Circuit and electromagnetism basics
Which correctly describes a series RLC circuit driven by a sinusoidal voltage at its resonant frequency?
- AThe impedance is at its maximum and the current at its minimum
- BThe L and C voltages are equal in magnitude and opposite in phase, cancelling so that the impedance is R alone
- CThe current lags the voltage by 90°
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Answer: B. The L and C voltages are equal in magnitude and opposite in phase, cancelling so that the impedance is R alone
At resonance XL = XC, so the voltages across L and C are equal in magnitude and 180° apart, and they cancel. The impedance falls to its minimum value R, the current is at its maximum, and it is in phase with the voltage. The individual L and C voltages are Q times the source voltage, which can be far larger than the source when Q is high, so component voltage ratings need attention. Maximum impedance is a property of parallel resonance.
- A. Maximum impedance and minimum current describe parallel resonance.
- B. Correct. The L and C voltages cancel, giving Z = R.
- C. At resonance the current is in phase with the voltage; a 90° lag describes a circuit with L alone.
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