# Homework Help: Two point charges

1. May 7, 2007

### kbyws37

Two point charges are located on the x−axis: a charge of +7.80 nC at x = 0 and an unknown charge q at x = 0.50 m. No other charges are nearby. If the electric field is zero at the point x = 1.0 m, what is q?

I separated the charges.
q1 = +7.80 nC
q2 = unknown

I am confused about the last part of the question where the electric field is zero.

Would I use the equation
E= k(Q) / r^2

2. May 7, 2007

### Hootenanny

Staff Emeritus
Yes you would use the defintion of the electric field to set up an equation of the form;

$$E_1 + E_2 = 0$$

3. May 7, 2007

### Vagrant

What are the electric field vectors at x=1m due to Q1 and Q2?

4. May 7, 2007

### kbyws37

I'm not getting the right answer.
I did...

0 = ((8.99x10^9)(7.80)) / 0.5^2) + ((8.99x10^9)(q)) / 0.5^2)

which does not equal zero.

I think I still need to do something else.. something about point x = 1.0m ?

5. May 7, 2007

### Hootenanny

Staff Emeritus
Indeed, how far is the first charge (q1) located from the point of zero electric field?

6. May 7, 2007

it would be
1.0 meter

7. May 7, 2007

### Vagrant

Check your expession for electric feild due to q1.

8. May 7, 2007

### Hootenanny

Staff Emeritus
That's correct, not 0.5m as you had previously