# Homework Help: Electric field & charges

1. Aug 27, 2011

### johnhunk

1. The problem statement, all variables and given/known data
http://imageshack.us/photo/my-images/94/mphelp.jpg/"

2. Relevant equations
coulomb law

3. The attempt at a solution

no idea.. maybe other E field is -0.3 to cancel it out?

Last edited by a moderator: Apr 26, 2017
2. Aug 27, 2011

### Tomer

Are you talking about part 1 or 2?
Are you trying to calculate the field at point A?

In that case, are you used to working with the coulomb law in it's vector form? It'll be much easier working in this form, with this formula:
The field E inflicted by a point charge q situated at point A on point B is:
$\vec{E} = \frac{kq}{d^{3}}\vec{AB}$

where d is the distance between A and B and K is the Coulomb constant.
Writing everything in vector form should be pretty simple.

If you're totally unfamiliar with the vector form, you have no choice but to calculate separately the x and y components of the electrical field at the point A from the two different charges. That would be 4 different calculations involving trigonometry and is pretty tiring.

If you're still stuck you'll get further help, but give it a fair try first.

Btw the x fields might cancel out and might not. You have to calculate it - it isn't trivial, since the charges aren't equal and so aren't the distances between them and point A.

Good luck!

3. Aug 27, 2011

### johnhunk

i mean part b.

4. Aug 28, 2011

### Tomer

According to your solution, there's a field acting both on the y and on the x axes. A charge located at point B would not be able to cancel out the x field, only the y field.

If I read something wrong and there was no x field cause it cancelled out, what you said is of course right - you need exactly the same y field in the opposite direction. You know the expression for an electrical field, use it to get q.