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
A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Question

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 
A) from a to b. 
B) from b to a. 
C) There is no induced current through the resistor.

Quizplus · Accepted Answer

According to Lenz's Law, the direction of the induced current will be such that it opposes the change in the magnetic field that caused it. When the current in the wire is quickly reduced to zero, the magnetic field inside the toroidal coil will change. The induced current will flow in a direction that produces a magnetic field that opposes the change in the original magnetic field. The induced current will flow counterclockwise around the toroidal coil (looking from above). This current will produce a magnetic field that opposes the original magnetic field, which was directed into the page. The induced current will therefore produce a magnetic field that is directed out of the page in the center of the toroidal coil. This will induce a clockwise current in the resistor, looking from above. Therefore, the induced current through the resistor will be from b to a.

In the Figure,a Straight Wire Carries a Current I