In a series LCR circuit at resonance, what is the phase difference between the applied voltage and the current?

zero
pi/4
pi/2
pi

Correct answer: zero

Solution

At resonance XL = XC, so the net reactance is zero and the impedance is purely resistive (Z = R). The phase angle phi satisfies tan(phi) = (XL - XC)/R = 0, so the voltage and current are in phase: phase difference is zero.

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