Alternating Current


The current in a series RCL circuit is given by
I = V / Z =

Notice that this has a maximum value for


since the (XL - XC) term in the denominator vanishes for XL = XC. From earlier equations for XL and XC, this means

2 f L =

Consider the capacitance and inductance to be fixed for a given circuit while the frequency can be varied. The current will be a maximum for the frequency that fits this equation; solving for the frequency and labeling it fo, we have

fo =

fo is known as the resonance frequency of the circuit. At resonance frequency in an electrical system, the amplitude of the current becomes maximum. This corresponds to resonance in mechanical systems in which the amplitude of vibration becomes maximum at the resonance frequency.


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(c) Doug Davis, 2002; all rights reserved