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

According to Lenz's law, the direction of the induced current will be such that it opposes the change in magnetic flux. When the current in the wire is reduced to zero, the magnetic field inside the toroidal coil that was created by the current in the wire will also be reduced to zero. Hence, the magnetic flux through the toroidal coil will decrease. To oppose this decrease in flux, an induced current will flow in such a direction that it creates a magnetic field that opposes the decrease in flux. This can be achieved by creating a clockwise magnetic field inside the toroidal coil, as seen from the top. Therefore, the induced current through the resistor R will flow from b to a, creating a clockwise magnetic field inside the toroidal coil.

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