Series resonance:
One of the most significant phenomena in alternating current circuits, especially in the radio frequency engineering, is property of resonance. You have already learned that resonance is a condition which occurs when capacitive and inductive reactance cancel each other out. Resonant circuits and devices have different applications in the electricity and electronics.
The capacitive reactance, XC, and inductive reactance, XL, can at times be equal in magnitude. They are always opposite in their effect. In any circuit containing an inductance and capacitance, there will be a frequency at which XL = -XC. This is reso- nance. At times XL = -XC at one frequency; in some special devices it can occur at many frequencies. In general, if a circuit contains 1 coil and 1 capacitor, there will be 1 resonant frequency.
Refer to schematic diagram of the figure given below. You might recognize this as a series RLC circuit. At some specific frequency, XL = -XC. This is inevitable, if L and C are nonzero and finite. This is resonant frequency of circuit. It can be abbreviated fo.
Figure: A series RLC circuit.
At fo, the effects of the capacitive reactance and inductive reactance cancel out each other. The result is that circuit appears as pure resistance, with the value very close to R. If R = 0, which menas, the resistor is a short circuit, then circuit is called as series LC circuit, and impedance at resonance will be very low. The circuit will offer practically no opposition to flow of alternating current at frequency fo. This condition is series resonance.