文档介绍：Source: ELECTRICAL EQUIPMENT HANDBOOK
CHAPTER 1
FUNDAMENTALS OF
ELECTRIC SYSTEMS
CAPACITORS
Figure illustrates a capacitor. It consists of two insulated conductors a and b. They carry
equal and opposite charges ϩq and Ϫq, 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 ϭ 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 ϭ 1 C/V
It is important to note that
dq dV
ᎏϭ C ᎏ
dt dt
but since
dq
ᎏϭ i
dt
Thus,
dV
i ϭ C ᎏ
dt
This means that the current in a capacitor is proportional to the rate of change of the voltage
with time.
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FUNDAMENTALS OF ELECTRIC SYSTEMS
CHAPTER ONE
FIGURE Two insulated conductors, totally isolated from their surroundings
and carrying equal and opposite charges, form a capacitor.
In industry, the following submultiples of farad are used:
Ϫ
● Microfarad (1 F ϭ 10 6 F)
Ϫ
● Picofarad (1 pF ϭ 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, which
are normally called plates. The electric energy stored in the capacitor is given by
1 q2
U ϭᎏᎏ
E 2 C
● To reduce voltage fluctuations in electronic power suppli