Resonance in an AC Circuit

LEARNING OBJECTIVES

By the end of the section, you will be able to:

  • Determine the peak ac resonant angular frequency for a RLC circuit
  • Explain the width of the average power versus angular frequency curve and its significance using terms like bandwidth and quality factor


In the

series circuit of Figure 12.3.1, the current amplitude is, from Equation 12.3.2,



(12.5.1)



If we can vary the frequency of the ac generator while keeping the amplitude of its output voltage constant, then the current changes accordingly. A plot of

versus

is shown in Figure 12.5.1.

(Figure 12.5.1)



Figure 12.5.1 At an
circuit’s resonant frequency,
the current amplitude is at its maximum value.



Figure 12.5.1 has a similar appearance to the plot of a damped harmonic oscillator’s variation in amplitude with respect to the angular frequency of a sinusoidal driving force. This similarity is more than just coincidence, as shown by the application of Kirchhoff’s loop rule to the circuit of Figure 12.3.1. This yields

(12.5.2)

or

where we substituted

for

Equation 12.5.2 has the general form of the differential equation for damped harmonic motion, demonstrating that the driven

series circuit is the electrical analog of the driven damped harmonic oscillator.



The resonant frequency

of the

circuit is the frequency at which the amplitude of the current is a maximum and the circuit would oscillate if not driven by a voltage source. By inspection, this corresponds to the angular frequency

at which the impedance

in Equation 12.5.1 is a minimum, or when





and



(12.5.3)



This is the resonant angular frequency of the circuit. Substituting ω0ω0 into Equation 12.3.1, Equation 12.3.2, and Equation 12.3.3, we find that at resonance,





Therefore, at resonance, an

circuit is purely resistive, with the applied emf and current in phase.



What happen to the power at resonance? Equation 12.4.3 tells us how the average power transferred from an ac generator to the

combination varies with frequency. In addition,

reaches a maximum when

which depends on the frequency, is a minimum, that is, when

and

Thus, at resonance, the average power output of the source in an

series circuit is a maximum. From Equation 12.4.3, this maximum is



Figure 12.5.2 is a typical plot of

versus

in the region of maximum power output. The bandwidth

of the resonance peak is defined as the range of angular frequencies

over which the average power

is greater than one-half the maximum value of

The sharpness of the peak is described by a dimensionless quantity known as the quality factor

of the circuit. By definition,



(12.5.4)



where

is the resonant angular frequency. A high

indicates a sharp resonance peak. We can give

in terms of the circuit parameters as



(12.5.5)



Figure 12.5.2 Like the current, the average power transferred from an ac generator to an RLC circuit peaks at the resonant frequency.



Resonant circuits are commonly used to pass or reject selected frequency ranges. This is done by adjusting the value of one of the elements and hence “tuning” the circuit to a particular resonant frequency. For example, in radios, the receiver is tuned to the desired station by adjusting the resonant frequency of its circuitry to match the frequency of the station. If the tuning circuit has a high

it will have a small bandwidth, so signals from other stations at frequencies even slightly different from the resonant frequency encounter a high impedance and are not passed by the circuit. Cell phones work in a similar fashion, communicating with signals of around

that are tuned by an inductor-capacitor circuit. One of the most common applications of capacitors is their use in ac-timing circuits, based on attaining a resonant frequency. A metal detector also uses a shift in resonance frequency in detecting metals (Figure 12.5.3).

(Figure 12.5.3)



Figure 12.5.3 When a metal detector comes near a piece of metal, the self-inductance of one of its coils changes. This causes a shift in the resonant frequency of a circuit containing the coil. That shift is detected by the circuitry and transmitted to the diver by means of the headphones.



EXAMPLE 12.5.1

Resonance in an
Series Circuit

(a) What is the resonant frequency of the circuit of Example 12.2.1? (b) If the ac generator is set to this frequency without changing the amplitude of the output voltage, what is the amplitude of the current?



Strategy

The resonant frequency for a

circuit is calculated from Equation 12.5.3, which comes from a balance between the reactances of the capacitor and the inductor. Since the circuit is at resonance, the impedance is equal to the resistor. Then, the peak current is calculated by the voltage divided by the resistance.



Solution

a. The resonant frequency is found from Equation 12.5.3:





b. At resonance, the impedance of the circuit is purely resistive, and the current amplitude is





Significance

If the circuit were not set to the resonant frequency, we would need the impedance of the entire circuit to calculate the current.



EXAMPLE 12.5.2

Power Transfer in an
Series Circuit at Resonance

(a) What is the resonant angular frequency of an

circuit with

and

?

(b) If an ac source of constant amplitude

is set to this frequency, what is the average power transferred to the circuit? (c) Determine

and the bandwidth of this circuit.



Strategy

The resonant angular frequency is calculated from Equation 12.5.3. The average power is calculated from the rms voltage and the resistance in the circuit. The quality factor is calculated from Equation 12.5.5 and by knowing the resonant frequency. The bandwidth is calculated from Equation 12.5.4 and by knowing the quality factor.



Solution

a. The resonant angular frequency is





b. At this frequency, the average power transferred to the circuit is a maximum. It is





c. The quality factor of the circuit is





We then find for the bandwidth



Significance

If a narrower bandwidth is desired, a lower resistance or higher inductance would help. However, a lower resistance increases the power transferred to the circuit, which may not be desirable, depending on the maximum power that could possibly be transferred.



CHECK YOUR UNDERSTANDING 12.6

In the circuit of Figure 12.3.1,

and

(a) What is the resonant frequency? (b) What is the impedance of the circuit at resonance? (c) If the voltage amplitude is

what is

at resonance? (d) The frequency of the AC generator is now changed to

Calculate the phase difference between the current and the emf of the generator.



CHECK YOUR UNDERSTANDING 12.7

What happens to the resonant frequency of an

series circuit when the following quantities are increased by a factor of

: (a) the capacitance, (b) the self-inductance, and (c) the resistance?



CHECK YOUR UNDERSTANDING 12.8

The resonant angular frequency of an

series circuit is

An ac source operating at this frequency transfers an average power of

to the circuit. The resistance of the circuit is

Write an expression for the emf of the source.



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