Power in an AC Circuit
By the end of the section, you will be able to:
- Describe how average power from an ac circuit can be written in terms of peak current and voltage and of rms current and voltage
- Determine the relationship between the phase angle of the current and voltage and the average power, known as the power factor
A circuit element dissipates or produces power according to
is the current through the element and
is the voltage across it. Since the current and the voltage both depend on time in an ac circuit, the instantaneous power
is also time dependent. A plot of
for various circuit elements is shown in Figure 12.4.1. For a resistor,
are in phase and therefore always have the same sign (see Figure 12.2.2). For a capacitor or inductor, the relative signs of
vary over a cycle due to their phase differences (see Figure 12.2.4 and Figure 12.2.6). Consequently,
is positive at some times and negative at others, indicating that capacitive and inductive elements produce power at some instants and absorb it at others.
Because instantaneous power varies in both magnitude and sign over a cycle, it seldom has any practical importance. What we’re almost always concerned with is the power averaged over time, which we refer to as the average power. It is defined by the time average of the instantaneous power over one cycle:
is the period of the oscillations. With the substitutions
this integral becomes
Using the trigonometric relation
Evaluation of these two integrals yields
Hence, the average power associated with a circuit element is given by
In engineering applications,
is known as the power factor, which is the amount by which the power delivered in the circuit is less than the theoretical maximum of the circuit due to voltage and current being out of phase. For a resistor,
so the average power dissipated is
A comparison of
is shown in ??(d). To make
look like its dc counterpart, we use the rms values
of the current and the voltage. By definition, these are
We may then write for the average power dissipated by a resistor,
This equation further emphasizes why the rms value is chosen in discussion rather than peak values. Both equations for average power are correct for Equation 12.4.2, but the rms values in the formula give a cleaner representation, so the extra factor of
is not necessary.
Alternating voltages and currents are usually described in terms of their rms values. For example, the
from a household outlet is an rms value. The amplitude of this source is
Because most ac meters are calibrated in terms of rms values, a typical ac voltmeter placed across a household outlet will read
For a capacitor and an inductor,
we find from Equation 12.4.1 that the average power dissipated by either of these elements is
Capacitors and inductors absorb energy from the circuit during one half-cycle and then discharge it back to the circuit during the other half-cycle. This behaviour is illustrated in the plots of Figure 12.4.1, (b) and (c), which show
oscillating sinusoidally about zero.
The phase angle for an ac generator may have any value. If
it absorbs power. In terms of rms values, the average power of an ac generator is written as
For the generator in an
Hence the average power of the generator is
This can also be written as
which designates that the power produced by the generator is dissipated in the resistor. As we can see, Ohm’s law for the rms ac is found by dividing the rms voltage by the impedance.
Power Output of a Generator
An ac generator whose emf is given by
is connected to an
circuit for which
(a) What is the rms voltage across the generator? (b) What is the impedance of the circuit? (c) What is the average power output of the generator?
The rms voltage is the amplitude of the voltage times
The impedance of the circuit involves the resistance and the reactances of the capacitor and the inductor. The average power is calculated by Equation 12.4.3, or more specifically, the last part of the equation, because we have the impedance of the circuit
the rms voltage
and the resistance
the rms voltage across the generator is
b. The impedance of the circuit is
c. From Equation 12.4.3, the average power transferred to the circuit is
If the resistance is much larger than the reactance of the capacitor or inductor, the average power is a dc circuit equation of
replaces the rms voltage.
CHECK YOUR UNDERSTANDING 12.4
An ac voltmeter attached across the terminals of a
ac generator reads
Write an expression for the emf of the generator.
CHECK YOUR UNDERSTANDING 12.5
Show that the rms voltages across a resistor, a capacitor, and an inductor in an ac circuit where the rms current is
are given by
respectively. Determine these values for the components of the
circuit of Equation 12.4.1.
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Introduction to Electricity, Magnetism, and Circuits by Daryl Janzen is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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