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S E C T I O N 3 3 . 7 • Resonance in a Series RLC Circuit 1049 Quick Quiz 33.8 An AC source drives an RLC circuit with a fixed voltage amplitude. If the driving frequency is # 1 , the circuit is more capacitive than inductive and the phase angle is * 10°. If the driving frequency is # 2 , the circuit is more inductive than capacitive and the phase angle is $ 10°. The largest amount of power is 1 (b) # 2 (c) The same amount of power is delivered at both frequencies. 33.7 Resonance in a Series RLC Circuit A series RLC circuit is said to be in resonance when the current has its maximum value. In general, the rms current can be written (33.33) where Z is the impedance. Substituting the expression for Z from Equation 33.25 into (33.34) Because the impedance depends on the frequency of the source, the current in the 0 at which X L * X C " 0 is called the resonance frequency of the circuit. To find # 0 , we use the condition X L " X C , from which we obtain # 0 L " 1/# 0 C, or (33.35) This frequency also corresponds to the natural frequency of oscillation of an LC circuit # 0 " 1 √ LC I
rms " ∆V
rms √ R
2 $ (X L * X C ) 2 I
rms " ∆V
rms Z Equation 33.31 shows that the power delivered by an AC source to any circuit depends on the phase—a result that has many interesting applications. For example, Calculate the average power delivered to the series RLC Solution First, let us calculate the rms voltage and rms max and I max from Example 33.5: I
rms " I
max √ 2 " 0.292 A √ 2 " 0.206 A ! V
rms " ! V
max √ 2 " 150 V √ 2 " 106 V Example 33.6 Average Power in an RLC Series Circuit Because - " * 34.0°, the power factor is cos (* 34.0°) " ! av " I rms ! V rms cos - " (0.206 A)(106 V)(0.829) " We can obtain the same result using Equation 33.32. 18.1 W Resonance frequency |