Question

The diagram shows a charged ring centered on the origin O, oriented perpendicular to the horizontal axis that is rotating with constant velocity about the horizontal axis. Point P is a location on the horizontal axis to the right of O.
The rotating charge creates and electric field at point P that is directed to the left and a magnetic field at point P that is directed to the right.
What is the sign of the charge on the ring, and in which direction (as indicated on the diagram) is the ring rotating?

• A. The charge is positive and the ring is rotating in the CW direction.
• B. The charge is positive and the ring is rotating in the CCW direction.
• C. The charge is negative and the ring is rotating in the CW direction.
• D. The charge is negative and the ring is rotating in the CCW direction.

Answer 1

Answer: (C)

The charge is negative and the ring is rotating in the CW direction.

As the electric field is directed toward the ring (left) therefore ring is negatively charged because electric field lines goes toward the negative charge .

Now as the magnetic field is toward right , it means right face of the ring is north pole (as magnetic field lines come out from north pole of a magnet) and it will be north pole when current in ring has counter-clockwise direction , but ring is negatively charged so it should rotate in clockwise (CW) direction (opposite to current) .