Q.How will you define Q-factor?

Definition. At resonance in a series RLC circuit, the current -- and hence the voltages developed across L and C individually -- both rise to values far LARGER than the applied source voltage itself. The Q-factor (quality factor) is defined as the ratio of the voltage across L (equivalently, across C) at resonance to the applied voltage:

Since the circuit is purely resistive at resonance, the applied voltage there equals the voltage across R, giving

(using $\omega_r=1/\sqrt{LC}$ for the last step). …