Electric Charges Manual - ELECTRIC CHARGES Physics 241/261...

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Nikola Tesla taking notes in his Colorado Springs lab in December 1899. Photo by Dickenson V. Alley (copyright expired). ELECTRIC CHARGES Physics 241/261 Fall 2014 1 Introduction Many of the objects we rely on daily—such as cars, TVs, and computers—rely on the behavior of the electromagnetic force to function. Here, we will begin to study this force by examining the behavior of non-moving charges and charges in equilibrium, a field of physics called electrostatics . We will explore three different methods to manipulate charges (see Figure 1): 1. Charging by conduction (or direct contact): A charged object is brought into contact with an uncharged one. Charge spreads across both objects, which are then separated. See Figure 1(a). 2. Polarization: A charged object near a neutral object separates positive and negative charges slightly in the neutral object, as seen in Figure 1(b). As a result, the charged object and the oppositely charged near side of the neutral object will be attracted to each other. The neutral object remains neutral overall. 3. Charging by induction: A wire connected to the ground can be used to drain off extra charge from one side of a polarized object, leaving it with a net charge, as seen in Figure 1(c). + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + F F ) c ( ) b ( ) a ( Figure 1: (a) Charging by conduction, or direct contact. (b) Polarization. (c) Charging by induc- tion. 1
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As you work through the lab, think about where the charge is going and which of these effects are important. To perform our experiments, we first will learn how to use an electroscope (Section 4) and how to determine charge polarity (Section 5). We will then examine how charges distribute themselves on a conductor, in this case a metal can (Section 6). We will also see how to charge an object both with conduction (Section 7) and through induction (Section 8). 2 Theory Before starting this lab, you should be familiar with the following physical concepts. If you need to review them, or if you haven’t yet discussed them in your lecture course, consult the indicated sections in Young & Freedman, University Physics. Electric charge, §21.1 Inductors, conductors, charging by induction, polarization §21.2 You may also find it useful to review §21.3. 3 Equipment electroscope with built-in or external deflection scale UMHVPS low-current, high-voltage power supply (To avoid risk of shock, do not substi- tute with any other power supply!) insulated disk; insulated platform; round can banana cables and alligator clips 3.1 Electroscope The electroscope, used to hold and measure charge, is shown in Figure 3. The round plate on top of the electroscope is attached to the movable vane inside the electroscope. These are insulated from the outer case. When there is charge on the top plate and movable vane, the vane will turn on its hinge as the charges repel each other and attempt to spread out. A greater amount of charge will cause the vane to deflect to a greater degree.
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