# Homework Help: Calculating Electric Potential Differences

1. Feb 18, 2010

### G-reg

1. The problem statement, all variables and given/known data
When an electron moves from A to B along an electric field line in the figure below, the electric field does 2.00 10-19 J of work on it.

What are the electric potential differences:
(a) VB - VA

(b) VC - VA

(c) VC - VB

2. Relevant equations
Ok so I know that the equation to use in this case is that of, $$\Delta$$V = $$\frac{-W}{q}$$

3. The attempt at a solution
I understand how to choose the equation I've chosen and how to get the right answer. What I don't understand is why the answers to (a) and (b) aren't negative when the equation shows that they should. Can anyone help?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 18, 2010

### collinsmark

I don't have all the problem details (not enough to actually work out the problem). But allow me to speculate: Forgot the negative charge of the electron maybe? There's enough information for me to do the first part.

The given wording was "When an electron moves from A to B [...] the electric field does [positive value] J of work on it."

If the above statement applied to a test charge it means the V(A) is greater than V(B). But we're not dealing with a positive test charge. We are dealing with a negative electron . So given the negative charge, the statement implies, V(B) is greater than V(a). Therefore,

V(B) - V(A) is positive.

Last edited: Feb 18, 2010