Figure 18.4 Basic parallel-plate capacitor.The battery in Figure 18.4 provides a voltage across the plates; once you’ve charged the capacitor,
you disconnect the battery. The space between the plates prevents any charges from jumping from one
plate to the other while the capacitor is charged. When you want to discharge the capacitor, you just
connect the two plates with a wire.
The amount of charge that each plate can hold is described by the following equation:
Q is the charge on each plate, C is called the “capacitance,” and V is the voltage across the plates. The
capacitance is a property of the capacitor you are working with, and it is determined primarily by the size
of the plates and the distance between the plates, as well as by the material that fills the space between
the plates. The units of capacitance are farads, abbreviated F; 1 coulomb/volt = 1 farad.
The only really interesting thing to know about parallel-plate capacitors is that their capacitance can
be easily calculated. The equation is:
In this equation, A is the area of each plate (in m^2 ), and d is the distance between the plates (in m). The
term ε 0 (pronounced “epsilon-naught”) is called the “permittivity of free space.” This term will show up
again soon, when we introduce the constant k . The value of ε 0 is 8.84 × 10−12 C/V·m, which is listed on
the constants sheet.
Capacitors become important when we work with circuits. So we’ll see them again in Chapter 19 .
Point Charges
As much as the writers of the AP exam like parallel plates, they love point charges. So you’ll probably be
using these next equations quite a lot on the test.