Power in an AC Circuit

LEARNING OBJECTIVES

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

where

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,

and

are in phase and therefore always have the same sign (see Figure 12.2.2). For a capacitor or inductor, the relative signs of

and

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.

(Figure 12.4.1)



Figure 12.4.1 Graph of instantaneous power for various circuit elements. (a) For the resistor,
whereas for (b) the capacitor and (c) the inductor,
(d) For the source,
which may be positive, negative, or zero, depending on



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:

where

is the period of the oscillations. With the substitutions

and

this integral becomes

Using the trigonometric relation

we obtain

Evaluation of these two integrals yields

and

Hence, the average power associated with a circuit element is given by

(12.4.1)



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

and

is shown in ??(d). To make

look like its dc counterpart, we use the rms values

and

of the current and the voltage. By definition, these are

where

With

and

we obtain

We may then write for the average power dissipated by a resistor,

(12.4.2)



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,

and

respectively. Since

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

circuit,

and



Hence the average power of the generator is

(12.4.3)


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.



EXAMPLE 12.4.1

Power Output of a Generator

An ac generator whose emf is given by

is connected to an

circuit for which

and

(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?



Strategy

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



Solution

a. Since

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



Significance

If the resistance is much larger than the reactance of the capacitor or inductor, the average power is a dc circuit equation of

where

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

and

respectively. Determine these values for the components of the

circuit of Equation 12.4.1.



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