The Q factor of a series resonant circuit is defined as the ratio of the voltage across a coil or capacitor to the applied voltage. Q is equal to voltage across L or C divided by applied voltage.

Voltage across L is equal to i omega not L. where omega not is the angular frequency of the a.c. at resonance. The applied voltage at resonance is the potential drop across R, because the potential drop across L is equal to the drop across C and they are 180 degree out of phase. Therefore they cancel out and only potential drop across R will exist.

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