# MP01b - Print View PHYS 2212 G/H Spring 2008 MP01b Due at...

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[ Print View ] PHYS 2212 G/H Spring 2008 MP01b Due at 9:00am on Tuesday, January 15, 2008 View Grading Details Electric Charge This assignment will receive a 30% bonus if it is completed before 9:00 AM, Friday, January 11. A Test Charge Determines Charge on Insulating and Conducting Balls 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? Part A.1 What is the net charge on ball A? Part not displayed Part A.2 What is the net charge on ball B? Part not displayed 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? Part not displayed ANSWER: strongly attractive strongly repulsive weakly attractive neither attractive nor repulsive Recall that ball C is composed of insulating material, which can be polarized in the presence of an external charged object such as ball A. Once polarized, there will be a weak attraction between balls A and C, because the positive and negative charges in ball C are at slightly different average distances from ball A.

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