# Homework Help: Electric force in an electric field.

1. Sep 10, 2013

### MostlyHarmless

Generally I would type out the question myself rather than just screenshotting it but since I'm on my phone I elected to simply attach pictures of both the question and my work.

Part a and b I was able to do, but what I'm having a hard time with is part c. I know how to find the electric force between 2 charges, but I'm lost as to how I'm supposed to incorporate the third charge. I feel like I should be using some of the values I already calculated but I just don't know where to start on part c.

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2. Sep 10, 2013

### Haye

You know how to calculate the force between two charges. You can just calculate the force that act on the last charge from the first one, and then the force it feels from the second one. Then you can just add up the two forces. Just make sure you know you're adding up vectors.

3. Sep 10, 2013

### barryj

You have done the hard part, assuming part a and b are correct. Since you have the E field at point P. how is force, e, and Q related?

4. Sep 10, 2013

### MostlyHarmless

Fe= qE

The addition of a non arbitrary charge at point p doesn't change the electric field vector? Does that mean my diagram is wrong for part c?

5. Sep 10, 2013

### MostlyHarmless

@Haye: I was pretty sure that's what I was supposed to be doing, I just didn't know how to calculate E with that third charge there.

6. Sep 10, 2013

### Haye

I am not sure how to explain it properly, but the third point charge can't really affect itself, can it. So you just calculate the electric field at that point as if it wasn't there.

7. Sep 10, 2013

### MostlyHarmless

That makes enough sense for me to complete the problem :) thank you both for the help.

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