A capacitor acts much like the storage tank in a compressed air system.
In a compressed air system air is forced into the tank by the
compressor. As this is done the pressure in the tank rises. When the
pressure in the tank equals the pressure supplied by the compressor,
the tank will stop filling. A capacitor acts much the same way. Forcing
electricity into a capacitor will charge the capacitor, increasing its
voltage. When the voltage in the capacitor equals the voltage of the
voltage source (a battery, etc.) the capacitor will stop charging. Once
full, a compressed air tank can be disconnected from the compressor and
it will store energy as air under pressure until the pressure is
released. A capacitor acts the same way. Once a capacitor is charged,
energy is stored in the electrostatic field between the conductors.
However, it acts just as if the energy were stored as a fluid under
pressure. In this state, the capacitor can be disconnected from the
charging circuit and it will remain charged until a circuit is provided
to allow it to discharge.1
Although a capacitor is essentially an open circuit (two conductors
separated by an insulator), while it is charging, current will appear
to flow through it. As electrical energy accumulates on one plate, it
forces electrical energy from the other. This gives the appearance of
electricity flowing through the capacitor. Once the capacitor is
charged to the source voltage, the flow of current stops and the
capacitor acts like the open circuit it is. A charged capacitor can
also be seen to be much like a battery. However, since a capacitor
doesn’t have the chemical energy stores that a battery has, it will
discharge many times faster. New "super capacitors", such as Electric
Double-Layer Capacitors (EDLCs), have such high capacities that they
are can store nearly as much energy as a battery. They may eventually
replace batteries in some areas such as electric and hybrid vehicles
(the high self-discharge rate of EDLCs must be overcome before this
will be practical). They are already frequently used to provide
temporary power to equipment while batteries are changed.
