# Electric Potential - Conceptual question

1. Oct 16, 2011

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

At a certain distance from a point charge, the magnitude of the electric field is 500 V/m and the electric potential is -3.00 kV. (a) What is the distance to the charge? (b) What is the magnitude of the charge?

2. Relevant equations

3. The attempt at a solution

a) 6 meters
b) -2 micro-Coulombs
*These are correct, I mention this fact cause I don't want you to have to waste your time checking so.

My question is: for part b, if you solve for Q, the magnitude of charge, from E instead of V (E = kq/r^2), what is the visual concept behind why the charge is negative? Because obviously if you solve from V, you will get a negative 2 uC.

Of course the book asks for magnitude anyway, but the answer key gives the negative sign as well. I want to understand the concept behind it.

2. Oct 16, 2011

### Staff: Mentor

You cannot tell the sign of Q just using E, since you were not told the direction of the field.

3. Oct 16, 2011