Two circular concentric loops of wire lie on a tabletop, one inside the other. T

Question

Two circular concentric loops of wire lie on a tabletop, one inside the other. The inner loop has a diameter of 20.0 cm and carries a clockwise current of 12.0 A, as viewed from above, and the outer wire has a diameter of 30.0 cm.

What must be the magnitude of the current in the outer loop so that the net magnetic field due to this combination of loops is zero at the common center of the loops?

Express your answer with the appropriate units.

What must be the direction (as viewed from above) of the current in the outer loop so that the net magnetic field due to this combination of loops is zero at the common center of the loops? clockwise or counterclockwise

Explanation / Answer

Part A

   the B field is proportional to the current andinv prop to the radius. Since the radius of the outer wire is 30/20 = 1.5 times greater than that of the inner wire, the current of the outerwire must also be 1.5 times greater. So

   current of outer wire =  1.5 * 12.0 = 18 Amps

Part B

If the current in the inner wire is clockwise, the current inthe outer wire must be counterclockwise.

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