FIGURE 26.20 shows two electrodes that have been charged to $\pm Q$. Their net charge is zero, but something has separated positive and negative charges. Consequently, there is a potential difference $\text{Δ}V$ between the electrodes.

FIGURE 26.20 Two equally but oppositely charged electrodes form a capacitor.

It seems plausible that $\text{Δ}V$ is directly proportional to Q. That is, doubling the amount of charge on the electrodes will double the potential difference. We can write this as $Q=C\text{Δ}V$, where the proportionality constant

is called the capacitance of the two electrodes. The two electrodes themselves form a capacitor, so we’ve written a subscript C on $\text{Δ}V{}_{\text{C}}$ to indicate that this is the capacitor voltage, the potential difference between the positive and negative electrodes.

The SI unit of capacitance is the farad, named in honor of Michael Faraday. One farad is defined as

One farad turns out to be an enormous amount of capacitance. Practical capacitors are usually measured in units of microfarads $(\mu \text{F})$ or picofarads $(1\text{pF}={10}^{-12}\text{F})$.

Turning Equation 26.14 around, we see that the amount of charge on a capacitor that has been charged to $\text{Δ}V{}_{\text{C}}$ is

The amount of charge is determined jointly by the potential difference and by a property of the electrodes called capacitance. As we’ll see, capacitance depends only on the geometry of the electrodes.