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Physics - Capacitance

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

voltage.

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 [1]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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