# Homework Help: Finding the electrical

1. Jan 15, 2012

### bobsmith76

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

The electric potential difference due to a point charge is 4.8 V at a distance of 4.2 cm from the charge. What will be the electric potential energy of the system if a second charge of +6.0 μC is placed at that location?

2. Relevant equations

V = (EQ)/q
V = kq/r
ΔV = W/q

3. The attempt at a solution

I've tried all three equations but none of them give me the right answer.

I try to find the charge of the first point

step 1. 4.8 = kx/.042

step 2. .042(4.8) = kx

step 3 (.042(4.8))/k = x

step 4 = 2.24 * 10^-11 (that seems like a ridiculously small number, most charges are 10^-6. Adding 6*10^-6 will do nothing to the number, so I must be doing something wrong. I just can't figure out what. Even if I do get the right number and I add the charge I can't figure out what equation to use next.

2. Jan 15, 2012

### cupid.callin

Write down formula of potential energy and potential side by side,
then you'll know a much simpler method, and btw your calculation are fine, it doesn't mater how small a charge is

smallest possible free charge is e = 1.6 * 10-19

3. Jan 15, 2012

### Staff: Mentor

Make sure that you know the difference between the potential at a location and potential energy of the system as a whole. The problem statement suggests that the second charge is being brought to the location where the potential due to the first charge is 4.8V, 4.2 cm from the first charge; it's not being added to the first charge.

Last edited: Jan 15, 2012