A Long Vertical Wire Carries a Steady 70 a Current

question 76

Multiple Choice

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-Ω 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? (μ₀ = 4π × 10^-7 T ∙ m/A)

• A) 0.63 μA, from a to b
• B) 0.63 μA, from b to a
• C) 0.32 μA, from a to b
• D) 0.32 μA, from b to a
• E) 1.9 μA, from b to a

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Definitions:

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