# Homework Help: Deriving equation for electrostatic force.

1. May 25, 2007

### Checkfate

1. The problem statement, all variables and given/known data
Basically I am given an image showing a test charge tied to a string where there is the ball is stationary. I need to use the fact that there is no net force on the ball to derive an equation for the electrostatic force in terms of g, the angle, and the mass of the ball. Unfortunately I am stuck. :(

2. Relevant equations

3. The attempt at a solution

Well I know that the vertical portion of $$F_{T}$$ is equal in magnitude to mg, and the horizontal portion of $$F_{T}$$ is equal in magnitude to the electromagnetic force. But I don't see how to tie it together to derive an equation.

2. May 25, 2007

### neutrino

How about trying to eliminate F_T from the equations.

3. May 25, 2007

### Checkfate

Ok sure let me try ;-).

If I am speaking about magnitudes only,

$$F_{T}^{2}=F_{g}^{2}+F_{e}^{2}$$

Or

$$F_{e}^{2}=F_{g}^{2}-F_{T}^{2}$$

But $$F_{T}=\frac{F_{E}}{Sin\Theta}$$

So

$$\frac{F_{e}^{2}}{(Sin\Theta)^{2}}=F_{e}^{2}+F_{g}^{2}$$

$$F_{e}^{2}=F_{e}^{2}(Sin\Theta)^{2}+F_{g}^{2}(Sin\Theta)^{2}$$

moving the fe^2(sin(theta))^2 to the left side then factoring out 1-sin(theta)^2 from the left side and converting it to cos(theta)^2 I get:

$$F_{e}=\sqrt{(tan\Theta)^{2}m^{2}g^{2}}$$

Does it look right?

Last edited: May 25, 2007
4. May 25, 2007

### neutrino

Yes that's right. But a simpler way, actually the simplest, would have been to divide the expression for F_e by that of F_g.