As shown in the figure,one end of a metal bar is in contact with a circular rail and the other end is pivoted at P.A steady uniform,magnetic field B into the page is present.As the bar rotates about point P in a clockwise direction,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,one end of a metal bar is in contact with a circular rail and the other end is pivoted at P.A steady uniform,magnetic field B into the page is present.As the bar rotates about point P in a clockwise direction,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

As the metal bar rotates in a clockwise direction, the perpendicular motion of the bar across the uniform magnetic field generates an EMF that induces a current in the circuit. According to Lenz's Law, the direction of the induced current will oppose the change that produces it. Thus, the induced current will create a magnetic field that opposes the original magnetic field. Since the original field is into the page, the induced magnetic field will be out of the page (to the right of the figure). This direction of the induced magnetic field pushes negative charges towards point a, creating a potential difference across the resistor R. The induced current will flow from b to a, in the direction opposite to the conventional current. Therefore, the answer is B.

As Shown in the Figure,one End of a Metal Bar