Cracking the SAT Physics Subject Test

5. Two large, flat plates—one carrying a charge of +Q, the other
   −Q—are separated by a distance d. The electric field between the
   plates, E, is uniform. Determine the potential difference between the
   plates.

Potential vs.

Potential Energy

Remember that everything

naturally tends toward

lower potential energy.

Positive charges tend

toward lower potential

and negative charges tend

toward higher potential.

Here’s How to Crack It

Imagine a positive charge q moving from the positive plate to the negative plate.

Since the work done by the electric field is

WE,+→ = FEd = qEd

the potential difference between the plates is