# Homework Help: Point charge

1. Jul 29, 2008

### kitkat2950

A proton is moved at a constant velocity towards a stationary +3micro coloumb point charge that cannot move. The proton is initially 50 cm away from the point charge and the proton is moved until it is 35 cm away from the point charge.
A) How much work was done on the proton?
B) If released from rest when 35 cm away from the point charge, what will be the speed of the proton at infinity.

For A, I know W=qV where q is the charge and i think v is the change in electric potential energy. And I thought that V=kq/r and k=9E9. I did V=(9E9)(1.6e-19C)/.35m and V=(9E9)(1.6E-19C)/.5m. Then I subtracted to find the change in potential energy. I then multiplied that by the charge, but my answer was wrong. The right answer is 3.7E-15.

For B, I think once I find V, I can figure out the rest.

2. Jul 29, 2008

### Hootenanny

Staff Emeritus
Which quantity did you multiply the change in potential by?

3. Jul 29, 2008

### Defennder

Well it is a point charge you are dealing with. So V should be relatively easy to calculate.

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