Page 323 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 26

Capacitance

CAPACITANCE

A capacitor is a system that stores energy in the form of an electric field. In its simplest form, a capacitor consists
of a pair of parallel metal plates separated by air or other insulating material.
The potential difference V between the plates of a capacitor is directly proportional to the charge Q on either
of them, so the ratio Q/V is always the same for a particular capacitor. This ratio is called the capacitance C of
the capacitor:
Q
C =
V
charge on either plate
Capacitance =
potential difference between plates
The unit of capacitance is the farad (F), where 1 farad = 1 coulomb/volt. Since the farad is too large for
practical purposes, the microfarad and picofarad are commonly used, where
1 microfarad = 1 µF = 10 −6 F
1 picofarad = 1pF = 10 −12 F

A charge of 10 −6 C on each plate of 1-µF capacitor will produce a potential difference of V = Q/C = 1V
between the plates.

PARALLEL-PLATE CAPACITOR
A capacitor that consists of parallel plates each of area A separated by the distance d (Fig. 26-1) has a capaci-
tance of
A
C = Kε 0
d
d
+Q −Q
A

V

Fig. 26-1. (From Modern Technical Physics, 6th Ed., Arthur Beiser, c 1992. Reprinted by permission of Pearson
Education, Inc.)
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