SAT Physics Subject Test

Chapter 9 Electric Potential and 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 make 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.

The most common conductors 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, Δ V, for any capacitor, is called its capacitance (C).

C = 

For a parallel-plate capacitor, we have

C = 

Capacitance is a measure of the capacity for holding charge. The greater the capacitance, the more charge can be stored on the plates at a given potential difference. The capacitance of any capacitor depends only on the size, shape, and separation of the conductors. From the definition C =Q/V, the units of C are coulombs per volt. One coulomb per volt is renamed one farad (abbreviated F): 1 C/V = 1 F .

Questions 6-7

A 10-nanofarad parallel-plate capacitor holds a charge of magnitude 50 C on each plate.

  6. What is the potential difference between the plates?

  7. If the plates are separated by a distance of 0.885 mm, what is the area of each plate?

Here’s How to Crack It

6.     From the definition, C = , we find that

7.     From the equation C = , we can calculate the area, A, of each plate.