fo is known as the resonance frequency of the circuit. At resonance frequency in an electrical system, the amplitude of the current becomes maximum.
As the frequency is changed -- either increased or decreased -- from the resonance frequency, the current decreases as shown here.
At resonance, with the (XL - XC) term in this equation,
I = V / Z =
equal to zero, the current is determined solely by the resistance
Ires = V / R
so the current at resonance will be greater in a circuit with small resistance and will be less in a circuit with large resistance.
Since the power used in a circuit is given by
P = I2 R
the power also depends very strongly on the frequency. A graph of power versus frequency looks similar to the current versus frequency graph of the figure above.
When you tune in a radio station you are probably rotating a variable capacitor like we have seen in class.
As the capacitance C of the tuning circuit is changed, the resonance frequency of the circuit
is changed. The antenna of the receiver acts as an AC source with all sorts of frequencies superimposed at the same time. The voltage -- called a signal -- that it provides at this resonance frequency fo will provide a relatively large current which provides the program you listen to. The voltages -- or signals -- at other frequencies will provide very little current. The sensitivity of a tuning circuit is directly related to the "sharpness" of its resonance curve, just as shown in the figure above.
(c) Doug Davis, 2002; all rights reserved
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