Cracking the SAT Physics Subject Test

CAPACITANCE

You may see a question on capacitors on the SAT Physics Subject Test, so let’s
discuss them now. Consider two conductors, separated by some distance, that carry
equal but opposite charges, +Q and −Q. This pair of conductors makes up a system
called a capacitor. Work must be done to create this separation of charge, and, as a
result, potential energy is stored. Capacitors are basically storage devices for
electrical potential energy.

Capacitors

It may help to think of a

capacitor as a dam. Water

builds up in one section

of the dam before it can

overflow and pass to the

next section.

The most common capacitors are parallel metal plates or sheets, and these
capacitors are called parallel-plate capacitors. We’ll assume that the distance d
between the plates is small compared to the dimensions of the plates, and in this
case, the electric field between the plates is uniform.

The ratio of Q to the voltage, V, for any capacitor, is called its

capacitance (C).

For a parallel-plate capacitor, we have

Determining

Capacitance

Capacitance shows the

relationship between

Q, the charge on the

capacitor, and the potential

difference, V. It does

not determine that charge,