Simple (?) electrostatics problem

Thread Starter


Joined Jan 26, 2008
I'm struggling through a data set put together (I suspect) by a non-physicist. The following is either a poorly written problem, or I'm missing the point completely. Please advise.

Given three infinitely large parallel plates with charge densities as follows:

Plate 1: -8 * 10^-9C/m^2
Plate 2: same as 1
Plate 3: -2 * 10^-9C/m^2

Point X is located between plates 1 and 2, point Y is located between plates 2 and 3. The question is,

"a. Which (point) has the higher potential energy for an electron?
b. Which has the higher electrical potential for an electron?"

To part b, my instinct is to say Y, because a test charge at Y "sees" a potential difference, while a charge at X does not. But here's the rub: Does a test charge at Y also "see" the charge on plate 1, or is this shielded by plate 2? If not, the answer would still be Y.

A bigger question is, what's the distinction between the potential energy and the electrical potential? I would think they're one and the same. Is the tutor simply playing a head game here?



Joined Dec 1, 2007
A couple things here, electrical potential energy is the measure of how badly a charge wants to move. It is the strength of the electric field multiplied by the strength of the charge (quantitatively).

The electric potential differs from the potential energy because it does not depend on a specific charged particle in the electric field. It is, more or less, the strength of the electric field. If you have an electric field, you can find the potential energy of an electron in the field. If you divide the energy by the charge of the particle, you are left with a potential. This is the number, when multiplied by any charge in the field, that will give you a potential energy.

The typical unit of electrical potential is volt.