Basic Electrical Theory
UNITS OF ELECTRICAL MEASUREMENT
Using Ohm’s Law for the value of voltage (E),
E = I x R
and using substitution laws,
P = I x ( I x R)
power can be described as the current (I) in a circuit squared times the resistance (R) of the
circuit. Equation (1-7) is the mathematical representation of this concept.
P = I2R
(1-7)
Inductance
Inductance is defined as the ability of a coil to store energy, induce a voltage in itself, and
oppose changes in current flowing through it. The symbol used to indicate inductance in
electrical formulas and equations is a capital L. The units of measurement are called henries.
The unit henry is abbreviated by using the capital letter H. One henry is the amount of
inductance (L) that permits one volt to be induced (VL) when the current through the coil changes
at a rate of one ampere per second. Equation (1-8) is the mathematical representation of the rate
of change in current through a coil per unit time.
(1-8)
DI
Dt
Equation (1-9) is the mathematical representation for the voltage VL induced in a coil with
inductance L. The negative sign indicates that voltage induced opposes the change in current
through the coil per unit time (DI/Dt).
(1-9)
VL
L
DI
Dt
Inductance will be studied in further detail later in this text.
Capacitance
Capacitance is defined as the ability to store an electric charge and is symbolized by the capital
letter C. Capacitance (C), measured in farads, is equal to the amount of charge (Q) that can be
stored in a device or capacitor divided by the voltage (E) applied across the device or capacitor
plates when the charge was stored. Equation (1-10) is the mathematical representation for
capacitance.
(1-10)
C
Q
E
Rev. 0
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