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The average power in an AC circuit is P = E_rms * I_rms * cos(phi), where phi is the phase angle between voltage and current. For a series LCR circuit operating at resonance, what is the value of cos(phi)?
- zero
- 1
- 1/2
- 1/sqrt(2)
Correct answer: 1
Solution
At resonance in a series LCR circuit, X_L = X_C, so the net reactance is zero and the impedance equals R (purely resistive). Hence the current and voltage are in phase: phi = 0 and cos(phi) = cos 0 = 1. The power factor is unity, giving maximum power transfer.
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