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Figure 11-4 An air spaced variable capacitor
Charging and discharging a capacitor

The interesting stuff starts to happen when we connect an emf to the plates of a
capacitor. Have a look at the test circuit of Figure 11-5.

Figure 11-5 A capacitor charge-discharge circuit

The capacitor can be switched so that it will be connected to the 10-volt
battery when the switch is thrown to the left, or when the switch is thrown to
the right, it will be connected to the resistor. The capacitor can be connected
by the switch to the battery or the resistor, but not both at the same time.

The negative terminal of the battery has an excess of electrons on it, created
by the chemical action within the battery. The positive terminal has a
deficiency of electrons. Now recall that unlike charges attract. If there was
any way for the electrons to get from the negative terminal of the battery to
the positive, they would.

Now, I just want you to imagine a battery by itself with two terminals. There
is an electrostatic field between the two terminals of any battery created by
the unlike charges on each terminal. In other words, there is a very slight
tugging from the positive terminal and a very slight pushing from the
negative terminal in a vain attempt to move electrons from the negative to the
positive terminal.
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