CAPACITORS

Figure 1.1 illustrates a capacitor. 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:

• q,n the magnitude of the charge on each conductor

• V, the potential difference between the conductors

Figure 1.1: Two insulated conductors, totally isolated from their surroundings and carrying equal and opposite charges, form a capacitor.

The parameters q and V are proportional to each other in a capacitor, or q = CV, where C is the constant of proportionality. It is called the capacitance of the capacitor. The capacitance 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 (C/V) 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 (1 ?F = 10 ^?6F

• Picofarad (1 pF = 10 ^?12F

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, which are normally called plates. The electric energy...