Ch 25 HW Problem 25.9 25.9. Model: Use the charge model and the model of a conductor as a material through which electrons move. Solve: The first step shows two neutral metal spheres touching each other. In the second step, the negative rod repels the negative charges which will retreat as far as possible from the top of the left sphere. Note that the two spheres are touching and the net charge on these two spheres is still zero. While the rod is there on top of the left sphere, the right sphere is moved away from the left sphere. Because the right sphere has an excess negative charge then, by charge conservation, the left sphere has the same magnitude of positive charge. Upon separation, the negative charge is trapped on the right sphere, as shown in the third step. As the two spheres are moved apart farther and the negatively charged rod is moved away from the spheres, the charges on the two spheres redistribute uniformly over the entire surface spheres. Thus, we are left with two oppositely charged spheres. 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 charge equal to either +Q, -Qor 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, weaklyattracted to C, and strongly attracted to D. Assume throughout this problem that the balls are brought very close together. Part AWhat is the nature of the force between balls A and B?
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