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 forces acts as a restoring force and is `cos theta` component, proportional to displacement. When it reaches the plane XY, the resultant force is zero and the mass moves down due to inertia. Thus oscillation is set.
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