# HW1 - Physics 220 Assignment 1 Assignment is due at 11:59pm...

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Physics 220 Assignment 1 Assignment is due at 11:59pm on Thursday, September 14, 2006 Section 1.1: A Test Charge Determines Charge on Insulating and Conducting Balls Description: Determine the force between pairs of conducting and/or insulating balls of unknown charge by analyzing the forces between each ball and a positive test charge. Learning Goal: To understand the electric force between charged and uncharged conductors and insulators. When a test charge is brought near a charged object, we know from Coulomb's law that it will experience a net force (either attractive or repulsive, depending on the nature of the object's charge). A test charge may also experience an electric force when brought near a neutral object. Any attraction of a neutral insulator or neutral conductor to a test charge must occur through induced polarization. In an insulator, the electrons are bound to their molecules. Though they cannot move freely throughout the insulator, they can shift slightly, creating a rather weak net attraction to a test charge that is brought close to the insulator's surface. In a conductor, free electrons will accumulate on the surface of the conductor nearest the positive test charge. This will create a strong attractive force if the test charge is placed very close to the conductor's surface. Consider three plastic balls (A, B, and C), each carrying a uniformly distributed unknown charge (which may be zero), and an uncharged copper ball (D). A positive test charge (T) experiences the forces shown in the figure when brought very near to the individual balls. The test charge T is strongly attracted to A, strongly repelled from B, weakly attracted to C, and strongly attracted to D. Assume throughout this problem that the balls are brought very close together. Part A What is the nature of the force between balls A and B?

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Part A.1 What is the net charge on ball A? Since the test charge is positively charged, and there is a strongly attractive force between ball A and the test charge, what must be the nature of the net charge of ball A? ANSWER: positive negative zero Part A.2 What is the net charge on ball B? Since the test charge is positively charged, and there is a strongly repulsive force between ball B and the test charge, what must be the nature of the net charge of ball B? ANSWER: positive negative zero ANSWER: strongly attractive strongly repulsive weakly attractive neither attractive nor repulsive Part B What is the nature of the force between balls A and C? Part B.1 What is the charge on ball C? Recall that ball C is composed of insulating material, which means that it can be polarized, but the charges inside are otherwise not free to move around inside the ball. Since the test charge experiences only a weak force due to ball C, what must be the nature of the net charge on ball C?
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