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Capacitors
plates of the capacitor now have equal, but opposite, electric
charges on them.
Current only flows during the time when the charges are building
up on the capacitor plates — not before and not after. It’s
also important to remember that current cannot flow through
the capacitor — a layer of insulator (known correctly as the
dielectric) lies between the plates, remember — current only
flows in the circuit around the capacitor.
If we now completely disconnect the charged capacitor from
the battery, the equal and opposite charges remain — in
theory — indefinitely. In practice, on the other hand, charge
is always lost due to leakage current between the plates.
You can try this for yourself, if you like: put a capacitor into
the breadboard then charge it up by connecting the battery
directly across it. Now disconnect the battery and leave the
capacitor in the breadboard for a time (overnight, say). Then
connect your meter across it to measure the voltage. You
should still get a reading, but remember that the resistance
of the meter itself will always drain the charge stored.
The size of the charge stored in a capacitor depends on two
factors; the capacitor’s capacitance (in farads) and the applied
voltage. The relationship is given by:
where Q is the charge measured in coulombs, C is the capacitance
and V is the voltage. From this we can see that a charge
of one coulomb is stored by a capacitor of one farad, when a
voltage of one volt is applied.
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