882_Physics ProblemsTechnical Physics

882_Physics ProblemsTechnical Physics - 222 *P31.24...

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222 Faraday’s Law *P31.24 Observe that the homopolar generator has no commutator and produces a voltage constant in time: DC with no ripple. In time dt , the disk turns by angle dd t θω = . The outer brush slides over distance rd θ . The radial line to the outer brush sweeps over area dA rrd r dt == 1 2 1 2 2 . The emf generated is ε =− N d dt BA εω ° F H G I K J 10 1 2 2 a f B dA dt Br cos (We could think of this as following from the result of Example 31.4.) The magnitude of the emf is FIG. P31.24 π = F H G I K J =⋅ L N M O Q P F H G I K J = 1 2 09 1 2 04 3200 2 60 241 2 2 .. . NsCm m r evm in rad rev sm V bg af A free positive charge q shown, turning with the disk, feels a magnetic force q vB × radially outward. Thus the outer contact is positive . *P31.25 The speed of waves on the wire is v T × = µ 267 298 3 Nm 31 0 k g ms. In the simplest standing-wave vibration state, d NN 064 2 . m λ = 128 m and f v
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