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Unformatted text preview: Physics 7b Midterm 2 Review Saturday, April 4, 2009 1. Use Gauss's law to prove that a point charge q in free space cannot be held in stable equilibrium by electrostatic forces alone. Hint: If a stable equilibrium position existed for q , what would the electric eld around that position look like? 2. A uniformly charged hoop with radius a and charge Q lies in the x-y plane and is centered on the origin. A uniformly charged rod with length l and charge q lies along the z -axis between z = b and z = b + l. Assume Q and q are both positive. a) b) Calculate the force exerted on the rod by the hoop. If the rod starts very far away along the z axis, with what velocity must it be red in order to make it through the hoop? Assume for this part that a l. 1 3. A point charge Q is surrounded by a spherical conducting shell with inner radius a and outer radius b. What is the force exerted by the charge on this shell. What if the shell was made of dielectric with constant ? Hint: Think about the energy of the shell, and what happens if it expands by a very small amount. 4. (Young and Freedman, 25-48) Four capacitors are initially uncharged and are connected as shown below with the switch S open. A potential dierence V is applied between points a and b. a) b) c) What is the potential dierence between points c and d? After the switch S is closed, what is the potential dierence between points a and c? How much charge owed through the switch when it was closed? 2 ...
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- Spring '08