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 is such that it opposes the change that produced it. When the current in the wire is quickly reduced to zero, the magnetic flux through the toroidal coil changes, which in turn induces a current in the coil to oppose the change. The induced current flows in such a direction that it creates a magnetic field that opposes the decrease in the magnetic field due to the current in the wire. This means that the induced current will flow in the same direction as the current in the wire was flowing before it was reduced, which is from b to a. Therefore, the answer is B.

In the Figure,a Straight Wire Carries a Current I