PHYS182B_196L_ Lab 2 - Electrostatic Charges UPDATED-1-1-1 (1)

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PHYSICS 182B/196L LAB REPORT – LAB 2: ELECTROSTATIC CHARGES Lab 2: Electrostatic Charges San Diego State University Department of Physics Physics 182B/196L Name: Partner(s): Introduction The purpose of this activity is to compare the results of three different methods of charging: (1) rubbing two objects together, (2) touching a charged object to a neutral one (charging by contact), and (3) grounding a neutral object while it is polarized (charging by induction). This experiment will also demonstrate the law of conservation of charge. Theory: Electric Charges Electric charge is a fundamental property of nature. It comes in two types, called positive and negative. Positive charge is the type of charge carried by protons. Negative charge is the type of charge carried by electrons. As nearly as can be measured (better than 1 part in 10 30 ), the magnitude of the charge on an electron is the same as the magnitude of the charge on a proton. Atoms normally have the same number of protons and electrons and this balance of charges makes them electrically neutral. Most objects are found in this neutral state. For an object to be positively charged, it has to have more protons than electrons. For an object to be negatively charged, it has to have more electrons than protons, disturbing the neutral charge balance. Forces between Charges Opposite charges always attract. Like charges tend to repel. At an elemental level, like charges always repel (electrons repel electrons, protons repel protons), but for macroscopic objects, non- symmetric charge distribution can result in an overall attraction between two objects that carry the same type of overall charge (positive or negative). Non-symmetrical charge distribution always results in an attraction between a charged object and an electrically neutral (overall) object. Charging: all charging processes involve the transfer of electrons from one object to another. In order for an object to become positively charged, it must lose some of its electrons. In order for an object to become negatively charged, it must acquire more electrons. 1. Charging by rubbing : When two initially neutral non-conducting objects are rubbed 1

PHYSICS 182B/196L LAB REPORT – LAB 2: ELECTROSTATIC CHARGES together, one of them will generally bind electrons more strongly than the other and take electrons from the other. The law of conservation of charge requires that the total amount of electrons be conserved. That is, electrons only move from one object to another, but no new electrons are created, nor do they disappear. Overall, the two objects when considered together still have zero net charge. 2. Charging by contact : When a charged object is touched to a neutral (or less charged) object, repulsive forces between the like charges result in some of the charge transferring to the less charged object so the like charges will be further apart. This effect is much larger for conducting objects. 3. Charging by induction : The protons and electrons inside any object respond to electric forces of attraction or repulsion. When an object is placed near a charged object, the charged object will exert opposite forces on the protons and the electrons inside the other object, forcing them to move apart from each other. One side of the object will become more positive than it was initially. The other side will become more negative, as electrons migrate internally. This condition is called polarization , a word that refers to the object having “poles,” or opposite sides with different electrical states, even though the object as a whole may still be neutral. If a conductor is touched to the polarized object, some of the charge will transfer to the conductor. If the conductor is then removed, the object now carries a net charge different from its initial charge. For example, consider the sequence in Figure 1. Figure 1a shows an isolated neutral conductor. In Figure 1b, a negatively charged object has been placed near the neutral conductor, which is now polarized. Another conductor (elliptical) is shown but is not yet involved. In Figure 1c, the elliptical conductor touches the polarized conductor and some of the negative charge transfers to the elliptical conductor due to repulsion from the negatively charged object. In Figure 1d, the elliptical conductor is removed, taking some negative charge with it and leaving a net positive charge on the round conductor. In Figure 1e the negatively charged object has been removed. The charge on the round conductor re-distributes, but the overall charge on the conductor is now positive. Note that we have ignored polarization of the elliptical conductor. 2 Figure 1

PHYSICS 182B/196L LAB REPORT – LAB 2: ELECTROSTATIC CHARGES Setup Make sure your setup is connected as shown in Figure 2. The alligator clip with the red band should be connected to the inner conductor. The ground (black alligator) should be connected to the outside conductor. The Electrometer is a device that can measure the voltage difference between small charges without affecting the charges. Voltage is an electrical potential difference. The “ice pail” is the inner conducting mesh cylinder and is called an ice pail for historical reasons. When a charge Q is placed inside the inner cylinder, the cylinder becomes polarized with a charge almost equal to Q moving to the outside of the inner cylinder. With opposite-signed charges on the two cages, an electric field is present, resulting in an electrical potential difference. This voltage (the measurement on the Electrometer) between the inner and outer cylinders is directly proportional to charge on the outside of the inner conductor. This gives a way to directly measure the change inside the “ice pail.” Procedure 1. Turn on the Electrometer and set the Range to 100V. 2. Zero the charges on the Ice Pail by touching a finger to both the inner and out cylinder at the same time. Then remove the finger from the inner cylinder and then the finger from the outer cylinder. This process is known as “grounding” and simply means removing all (or most) of the excess charge. We generally do this by touching the system with a conductor that is much larger than the system, in this case our bodies. 3. Press the “ZERO” button on the Electrometer. This forces the Electrometer to read zero even if there is a charge on the Ice Pail. This is OK since we really are only interested in changes in the charge. 3 Figure 2

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PHYSICS 182B/196L LAB REPORT – LAB 2: ELECTROSTATIC CHARGES 4. You may need to redo the grounding and/or zeroing of the Ice Pail during the experiment. It is very easy to transfer charge to the ice pail by touching it or even getting too close to it with a charged object. It may even acquire a charge sitting on the table for a while. To see how sensitive the system is, stick a finger down the axis of the inner cylinder (without touching the cylinder.) Now rub your fingers through your hair, or on your shirt, or shuffle your shoes on the floor and try sticking your finger back into the Ice Pail. See any difference? What happens if you touch the Ice Pail? What’s the moral about where you put your hands during the experiment? Redo the grounding of the Ice Pail. 5. For practice, insert the disks of the Charge Producer wands one at a time (one with a dark plastic disk and the other with a white leather disk) into the inner basket. Where are your hands? If the disks are uncharged, the needle in the electrometer will stay at zero. If the needle moves, then there is residual charge in the wands. To remove the residual charge, touch the charge producers to the outer basket of the grounded ice pail. Sometimes if residual charge is hard to remove, you can breathe on the disks. The moisture from your breath will remove the charges. It is difficult to remove all the charge since the disks are non-conducting. If you see a voltage change of less than one volt, that is good enough. Procedure 1: Charging by Rubbing Objects Together. 1. Ground the ice pail, zero the electrometer and make sure there is no charge on the charge producing wands (see step7 under Setup A). 2. Hold one wand in each hand and lower them into the lower half of the inner basket, without letting them touch each other or the walls of the basket. Press the Zero button on the Electrometer. Record this reading on the Electrometer as “Initial” in row 1 of Table I. Table I: Electrometer Voltages Reading Run 1 (V) Run 2 (V) Run 3 (V) 1 Initial 2 After rub 3 After separation 4 Dark out 5 Both in 6 White out 7 Final 8 Both out 9 Sum 4

PHYSICS 182B/196L LAB REPORT – LAB 2: ELECTROSTATIC CHARGES 3. Do all of the following steps as rapidly as possible to minimize charge migration to the surroundings. 4. While inside the basket, briskly rub the two charge producers together and observe the needle in the monitor. Record this reading as “After rub” in row 2 of Table I. 5. Stop rubbing. Still inside the basket, separate the wands and observe the needle in the monitor. Record this reading as “After separation” in row 3 of Table I. 6. Take the dark wand out of the basket. Record this reading as “Dark out” in row 4 of Table I. 7. Insert the dark wand into the basket with the white wand (not touching). Record this reading as “Both in” in row 5 of Table I. 8. Remove the white wand. Record this reading as “white out” in row 6 of Table I. 9. Insert the white wand into the basket with the dark wand. Record this reading as “Final” in row 7 of Table I. 10. Remove both wands. Record this reading as “Both out” in row 8 of Table I. 11. Rub the two wands together and then remove the charge from the white one by touching it to the outside mesh cylinder. Ground the Ice Pail with your finger and zero the Electrometer. Repeat steps 2-10. 12. Rub the two wands together and then remove the charge from the black one. Ground the Ice Pail and zero the Electrometer. Repeat steps 2-10. Analysis A1 1. What can you immediately conclude about the charges on the white wand and the dark wand based on the signs of the voltages in Run 1? 2. Note that lines 3, 5, and 7 are exactly the same system so should have the same voltage. Any difference implies that there has been some charge lost or gained. In addition, line 8 should be zero unless some charge has transferred. Looking at these should allow you to estimate the uncertainties in the experiment and decide if numbers agree within the uncertainties. 3. Recall that the voltage is directly proportional to the charge. Thus if a voltage or 8 V implies 8 units of charge, a voltage of 12 V implies 12 units of charge. 4. What do lines 1-3 imply? 5

PHYSICS 182B/196L LAB REPORT – LAB 2: ELECTROSTATIC CHARGES 5. Add line 4 to line 6 (don’t forget the sign). Enter the sum on line 9. 6. Compare line 1 to line 9. What does this imply? 7. Define charge and types of charge ? 8. Discuss the methods by which we can transfer charge from one body to another? 9. Can 2 bodies with the same polarity of charge attract each other? If so, Describe how and under what condition that works? 10. Find the magnitude and direction of the force between two charges A and B with charge 2 C and 12 C on them respectively, and are placed 20 m away from each other. 6

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PHYS182B_196L_ Lab 2 - Electrostatic Charges UPDATED-1-1-1 (1)

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