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

Options

A. `-ve` 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

a

Explanation

The negative charge oscillates, the resultant force acts as a restoring force and proportional to displacement. When it reaches the plane `xy`, the resultant force is zero and the mass moves down due inertia. Thus oscillation is set.