Oscillations produced in a circuit that is acted upon by an external driving force, such as oscillations in a resonant circuit coupled to a fixed-frequency oscillator. The resulting oscillations have two components: a transient component, whose frequency is determined by the natural frequency of the circuit and decays rapidly, and a steady component, whose frequency equals that of the external driving force.
If a circuit is acted upon by an external voltage
applied at t = 0, then the steady-state solution is given by
where Z is the impedance and is given by
and ϕ, the phase angle, is given by
The phase differs from that of the applied voltage except when
i.e when
where ω0 is the natural frequency of the circuit. This is the resonance condition and the current is then a maximum. See also resonant frequency; free oscillations.