# Homework Help: Electric potential question (check my work)

1. Feb 21, 2012

### AnthroMecha

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

A beam of electrons with energy 1.20 keV (per electron) is directed towards a solid copper sphere of radius 1.25cm.
a) Determine the velocity of the electrons in the beam.
b) The beam continues to send electrons into the sphere until it is charged up enough to repel the electrons. What is the charge on the sphere at which it stops further electrons from hitting it?

2. Relevant equations

1/2mv^2=-qΔV
mass of an electron=9.12X10^-31kg
elementary charge=1.602X10^-19 C
3. The attempt at a solution

Not sure if I'm going part a right:
.5(9.12X10^-31kg)v=(-1.602X10^-19 C)(1.2KeV)

Solving for v (velocity) I get 2.19 X 10^33 m/s

Does this look right?

Not even sure what to do to answer part b.

Any help is appreciated!!

2. Feb 21, 2012

### Innoko

Sorry, I have no time to check it fully, but 10^33 if way to much for a speed. It's 10^25 times higher than the speed of light!

3. Feb 21, 2012

### AnthroMecha

Yeah, I knew that answer sounded ridiculous.