P2213_HW07_solns_Fall2008 - Physics 2213 HW #7 Solutions...

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- 1 - Physics 2213 HW #7 — Solutions Fall 2008 22.31 [Charge in a Cavity] Apply Gauss’s law and conservation of charge. 0 E = in a conducting material. (a) Gauss’s law says + Q on inner surface, so 0 E = inside metal. (b) The outside surface of the sphere is grounded, so no excess charge. (c) Consider a Gaussian sphere with the – Q charge at its center and radius less than the inner radius of the metal. This sphere encloses net charge – Q so there is an electric field flux through it; there is electric field in the cavity. (d) In an electrostatic situation 0 E = inside a conductor. A Gaussian sphere with the Q - charge at its center and radius greater than the outer radius of the metal encloses zero net charge (the Q - charge and the Q + on the inner surface of the metal) so there is no flux through it and 0 E = outside the metal. (e) No, 0 E = there. Yes, the charge has been shielded by the grounded conductor. There is nothing like positive and negative mass (the gravity force is always attractive), so this cannot be done for gravity. 22.48 [Concentric Spheres] Use a Gaussian surface that is a sphere of radius r and that is concentric with the sphere and shell. The volume of the insulating shell is ( 29 3 3 3 4 28 [2 ] 3 3 V R R R π = - = . (a)
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This note was uploaded on 10/25/2008 for the course PHYS 2213 taught by Professor Perelstein,m during the Fall '07 term at Cornell University (Engineering School).

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P2213_HW07_solns_Fall2008 - Physics 2213 HW #7 Solutions...

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