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E07-1 MASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Physics 8.02 Experiment 2: Faraday Ice Pail OBJECTIVES 1. To explore the charging of objects by friction and by contact. 2. To explore the charging of objects by electrostatic induction. 3. To explore the concept of electrostatic shielding. PRE-LAB READING INTRODUCTION When a charged object is placed near a conductor, electric fields exert forces on the free charge carriers in the conductor which cause them to move. This process occurs rapidly, and ends when there is no longer an electric field inside the conductor ( E inside conductor =0). The surface of the conductor ends up with regions where there is an excess of one type of charge over the other. For example, if a positive charge is placed near a metal, electrons will move to the surface nearest the charge, leaving a net positive charge on the opposite surface 1 . This charge distribution is called an induced charge distribution . The process of separating positive from negative charges on a conductor by the presence of a charged object is called electrostatic induction . Michael Faraday used a metal ice pail as a conducting object to study how charges distributed themselves when a charged object was brought inside the pail. Suppose we lower a positively charged metal ball into the pail without touching it to the pail . When we do this, positive charges move as far away from the ball as possible – to the outer surface of the pail – leaving a net negative charge on the inner surface. If at this point we provide some way for the positive charges to flow away from the pail, for example by touching our hand to it, they will run off through our hand. If we then remove our hand from the pail and then remove the positively charged metal ball from inside the pail, the pail will be left with a net negative charge. This is called charging by induction . In contrast, if we touch the positively charged ball to the uncharged pail, electrons flow from the pail into the ball, trying to neutralize the positive charge on it. This leaves the pail with a net positive charge. This is called charging by contact . Finally, when a positively charged ball approaches the ice pail from outside of the pail, charges will redistribute themselves on the outside surface of the pail and will exactly cancel the electric field inside the pail. This is called electrostatic shielding . 1 We will typically say that “positive charge flows outward” even though in metals it’s really electrons moving inward. This is a completely equivalent way of thinking about it for our purposes.
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E07-2 You will investigate all three of these phenomena—charging by induction, charging by contact, and electrostatic shielding—in this experiment.
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This note was uploaded on 02/27/2011 for the course PHYSICS 8 taught by Professor Dourmashkin during the Fall '10 term at MIT.

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