As shown in the figure, a straight wire carries a current I into the page. The wire passes through the center of a toroidal coil. If the current I is quickly reduced to zero, the direction of the induced current through the resistor R is
A) from a to b.
B) from b to a.
C) No current is induced.

Question

As shown in the figure, a straight wire carries a current I into the page. The wire passes through the center of a toroidal coil. If the current I is quickly reduced to zero, the direction of the induced current through the resistor R is 
A) from a to b. 
B) from b to a. 
C) No current is induced.

Quizplus · Accepted Answer

When the current in the wire is quickly reduced to zero, the magnetic field inside the toroidal coil is also reduced to zero. According to Lenz's Law, the direction of the induced current in the coil will be such that it opposes the change that produced it, which in this case is the decrease in the magnetic field. By applying the right-hand rule for the direction of induced current, we can see that the induced current will flow in a clockwise direction inside the coil when viewed from the side of the wire labeled "a", which corresponds to the current flowing from b to a through the resistor R. Therefore, the correct choice is B.

As Shown in the Figure, a Straight Wire Carries a Current