CAPACITORS

CHAPTER FIVE
CAPACITORS
5.1 INTRODUCTION
So far we have limited our study to resistive circuits. In this chapter, we shall introduce two new and important passive linear circuit elements: the capacitor and the inductor (the inductor is discussed in detail in Chapter 7). Unlike resistors, which dissipate energy, capacitors and inductors do not dissipate but store energy, which can be retrieved at a later time. For this reason, capacitors and inductors are called storage elements. We begin by introducing capacitors and describing how to combine them in series or in parallel. Later, we do the same for inductors.
5.2 CAPACITORS
A capacitor is a passive element designed to store energy in its electric field. Besides resistors, capacitors are the most common electrical components. Capacitors are used extensively in electronics, communications, computers, and power systems. For example, they are used in the tuning circuits of radio receivers and as dynamic memory elements in computer systems.
A capacitor consists of two conducting plates separated by an insulator (or dielectric).
In many practical applications, the plates may be aluminum foil while the dielectric may be air, ceramic, paper, or mica.
The amount of charge stored, represented by q, is directly proportional to the applied voltage v so that
q = Cv (5.1)
where C, the constant of proportionality, is known as the capacitance of the capacitor. The unit of capacitance is the farad (F), in honor of the English physicist Michael Faraday (1791–1867). From Eq. (5.1), we may derive the following definition.
Capacitance is the ratio of the charge on one plate of a capacitor to the voltage difference between the two plates, measured in farads (F).
Note from Eq. (5.1) that 1 farad = 1 coulomb/volt.