# Homework Help: Electromagnetism exam tomorrow

1. Feb 7, 2010

### nunos

Have exam tomorrow. I know I should have studied before and more, but please help. If you know the answer, please take some time to reply. Thanks very much.

1. The problem statement, all variables and given/known data

There are two charges, q1 = +4.5nC, and q2, ?, which are at distances d1 = 2.5cm and d2 = 2.0cm, from a point which has the tension of 3.2kV. What is the value of q2?

2. Relevant equations

$$E = \frac{kQ}{r^2}$$, I think that's all, but I most probably am wrong.

3. The attempt at a solution

This is certainly wrong. I have not even used one of the data given. Anyone, that's the best I was able to come up with.

$$\frac{kq_1}{r_1^2} = \frac{kq_2}{r_2^2}$$

which gives $$q_2 = 1.28 * 10^{-6}$$ which is a wrong answer, since the correct one is supposed to be $$3.5nC$$.

2. Feb 8, 2010

### CompuChip

Re: Electromagnetism

You have not used that the potential difference U is 3.2 kV.
What does potential have to do with (difference in) electric field E?