Cracking the SAT Physics Subject Test

What is its change in electrical potential energy?

Here’s How to Crack It

Since the field is uniform, the electric force that the charge feels, FE = qE, is

constant. Since q is positive, FE points in the same direction as E, and, as the figure

shows, they point in the same direction as the displacement, r. This makes the work
(W = Fd) done by the electric field equal to WE = FEr = qEr, so the change in the

electrical potential energy is

∆UE = −qEr

Notice that the change in potential energy is negative, which means that potential
energy has decreased; this always happens when the field does positive work. It’s
just like when you drop a rock to the ground: Gravity does positive work, and the
rock loses gravitational potential energy.

2. Solve the previous problem, but consider the case of a negative
   charge, −q.