Physics - Capacitance
The amount of charge that can be stored by a capacitor depends upon the
surface area of the plates, the distance separating the plates and the
material that is used to electrically insulate the plates. Obviously the
larger the surface area the more charge can be stored for a given applied
The amount of charge that is able to be stored by a given capacitor is
affected by the size of the voltage applied to it. It is therefore
convenient to talk about the capacitance of a capacitor in terms of how
much charge can be stored for a given applied voltage. This is allocated
the unit of the farad. However, most capacitors have capacitances of the
order of microfarads.
Charging a capacitor
A capacitor (initially uncharged) is connected to a DC supply as shown.
Remember that electric current really involves the flow of electrons from
the negative terminal of a supply to the positive terminal. So it is easier
to gain an understanding of the operation of capacitors by referring to
electron flow rather than conventional current. The plates are initially
uncharged, but so once they are connected to the battery electrons will
flow readily from the negative terminal of the battery due to the fact that
like charges repel. The electrons begin to accumulate on plate X. Plate X
therefore develops a negative charge. Simultaneously, the positive terminal
of the battery will attract electrons from plate Y leaving plate Y, with a
net positive charge.
Recall that initially the plates were uncharged, that is the voltage
across the capacitor was initially zero volts. Current initially flows
easily in the circuit as there is little resistance to the flow of charge.
As the number of electrons on plate X gradually increases, any more
electrons arriving at the plate will be repelled. The rate of flow of
charge in the circuit has reduced as shown in the diagram below. The
capacitor has stored charge until it has reached its limit and no more
current can flow. The voltage across the capacitor plates will now be equal
to the voltage across the battery. The voltage across the capacitor was
originally zero and initially increased at a rapid rate. However, as the
capacitor became more and more charged the rate at which the voltage across
it increased became slower. Hence both the current-time and voltage-time
graphs for the capacitor display exponential relationships.
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