Question

Four point positive charges of same magnitude $Q$ are placed at the four corners of a rigid square frame as shown in the figure. The plane of the frame is perpendicular to $z$-axis. If a negative charge $-q$ is placed at a distance $z$ away from the above frame ($z<<L$) then:

• A. negative charge oscillates along the z-axis.
• B. it moves away from the frame.
• C. it moves slowly towards the frame and stays in the plane of the frame.
• D. it passes through the frame only once.

Answer 1

The correct option is A negative charge oscillates along the z-axis.

Because the resultant force on the negative charge particle $q$,due to all the four charge $Q$ which are present on the corner, acts as restoring force. ( assuming no other force is present in the environment) so it will oscillate along z axis