8.02Experiment #1: Faraday Ice PailINTRODUCTIONGENERALIZED PROCEDUREEND OF PRE-LAB READINGIN-LAB ACTIVITIESEXPERIMENTAL SETUPMEASUREMENTSPart 1: Polarity of the Charge ProducersPart 2: Charging By Contact: Using the White Charge ProducerPart 3: Charging By Induction: Using the White Charge ProducerPart 4: Testing the shield: Using the White Charge ProducerPart 5: Repeat Part 2, 3, & 4 but now using the Blue Charge ProducerPart 5a: Repeat Part 2 but now using the Blue Charge ProducerPart 5b: Repeat Part 3 but now using the Blue Charge ProducerPart 5c: Repeat Part 4 but now using the Blue Charge Producer1 MASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Physics 8.02 Experiment #1: 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 causes them to move. This process occurs rapidly, and ends when there is no longer an electric field inside the conductor (Einside 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 (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.). 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. You will investigate all three of these phenomena—charging by induction, charging by contact, and electrostatic shielding—in this experiment. The Details: Gauss’s Law In the above situations, the excess charge on the conductor resides entirely on the surface, a fact that may be explained by Gauss’s Law. Gauss’s Law states that the electric flux through any closed surface is proportional to the charge enclosed inside that surface,enc0closedsurfaceqdε⋅=∫∫EA.E02-2 Consider a mathematical, closed Gaussian surface that is inside the ice pail. Note that in this top view of a Gaussian surface for the Faraday Ice Pail, the pail is shown as the thick grey cylindrical shell. Once equilibrium has been reached, the electric field inside the conducting metal walls of the ice pail is zero. Since the Gaussian surface is in a conducting region where there is zero electric field, the electric flux through the Gaussian surface is zero. Therefore, by Gauss’s Law, the net charge inside the Gaussian surface must be zero. For the Faraday ice pail, the positively charged ball is inside the Gaussian surface. Therefore, there must be an additional induced negative charge on the inner surface of the ice pail that exactly cancels the positive charge on the ball. It must reside on the surface because we could make the same argument with any Gaussian surface, including one which is just barely outside the inner surface. Since the pail is uncharged, by charge conservation there must be a positive induced charge on the pail which has the same magnitude as the negative induced charge. This positive charge must reside outside the Gaussian surface, hence on the outer surface of the pail. Note that the electric field in the hollow region inside the ice pail is not zero due to the presence of the charged ball, and that the electric field outside the pail is also not zero, due to the positive charge on its outer surface. Now suppose the ice pail is connected to a large conducting object (“ground”). The figure shows grounding the ice pail (left) and after removing the ground & charged ball (right). Now the positive charges that had moved to the surface of the ice pail can get even further away from each other by flowing into the ground. Now that there are no charges on the outer surface of the pail, the electric field outside the pail is zero and the pail is at the same “zero” potential as the ground (and infinity). If the wire to ground is then disconnected, the pail will be left with an overall negative charge. Once the positively charged ball is removed, this negative charge will redistribute itself over the outer surface of the pail. Finally, when a charged ball approaches the ice pail from outside of the pail, charges will redistribute themselves on the outside surface of the pail while the electric field inside the pail will remain zero, cut-off from any knowledge of what is going on outside by the enforced zero electric field inside the conductor. This effect is called shielding or “screening” and explains popular science demonstrations in which a person sits safely inside a cage while an enormous voltage is applied to the cage. This same effect explains why metal boxes are used to screen out undesirable electric
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