Physics Quizzes (1 - 4)

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QuUIZ 1 Two balls with identical charges g and masses m are placed one above the other in a vertical tube. The bottom ball is resting on the ground, and the upper ball is hovering at a height of y. The charged balls are in equilibrium under the combined forces of electrostatics and gravity. If half of the mass of the top ball (i.e., m/2) is transferred to the bottom ball, but the charges of the balls remain the same, the distance between the balls in the tube would become: ¥ « A y:the distance does not change because the charges have not changed. B v2y c.y/2 D 4/3/4y E 1.5y Answer Key:B Question 2 of 7 1.0 1.0 Points Consider the three point charges and their electric field lines in the attached figure. Considering what you know about how electric field lines are represented. which of the following statements are not true? A.The electric field is strongest near each of the charges. B. The top and bottom charges are equal in sign and magnitude. « C. The middle charge is positive and has the same magnitude as the other two charges. D. The electric field is strongest near each of the charges. Even if the top and bottom charges are at the same potential, they are not connected by an equipotential surface. + « E. The total amount of work done by the electric field in moving an additional test charge gy from the top to the bottom point charge depends on the sign of gp. Answer Key:C, E

Queston3of7 | 10 1.0 points Consider a parallel-plate capacitor with a separation d = 4.26 mm between the plates with an energy density of u;. We multiply the charge surface density o on each of the plates by a factor of z = 2.20, and move them a factor of y = 1.61 further apart. In the new configuration the energy density of the capacitor is . Find uy/u, = «# 5.24 . Give your answer to two decimals. Answer ey 244 Question4of7 | 00 1.0 Points Which of the following statements about the electric fields of charge distributions are not true? 4 A The electric field of a plane of charge s weaker than that of a point charge at all separations. & The electric field of an infinite plane of charge is proportional only to the charge surface density, and not to the istance. X ' C The electric field of a -tq dipole of length d at a distance r > d from the dipole is weaker than the electric field an infinite charged wire with a linear charge density of A = g/d at the same distance r. 0. The electric field strength of dipole drops off faster with distance than the electric field strength of a line of charge € The electric field of an electric dipole with charges -q is weaker than the electric field of a point charge g at distances much larger than the length of the ipole. Answer KeyA

Questonsof7 | 10 1.0 points A cylindrical capacitor consists of a positively charged metal core of radius @ and negatively charged outer metal shell of radius b. The core and the shell carry equal and opposite charges per unit length. The thickness of the outer shell can be ignored. Which of the following graphs best describes the electric field as a function of distance from the vertical axis of the cylindrical capacitor? E0) A0 v a b E) . r a b E(r)| . . a b E) \ EOT I e S a b e Answer KeyD

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Question6of7 00 1.0 Points Star Trek’s ship The Enterprise uses an ion propulsion drive. Much like a present-day conventional rocket, the ion drive works by conservation of linear momentum: fuel is expelled in one direction at a high relative velocity, and the rocket moves in the opposite direction with the total momentum of rocket-plus-fuel system unchanged. If current ion-drive technology is still used in the distant Star Trek future, the propellant would be *2Xe (xenon with atomic weight of 132) that is positively ionized so that it is missing some (n) electrons. Assume that n = 3 and that the ion propulsion engine accelerates the xenon ions from a near-zero starting velocity through a potential difference of AV = —144.5 volts in the propulsion chamber. What is the velocity of the xenon ions as they escape the propulsion chamber? Give vour answer in units of km/s to 2 decimals: v = ¥ 1.06e16 ks, Answer Key:25.07 Question70of7 0.0 1.0 Points Consider an electric potential defined by V(z,y,2) = %1:2: + y [volts]. What is the magnitude of the electric field E at (z,y, 2) = (6.45,9.84,6.03) m? Provide your answer in units of V/m to two decimals: E= 3 10.23 V/m. Answer Key:44.12 QUIZ 2

Question1of7 | 00 1.0 Points. Consider the circuit in the figure. Which combination of capital letters includes all points that would be considered nodes in the meaning of Kirchhoff's Node Law? B 1200 YA 2 A None are considered nodes % ® B Al capital letters: ABCDEF c ABDE D.CDEF ECF Answer KeyE

Question20f7 | 00 1.0 Points. Consider the pair of R-C circuits shown in Figures (2) and (5). The values of ¢, R, 2nd C are identical between the two Figures. Let e = 722V, R = 6912, 2nd C' = 6.65 uF, and the time constants of the two R-C circuits be , and 7, respectively. Whatisthe ratio 7, /n? Give your anstwer to two decimals: 7,/n= X 400 —v—I L (a) Answer Key0.25 Queston3of7 | 10 1.0 Points Electrons in an electric circuit pass through a resistor R. The wire on either side of the resistor has the same diameter. The voltage measured across the resistor is V. Which of the following statements is not true? v @ A The potential energy of an electron entering the resistor is lower than that of an electron leaving the resistor. 8. The power output of the resistor is V2/R C.If we doubled the thickness of the wire, but kept the voltage V" acros ame, the drift speed of the electrons in the wire would decrease by a factor of 4 D. The drift speed of the electrons on either side of the resistor is the same. £ If the resistor and the wires have the same diameters, then the current densities in them are identical, even if they are made of different materials Answer KeyA

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Question4of7 | 00 1.0 points Consider the two circuits in Figures (2) and (b). A is an ammeter and V is a voltmeter. The emf & and the light bulb are identical between the two circuits. Which of the following statements is not true? (a) £ ® Light bulb (b) & +| 1 Q@ @ g4 Light bulb If the emf was an ideal battery, the light bulb in both Figures would shine more brightly than if the emf was a real battery with an added internal resistance. &, IfAwas an ideal ammeter, the light bulb in Figure (2) would shine more brightly than if A was a real ammeter. C. IfV was an ideal voltmeter, the light bulb in Figure (b) would shine more brightly than if V' was a real voltmeter D. If A was an ideal ammeter, it would me: ure 2 higher current than if A was a real ammeter X ® E IfAand V are real, as opposed to ideal, ammeter and voltmeter, the light bulb in Figure (2) would shine more brightly than the light bulb in Figure (b). Answer KeyC

Consider two typical household copper wires: one is gauge 14 (so, diameter of 1.63 mm) and the other is gauge 12 (so, diameter of 2.05 mm). (Larger gauge means a thinner wire!) Which of the following statements is not true? A Ifwe doubled the diameter of the 14-gauge wire (which would make it gauge 8). we would decrease its resistance by a factor of 4 A 10.0 cm long 14-gauge wire has the same resistance a5 a 6.3 cm long 12-gauge wire. I we doubled the length of the 12-gauge wire, that would double its resistance. D. The conductivities of a 10 cm long 14-gauge wire and a 12.6 cm long 12-gauge wire are the same. £ Ifthe two wires have identical lengths then they have the same resistivity. (Recall that res: ity and resistance are not the same). Answer KeyB Queston6of7 10 1.0 Points Gold plating is often used in electronics to provide a corrosion-resistant electrically conductive layer on copper. In the process, gold ions Au* are deposited uniformly on the negatively charged copper element in the plating chamber. Suppose you want to deposit m = 0.74 gof pure gold. An Au* ion weighs 3.27 x 102 g and has a charge of ~e. Gold has a density of 19,300 kg'm?. Your apparatus uses a current of I = 3.95 A. How long do you need to run the Au* ion current to plate the surface of the copper element? Give your answer in seconds, to two decimals: ¢ v 9167s. Answer Key1.8 Question7of7 | 00 1.0 Points Consider the attached figure that shows four identical resistors, a light bulb, and an ideal battery. Let all resistors and the light bulb have identical resistances of R = 7.52 £2, and the emf of the battery be & = 1.6 V. When the switch S is open, the light bulb shines with power P, and when S is closed, it shines with power Py. Find the ratio of Py /Py. Give your answer to two decimals: Py /P = % 08¢ Answer Key0.87

QUIZ 3 Question 1 0of 7 1.0 1.0 Points Which one of the following statements about electric and magnetic fields is not true? A.Much like the electric field of an infinite charged sheet, the magnetic field of an infinite conducting sheet is independent of distance from the sheet. B. The dipole moments of electric and magnetic dipoles are such that they seek to align with the direction of any external electric or magnetic fields, respectively. < o C. An electric dipole that is held stationary in an external magnetic field experiences a torque seeking to align it with the magnetic field. D. The strength of the electric field of an electric dipole follows an = dependence on distance from the dipole, similar to the strength of the magnetic field of a magnetic dipole E. Earth’s magnetic field is such that Earth’s south magnetic pole is located near Earth’s north geographic pole. Answer Key:C Question 2 of 7 1.0 1.0 Points Consider the two long parallel wires shown in the figure. The distance between the wires is d = 0.16 m, and the force per unit length exerted by the wires on one another is F//L=1.68x10° N/'m. Current I, is 2.95 times larger than current I, ie., I; = 2.95,. What is current [,? Give vour answer to two decimals, in units of amps: [, = «# 2.13 A. I fi( D o <« o — Answer Key:2.13

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A kinked wire carries a current of = 19.3 A, as shown in the figure. The angle at the kink is 90 degrees. Take two z = 1.64 mm-long segments of wire (shown in red), each y = 4.7 cm from the kink Find the magnitude of the magnetic field B created by these fwo Segments at the point P that is halfivay in between Give your answer to two decimals, in units of micro-tesla: B— % 164.26 X107 T. Answer Key4.05 Queston4of7 1.0 1.0 Points Consider a solenoid with length I = 1.0 m, N = 1000 coils, and diameter of d = 10 mm. The solenoid carries a current of 1.0 A. Select the statement that is not true. A The magnetic field inside the solenoid is 1.3 mT. 6. The magnetic field right outside the solenoid is nearly zero. C If the resistance of a single coil is 1 m, the total power dissipated by the solenoid is 1 W v ® D.If the solencid is placed parallel to Earth's magnetic field, there will be a clockwise torque on it aiming to align the solenoid west to east £ If we doubled the length and the number of loops on the solenoid, the magnetic field will stay the same. Answer KeyD

Quest on50f7 | 10 1.0 Points. Consider the figure showing a straight current-carrying conductor next to current-carrying loop. - Select the statement that s not true. I A If we increased 1y I ® 5. The net magnetic force on the loop is upward, and there is a net torque on the loop. CIf1y > Iy, the magnetic field at th ater of the loop points into the p 0. The net magnetic force on the loop is upward. € The magnetic force on the wire is downward. Answer KeyB

Queston6of7 | 10 1.0 points A conducting object moving in a magnetic field will experience internal current flows known as eddy currents. Select the statement that is not true. A1 a moon with a uid (and so, conducting) interior is moving through its host planet’s magnetic field (e.g., Jupiter and clos in moon To), the moon’s interior will experience eddy current flows « @ 5. Despite the generated current flow, eddy currents do not generate extra heat because they encounter no internal resistance. A rectangular metallic comb moving through a magnetic field will experience weaker eddy currents than a rectangular metallic sheet with the same dimensions because the teeth of the comb prevent large eddy current flows. D.Eddy currents slow the motion of metal objects in the presence of external magnetic fields, and so are used a mechanism to slow down roller coasters or high-speed trains £ A metal scanner in an airport security checkpoint detes changes in the magnetic field induced by eddy currents in metal objects carried through the scanne: Answer Key's Question7of7 | 00 A rectangular conducting loop with dimensions of 1.3 x 1.8 m sits at an angle of 45 deg with respect to the direction of an external magnetic field of strength 1.8 T. The external magnetic field strength decreases at a rate of 30 mT/s. What is the magnitude of the emf & induced in the loop? Give your answer to one decimal, in units of millivolts: = % 30 mV. Answer Key495

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QUIZ 4 Question 1 of 7 1.0 Points Which of the following statements about simple harmonic motion is not true? A.For all simple harmonic oscillators, acceleration is proportional to the negative of the displacement from equilibrium. B. The frequency of oscillation of a mass-spring system is independent of the amplitude of oscillation C. The angular frequency w measures how many radians a simple harmonic oscillator covers in 1 sec, where a full cycle equals 27 radians. D. The oscillation period of a physical pendulum with moment of inertia I — M1 is the same as that of a simple pendulum with mass M and length [. + @ E. The kinetic energy oscillates at half the oscillation frequency of the displacement. Answer Key:E Question 2 of 7 1.0 Points Consider a simple pendulum on the surface of the Earth. The motion of the bob can be described as 8(t) = 1.5° sin (2.56¢+1.03). How long is the pendulum? Give your answer to two decimals, inmeters: [ = «# 1.50 m. Answer Key:1.5 Question 3 of 7 1.0 Points Which of the following statements about waves i3 not true? A. Longitudinal waves can be considered compression waves, since the oscillation of the particles is in the direction of motion of the wave. B. The transverse displacement of a travelling wave need not always follow a sinusoidal form. C. The equation y(z, t) = f(kz + wt — ¢) describes a wave travelling to the left D. Visible light 1s an example of an electromagnetic wave, but most of the electromagnetic wave spectrum is inaccessible to the human eye. ' @ E. The wave number k corresponds to the number of wavelength oscillations measured in radians per second Answer Key:E

Question 4 of 7 1.0 points In the following statements 7 is measored in meters, ¢ in seconds, and any argoments of rigonomefric functions are in radians. 1. 2{t) = 5 sin(3) represents SHM with a period of 2x/3.5. 2 2(t) = 3 sin® (21) represents SHM with an amplitude of 3 m 3.2(t) = 4sin(3t) represents SHM with 2 maximum velocity of 1 m's. 4.2(t) = cos(2t + 7/2) represents SHM in which maximum displacement and maximum velocity have a phase offset of 7 radians. Which of the above statements are not true? Aonly 1. B lands Answer Key.C Question 5 of 7 1.0 Points Amarker follows the circumfereace of a wheel a it spins ia the (z, y) plane around a fixed axis. The wheel has a radivs of 0.40 m and spiss 180 times per minute. Which combination of statements below i true? 1. The angular frequency of the moticn is 30 rad's. 2. The minimum velocity of the marker along the z-coordinate is- 3. The maximum acceleration of the marker along the y-coordinate is 142 mvs” j. 4. The period of one oscilation s 3.0's 5. The acceleration and the velocty of the marker along the z-coordinate are 180 degrees out o phase.

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Physics Quizzes (1 - 4)

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