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# hw3 - Physics 21 Fall 2008 Solution to HW-3 Electric Field...

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Physics 21 Fall, 2008 Solution to HW-3 Visualizing Electric Field Lines Answer several multi- ple choice questions about field lines. (a) For a sheet of negative charge, the field is (choice D) (b) This diagram is wrong because (choice A) field lines can- not cross each other, and because (choice B) symmetry re- quires the field lines to be parallel. (c) The field of an electric dipole is (choice B) (d) This diagram is wrong because (choice C) the field lines should be smooth curves, and the (choice D) field lines should always end on negative charges or at infinity. (e) From the diagram below we can see Q A > 0 and Q B < 0 from the direction of the field lines. The lines are more dense near Q A , so | Q A | > | Q B | . Only the second choice ( Q A = +7 q , Q B = - 3 q ) satisfies both these relations. Electric Field inside a conductor Answer several mul- tiple choice questions about the charges in a conducting rod suspended in an electric field E directed towards the right. (a) The (positively charged) nuclei experience a force to the right, and the (negative) electrons a force to the left. (b) The electrons move to the left, and the nuclei are al- most stationary (the nuclei move slightly to the right). The nuclei are much more massive than the electrons, so their acceleration a = F/m = eE/m is much less.

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