quiz 5 - Physics 2Ba quiz 5 solutions(Dated 13:13 Problem 1...

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Physics 2Ba quiz 5 solutions (Dated: February 14, 2009 13:13) Problem 1 A copper wire of length L and radius b is attached to another copper wire of length L and radius 2 b , forming one long wire of length 2 L . This long wire is attached to a battery, and a current is flowing through it. Relative to the electric field within the wire of radius b , the magnitude of the electric field within the wire of radius 2 b is A) two times stronger. B) four times stronger. C) two times weaker. D) four times weaker. E) equal. The correct answer is D . The current through each wire is the same and so we have I 1 = J 1 A 1 = J 2 A 2 = I 2 ± ± σ 1 E 1 A 1 = ± ± σ 2 E 2 A 2 . In the last line, we used the fact that material in each wire is the same, therefore they have the same conductivity. The ratio of the electric fields is E 2 = E 1 A 1 A 2 = E 1 πb 2 4 πb 2 = 1 4 E 1 Problem 2 The figure below shows a steady electric current passing through a wire with a narrow region. What happens to the drift velocity of the moving charges as they go from region A to region B and then to region C? A) The velocity increases all the time. B) The velocity decreases all the time. C) The velocity decreases from A to B and increases from B to C. D) The velocity increases from A to B and decreases from B to C. E) The velocity remains constant. The correct answer is D . The current through any part of the wire must be the same and since each region is made up of the same material, the free electron density, n e , is the same. We know that I = JA , where J is the current density and A is the cross-sectional area. As we move from region
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This note was uploaded on 03/28/2009 for the course PHYS 2b taught by Professor Schuller during the Winter '08 term at UCSD.

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quiz 5 - Physics 2Ba quiz 5 solutions(Dated 13:13 Problem 1...

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