CAPACITORS

Figure 26.1 illustrates a capacitor.1 It consists of two insulated conductors, a and b. They carry equal and opposite charges, + q and ? q, respectively. All lines of force that originate on a terminate on b.The capacitor is characterized by the following parameters:

Figure 26.1: Two insulated conductors ( a and b), totally isolated from their surroundings and carrying equal and opposite charges, form a capacitor.

• q, the magnitude of the charge on each conductor

• V, the potential difference between the conductors

Both q and V are proportional to each other in a capacitor, or q = CV. C is the constant of proportionality. It is called the capacitance of the capacitor. C depends on the following parameters:

• Shape of the conductors

• Relative position of the conductors

• Medium that separates the conductors

The unit of capacitance is the coulomb/volt or farad (F). Thus

It is important to note that:

but since

Thus,

This means that the current in a capacitor is proportional to the rate of change of the voltage with time.

In industry, the following submultiples of farad are used:

• Microfarad ( ?F): 1 ?F = 10 ^?6 F

• Picofarad (pF): 1 pF = 10 ^?12 F

Capacitors are very useful electric devices. They are used in the following applications:

• To store energy in an electric field. The energy is stored between the conductors of the capacitors, which are normally called plates. The...