Page 2 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 1
FUNDAMENTALS OF

ELECTRIC SYSTEMS

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, the magnitude of the charge on each conductor
● V, the potential difference between the conductors
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 capac-
itance 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
1 F 1 C/V
It is important to note that

dq dV
C
dt dt
but since

dq
i
dt
Thus,
dV
i C
dt
This means that the current in a capacitor is proportional to the rate of change of the voltage
with time.

1.1
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.