A Long Vertical Wire Carries a Steady 70 a Current

question 34

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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