# Electric Potential question

1. Feb 28, 2008

### jcpwn2004

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

An electron is released from rest 4m from an infinite plane of positive charge. The plane produces an electric field with a constant magnitude of 100 V/m. How fast is the electron moving when it collides with the plane? Mass of E = 9.11x10^-31kg.

2. Relevant equations

Not sure... Ui + Ki = Uf + Kf?

3. The attempt at a solution

2. Feb 28, 2008

### kkrizka

You are given the electric field, not the voltage at the point so you can't use conservation of energy. Remember, electric field * charge = force on charge. You can use this to find the force on the electron, and then use one of the kinematics equations so solve for the speed.

3. Feb 28, 2008

### jcpwn2004

Ok so I did F=EfxQ...(100 v/m)(1.6x10-19) = 1.6x10^-13 J. And then F=ma so A = (1.6x10^-17)/(9.11x10-31) = 1.756x10^13 m/s...

I'm lost what to do after that...what kinematic equation should i use?

4. Feb 28, 2008

### kkrizka

The one that involves initial, final velocities, acceleration and displacement. I believe it is
$$V_f^2 = V_i^2 + 2ad$$