In a typical capacitor, the electric field lines start on one conductor and terminate on the second conductor. The conductor at zero volts is often called the *reference conductor*. The ratio of charge on the first conductor to the charge on that of the second conductor is called *self-capacitance*. The capacitors used in a circuit all have self-capacitances.

When there is a group of conductors as in a circuit, the concept of capacitance must be extended to allow for cross coupling. As an example, consider traces over a ground plane as in Figure 1.2. The ground plane can be considered the reference conductor at zero volts.

Assume that one of the traces is at a voltage *V*_{1} and all the other conductors are at zero volts. By definition the self-capacitance *C*_{11} of trace 1 is the ratio of charge on trace 1 to the voltage on trace 1 or *Q*_{1}/*V*_{1}. When trace 1 is at potential *V*_{1}, some of the electric field lines terminate on trace 2. This represents an induced charge *Q*_{2} on this conductor. The ratio of *Q*_{2}/*V*_{1} = *C*_{21} is called a *mutual capacitance*. Since the field lines always terminate on charges of opposite polarity this ratio is always negative.

N.B.

Mutual capacitance is often called a *leakage* or *parasitic capacitance*.

Mutual capacitance can allow an induced current to flow in nearby circuits. ...

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