UNITS OF ELECTRICAL MEASUREMENT
Basic Electrical Theory
Example 3:
Find the applied voltage, when given circuit resistance of 100 W and circuit current
of 0.5 amps.
Solution:
Since circuit resistance and circuit current are known, use Ohm’s Law to solve for
applied voltage.
E = IR
E = (0.5 A)(100 W) = 50 V
Conductance
The word "reciprocal" is sometimes used to mean "the opposite of." The opposite, or reciprocal,
of resistance is called conductance. As described above, resistance is the opposition to current
flow. Since resistance and conductance are opposites, conductance can be defined as the ability
to conduct current. For example, if a wire has a high conductance, it will have low resistance,
and vice-versa. Conductance is found by taking the reciprocal of the resistance. The unit used
to specify conductance is called "mho," which is ohm spelled backwards. The symbol for "mho"
is the Greek letter omega inverted ( ). The symbol for conductance when used in a formula is
G. Equation (1-5) is the mathematical representation of conductance obtained by relating the
definition of conductance (1/R) to Ohm’s Law, Equation (1-4).
(1-5)
G
1
RESISTANCE
I
E
Example:
If a resistor (R) has five ohms, what will its conductance (G) be in mhos?
Solution:
G (or
)
1
R
1
5
0.2
Power
Electricity is generally used to do some sort of work, such as turning a motor or generating heat.
Specifically, power is the rate at which work is done, or the rate at which heat is generated. The
unit commonly used to specify electric power is the watt. In equations, you will find power
abbreviated with the capital letter P, and watts, the units of measure for power, are abbreviated
with the capital letter W. Power is also described as the current (I) in a circuit times the
voltage (E) across the circuit. Equation (1-6) is a mathematical representation of this concept.
P = I x E or P = IE
(1-6)
ES-01
Page 16
Rev. 0
