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

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 frame
d)it passes through the frame only once

Hello Ishaan,

I will assume that the negative charge is free to move without friction. I also assume we are to ignore gravity (or perhaps the system is oriented so that gravity acts along the z axis and does not change the following conclusion). Therefore the charge will accelerate toward the plane of the frame. Since the system has symmetry, the negative charge will not be more attracted to any one of the positive charges in preference over any of the others. Passing through the plane, the charge will begin decelerating. The charge will go a distance z past the plane where it'll stop. And reverse the process.

It will be much like a pendulum swinging through the point of least potential energy, stopping on either side, and swinging back the other way.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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