Capacitors

The combination of any two conductors separated by an insulator is called a capacitor. A capacitor is a device that can be made to store electric charge and you can compare it with a bucket used to store water.
In general a bigger capacitor can store more charge than a smaller one. The two conductors usually carry an equal and opposite charge such that the net charge on the capacitor as a whole is zero. If a capacitor is state as having a charge Q it means that the conductor at the higher potential has a charge + Q and that at the lower potential a charge - Q.

The charge can then be released later.

Practical capacitors come in three basic forms:
(a) as part of an integrated circuit
(b) as two parallel plates (described more fully later)
(c) as a cylinder made of a pair of rolled up plates

All capacitors have an insulator between the plates and this insulator may be air or another gas, waxed paper or an electrolyte. This insulating material is called a dielectric.

In just the same way that a certain volume of water can be stored in two different shaped beakers (see Figure 1(a)) the same amount of charge may be stored in two different capacitors (see Figure 1(b)). The water pressure at the base of one container is higher than the other and similarly the potential difference across one capacitor is greater than that across the other.


You have to be careful when handling capacitors as you cannot be sure that they are not still charged. One that is storing a large charge at a high potential can give you a nasty shock!

The simplest

type of capacitor is a pair of metal plates with air between them. If they are connected to a cell as shown in Figure 2 no steady current will flow because of the insulator (air), between the plates. However currents that change with time are possible. The following diagrams explain what happens when the capacitor is charged.


When the switch is closed, electrons flow from plate A to the cell and from the cell to plate B.

Eventually a charge Q is stored on each plate and the capacitor is said to be fully charged.
Notice that both plates have the same size of charge although they are of opposite sign.


The addition of a resistor (R) in the circuit (Figure 3) does not affect the final potential difference across the capacitor. However it will slow down the time it takes the capacitor to become fully charged because the current in the circuit during charging will be less.

We will return to charging later to look at the factors which affect the rate at which capacitors can be charged and discharged in much greater detail including a mathematical treatment.

Capacitance is measured in farads (F)

The capacitance of a capacitor is related to the potential difference across it (V) and the charge on each of its plates (Q) by the formula:

A farad is actually a very large unit. A pair of plates 1mm apart in a vacuum would have need to have an area of 1.13x10⁸m² if the capacitance was to be 1 farad. This means two square plates with sides about 11.3 km (6.5 miles) long! Most of the capacitors that you will meet will have capacitances of microfarads (µF, 10^-6F), nanofarads ( nF, 10^-9F) or picofarads (pF, 10^-12F). The capacitance of the Earth is about 4F.

Electrolytic capacitors have capacitances greater than 1 mF, they are polarised and must always be connected the right way round in the circuit otherwise they will explode! Smaller value capacitors can be unpolarised and may be connected either way round. The symbols for the two types of capacitor are shown in Figure 4.