Posted by **Crystal** on Thursday, February 3, 2011 at 3:31pm.

Twelve identical point charges are equally spaced around the circumference of a circle of radius 'R'. The circle is centered at the origin. One of the twelve charges, which happens to be on the positive axis, is now moved to the center of the circle.

Part A

Find the magnitude of the net electric force exerted on this charge.

Express your answer in terms of some or all of the variables q, R, and appropriate constants.

Part B

Find the direction of the net electric force exerted on this charge.

Express your answer as an integer. (in degrees counterclockwise from the positive x axis).

- Physics -
**bobpursley**, Thursday, February 3, 2011 at 4:01pm
This is a rather simple problem.

Do it with symettry.

First, find the E at the center if the all 12 charges are symettrical about the circle (wont it be zero?).

Next, then consider what if you put a charge at one position which is opposite, making a net charge of zero at that position.

Add then E from this added "virtual" charge. E=k(-q)/R^2 the negative sign means it is in the opposite direction as the original E.

Net E=kq/r^2, the E points toward the replaced charge

- Physics -
**Crystal**, Thursday, February 3, 2011 at 4:13pm
E=kq/r^2 doesn't seem to be the correct answer for Part A.. help?

Ofcourse, for part B, 0 degrees makes sense and is correct as you have explained.

- Physics -
**Crystal**, Thursday, February 3, 2011 at 4:15pm
Never mind, got it!

Thanks!!

- Physics -
**Anonymous**, Thursday, February 3, 2011 at 4:50pm
i hv the same question i understand part b but i am getting wrong answer for part a, what was the equation you used crystal?

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