When there is a flow of charge q in an electric circuit containing a resistor R, capacitor C and inductor
L, there is a direct analogy with the moving parts of a mechanical oscillator comprising a mass m and a spring
with spring constant k. If a series LRC circuit is connected across a voltage source given by V = Vo cos ωt, the
flow of charge through the circuit is given by the following second order differential equation:
Lq¨+ Rq˙ +
1
C
q = V0 cos ωt.
(a) Determine the correspondence between the parameters of a driven mechanical oscillator (as discussed in
the notes) and the above driven electrical oscillator.
(b) Calculate the quality factor Q of the electrical circuit in terms of the coefficients of the above differential
equation.
(c) Show that, in the case of small or light damping, Q can be written as Q = R0/R, where R0 =
p
L/C is
the characteristic impedance of the circuit.
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