains? 51. A camera with a 100 mm focal length lens is used to photograph the sun and moon. What is the height of the image of the sun on the film, given the sun is 1.40 × 10 6 km in diameter and is 1.50 × 10 8 km away?
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52. Combine thin lens equations to show that the magnification for a thin lens is determined by its focal length and the object distance and is given by = /( o ) . 25.7 Ima ge Formation by Mirrors
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53. What is the focal length of a makeup mirror that has a power of 1.50 D? 54. Some telephoto cameras use a mirror rather than a lens. What radius of curvature mirror is needed to replace a 800 mm focal length telephoto lens? 55. (a) Calculate the focal length of the mirror formed by the shiny back of a spoon that has a 3.00 cm radius of curvature. (b) What is its power in diopters? 56. Find the magnification of the heater element in Exam ple 25.9 . Note that its large magnitude helps spread out the reflected energy. 57. What is the focal length of a makeup mirror that produces a magnification of 1.50 when a person’s face is 12.0 cm away? Explicitly show how you follow the steps in the Problem-Solvin g Strategy for Mirrors . 58. A shopper standing 3.00 m from a convex security mirror sees his image with a magnification of 0.250. (a) Where is his image? (b) What is the focal length of the mirror? (c) What is its radius of curvature? Explicitly show how you follow the steps in the Problem-Solvin g Strategy for Mirrors . 59. An object 1.50 cm high is held 3.00 cm from a person’s cornea, and its reflected image is measured to be 0.167 cm high. (a) What is the magnification? (b) Where is the image? (c) Find the radius of curvature of the convex mirror formed by the cornea. (Note that this technique is used by optometrists to measure the curvature of the cornea for contact lens fitting. The instrument used is called a keratometer, or curve measurer.) 60. Ray tracing for a flat mirror shows that the image is located a distance behind the mirror equal to the distance of the object from the mirror. This is
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stated i = – o , since this is a negative image distance (it is a virtual image). (a) What is the focal length of a flat mirror? (b) What is its power? 61. Show that for a flat mirror i = ℎ o , knowing that the image is a distance behind the mirror equal in magnitude to the distance of the object from the
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mirror. 62. Use the law of reflection to prove that the focal length of a mirror is half its radius of curvature. That is, prove that = /2 . Note this is true for a spherical mirror only if its diameter is small compared with its radius of curvature. 63. Referring to the electric room heater considered in the first example in this section, calculate the intensity of IR radiation in W/m 2 projected by the concave mirror on a person 3.00 m away. Assume that the heating element radiates 1500 W and has an area of 100 cm 2 , and that half of the radiated power is reflected and focused by the mirror. 64. Consider a 250-W heat lamp fixed to the ceiling in a bathroom. If the filament in one light burns out then the remaining three still work. Construct a problem in which you determine the resistance of each filament in order to obtain a certain intensity projected on the bathroom floor. The ceiling is 3.0 m high. The problem will need to involve concave mirrors behind the filaments. Your instructor may wish to guide you on the level of complexity to consider in the electrical components.
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