When the resistance in an LCR circuit is small, the circuit is very responsive to changes in frequency (figure 1-5). That

is, the resonance curve is sharp and current at resonance is quite large, being limited only by R. If the resistance is

large, in comparison with the reactance, the total impedance change as the frequency passes through resonance is small,

the resonant peak is broad, and the current at resonance is quite small. A series resonant circuit with small resistance is

said to have high gain Q (not to be confused with the Q of electric charge). Q is determined by the ratio

(Q is the ratio of apparent power to real power.)

When the Q of a series resonant circuit is known, it may be used to enable an easy solution for the voltage across L or

C at resonance. The voltage across either reactance by Ohm's Law is

This is the phenomenon of resonance, that is, the applied voltage is multiplied by the Q gives the voltage across the coil

or capacitor.