# Homework Help: Electric Potential

1. Feb 11, 2010

### melst

1. The problem statement, all variables and given/known data
A point charge Q resides at the origin. A partical of mass 0.001kg carries a charge of 6 microC. The particle is released from rest at x = 3m. Its kinetic energy as it passes x = 0.5m is 0.4J. The coulomb constant is 8.99x10^9Nm^2/C^2.
Find the charge Q. Answer in microC.

2. Relevant equations
I use kqQd = -k

3. The attempt at a solution
(8.99x10^9Nm^2/C^2)(6 microC)Q[(1/0.5)-(1/3]) = -0.4
Q=-4.44x10^-12

I don't know what I did wrong and is kinda confused. Can anyone help???

2. Feb 11, 2010

### Jebus_Chris

Use conservation of energy.
$$U = k\frac{q_1q_2}{r}$$

3. Feb 11, 2010

### melst

so is the mass not important??

4. Feb 11, 2010

### Jebus_Chris

Mass is important for the kinetic energy calculation. You never have to use mass though since they give you the kinetic energy. Just write an equation for conservation of energy. You start with all potential and then some of the potential energy turns into kinetic energy.