Motional Emf: A long vertical wire carries a steady 70 A current. As shown in the figure, a pair of horizontal rails are 0.20 m apart. A 20-? resistor connects points a and b, at the end of the rails. A bar is in contact with the rails, and is moved by an external force with a constant horizontal velocity of 0.90 m/s to the right, as shown. The bar and the rails have negligible resistance. At the instant that the bar is 0.20 m from the wire, what are the induced current in the resistor and its direction through the resistor? (?0 = 4? × 10-7 T ? m/A)
A. 0.63 ?A, from a to b
B. 0.63 ?A, from b to a
C. 0.32 ?A, from a to b
D. 0.32 ?A, from b to a
E. 1.9 ?A, from b to a
Answer: A
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