# Electric force between two charges on z-axis

1. Oct 28, 2005

### Reshma

Question:
Two charges -2q and +q are located on the z-axis at distance 'd' and '3d' respectively. The x-y plane is a grounded conductor. Find the force on the charge +q.
My work:
Let the force be F. Distance between the charges is '2d'.
$$F = \frac{1}{4\pi \epsilon_{0}}\left(\frac{(-2q)(q)}{{(2d)}^2}\right)$$
Can I apply the Coulomb's equation here the way I have done? Will the grounded x-y plane have any effect on the force?

2. Oct 28, 2005

### Galileo

A better question is: does the charge below the grounded conductor have any effect on the force?

3. Oct 29, 2005

### Reshma

Umm...the charges mentioned here are above the grounded plane. The grounded plane has a zero potential. Are you talking about image charges?

4. Oct 30, 2005

### Reshma

Can someone help me proceed?

5. Oct 30, 2005

### Physics Monkey

You know the grounded conductor will "draw up charge from infinity" so that it can maintain a constant potential over its surface. This charge will exert a force, right? How can you describe the field produced by a conducting sheet in the presence of point charges?

6. Nov 5, 2005

### Reshma

Took me a while to read up on the theory. I am convinced this is an image problem. Well, the electric field is normal to the surface of the conducting sheet, right?
$$\vec E = \frac{\partial V}{\partial z}\hat z$$