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

By Faraday's law of electromagnetic induction, a change in the magnetic flux through a loop induces an electromotive force (emf) in the loop. When the current in the wire passing through the toroid is rapidly reduced to zero, the magnetic field inside the toroid changes accordingly. This change in the magnetic field induces an emf in the toroidal coil, which causes a current to flow in the resistor R. The direction of the induced current is such that it opposes the change in magnetic flux that caused it, according to Lenz's law. In this case, the induced current creates a magnetic field that opposes the decrease in the magnetic field inside the toroid. Therefore, the induced current flows from b to a, opposing the direction of the initial current in the wire. So, the correct answer is B.

In the Figure, a Straight Wire Carries a Current I