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**Finite Number of Charges around a regular Polygon**

I know this is probably an easy one, but I'm just unsure...

There are 13 equal charges,

*q*, that are placed at the corners of a regular 13-sided polygon.

a) What is the Force on a test charge Q at the centre?

What I've been donig is simply saying that all but three of the charges will cancel with the remaining three charges having a Y shape. then here, the total force

**F**=2Fcos(theta) -F and then calculated theta as 2pi/13 (F is the force of one point charge).

Is this right? it seems very complicated for what at first glance seems like a simple question.

b) If one of the 13 charges is removed, what is the force on Q?

In this case I have:

**F**=Fcos[theta] - F. Again, this just doesn't "feel" right.

Can anyone help?

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