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# Q6S - a carrying current I out of the page Cable B has a...

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Quiz 6 Solutions Check the best answer. 1. An inductor has self-inductance L . Current I flows through it. Which of the following is wrong? The inductor stores magnetic field energy 1 2 LI 2 . If the current is decreasing, the inductor is putting magnetic field energy into the circuit. The potential always decreases as you pass through the inductor in the direction of the current. [Depends on dI / dt .] One of the above is not true. Check T or F depending on whether the statement is true or false. 2. A charged particle initially at rest in a magnetic field that is increasing with time remains at rest because a magnetic field exerts no force on a charge at rest. T F [The induced E-field can make the charge move.] Physics 54 Summer 2011 1

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3. Two coaxial cables have equal length and both have a thin cylindrical outer conductor of radius b carrying current I into the page. Cable A has a thin hollow cylindrical inner conductor of radius
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Unformatted text preview: a , carrying current I out of the page. Cable B has a solid cylindrical inner conductor, also of radius a and carrying total current I out of the page, but distributed uniformly over its cross- section. a. Compare the B-Feld magnitudes in the two cables at a distance r from the symmetry axis, where a < r < b . Are they the same? If not, which is larger? How do you know? b. Do the same for r < a . c. Which cable has the larger self-inductance? How do you know? a. By Ampere’s law, at distance r the Feld has magnitude μ I /2 π r in both cases. b. In cable A the Feld is zero for r < a , but not in case B. c. The stored energy in the B-Feld is the same for both everywhere except for r < a , where it is zero for cable A but positive for cable B. Thus cable B has more stored energy for the same current, and thus has larger self-inductance. A B Physics 54 Summer 2011 2...
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