Lenz's law: In the figure, a straight wire carries a current I. The wire passes through the center of a toroidal coil. If the current in the wire is quickly reduced to zero, the induced current through the resistor R is

Question

Quizplus · Accepted Answer

According to Lenz's law, the induced current in a closed loop is always in such a direction as to oppose the change which produced it. When the current in the wire is reduced to zero, the magnetic field in the toroidal coil starts collapsing. To maintain the magnetic flux through the toroidal coil, an induced current is generated in the opposite direction to the original current, which means in a direction from b to a. Therefore, the answer is B.

Lenz's Law: in the Figure, a Straight Wire Carries a Current