An alternating e.m.f. of frequency v= 12πLC is applied to a series LCR circuit. For this frequency of the applied e.m.f.

The circuit is at resonance and its impedance is made up only of a reactive part

The current in the circuit is in phase with the applied e.m.f. and the voltage across R equals this applied emf

The sum of the p.d.’s across the inductance and capacitance equals the applied e.m.f. which is 180o ahead of phase of the current in the circuit

The quality factor of the circuit is  ωL/R or 1/ωCR  and this is a measure of the voltage magnification (produced by the circuit at resonance) as well as the sharpness of resonance of the circuit