1 A spherical conductor A contains two spherical cavities. The tout charge on the conductor itself is zero. However, there is a point charge q_t, at the center of one cavity and ri_e at the center of the other. A considerable distance r away is another charge cf. What force acts on each of the four objects, A, ca_r, ti_n q₄? Which answers, if any, arc only approximate, and depend on r being relatively large?

9.5 A charge Q is located It cm above a conducting plane, just as in Fig. 3.8a. Asked to predict the amount of work that would have to be done to move this charge out to infinite distance from the plane, one student says that it is the same as the work required to separate to infinite distance two charges Q and -Q which are initially 2h cm apart, hence IV = 12h. Another student calculates the force that acts on the charge as it is being moved and integrates F dx, but gets a different answer. What did the second student get, and who is right?

3.6 By solving the problem of the point charge and the plane con¬ductor we have, in effect, solved every problem that can be constructed from it by superposition. For instance, suppose we have a straight wire 200 meters long uniformly charged with 10) esu per centimeter of length, running parallel to the earth at a height of 5 meters. What is the field strength at the surface of the earth, immediately below the wire? (For steady fields the earth behaves like a good conductor.) What is the electrical force acting on the wire?