The inductance reactance increases with the increase of frequency, while the capacitance reactance increases with the increase of frequency and decreases with high frequency. Because these opposite characteristics of LC combination, there must be a frequency XL equal to XC, because one is increasing and the other is decreasing. In this case, the reactants are equal, and the opposite is called resonance. The AC circuit is a series resonance (also known as series variable frequency resonance) circuit.

In an RLC series resonant circuit, a frequency at XL=XC is called the resonant frequency. The frequency is defined as

At this frequency, the circuit is in a resonant state, and the input voltage and current are in a resonant state phase. At resonance, the circuit is essentially resistive, with its minimum value being the maximum value of impedance and current. This can be solved to obtain a series of RLC circuits:

As for voltage, the shape of the voltage VR on the resistor is exactly the same as the current, because its different constant r, Vs is a maximum resonance. Vc and VL are equal at resonance (fR) due to XL=XC, but note that they are not maximum at the resonance frequency. At frequencies lower than fR, VC>VL; The frequency is higher than fR, VL>VC.

Establish the following circuit