51. Two coils of wire are placed close together. Initially, a current of 2.5 A e

Question

51. Two coils of wire are placed close together. Initially, a current of 2.5 A exists in one of the coils, but there is no current in the other. The current is then switched off in a time of 3.7 x 10-2 s. During this time, the average emf induced in the other coil is 1.7 V. What is the mutual inductance of the two-coil system? 66. In a television set the power needed to operate the picture tube comes from the secondary of a transformer. The primary of the transformer is connected to a 120-V receptacle on a wall. The picture tube of the television set uses 91 W, and there is 5.5 mA of current in the secondary coil of the transformer to which the tube is connected. Find the turns ratio N/Np of the transformer.

Explanation / Answer

51. induced emf = M d(i)/dt

1.7 = M (2.5)/(3.7 x 10^-2)

M = 0.025

66. Vp = 120 Volt

W = V I

91 = (Vs) (5.5 x 10^-3)

Vs =16545 Volt

Ns / Np = Vs / Vp = 138 turns

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