ResonanceResonance in AC circuits implies a special frequency determined by the values of the resistance , capacitance , and inductance . For series resonance the condition of resonance is straightforward and it is characterized by minimum impedance and zero phase. Parallel resonance , which is more common in electronic practice, requires a more careful definition. 
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Series ResonanceThe resonance of a series RLC circuit occurs when the inductive and capacitive reactances are equal in magnitude but cancel each other because they are 180 degrees apart in phase. The sharp minimum in impedance which occurs is useful in tuning applications. The sharpness of the minimum depends on the value of R and is characterized by the "Q" of the circuit.

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Selectivity and Q of a CircuitResonant circuits are used to respond selectively to signals of a given frequency while discriminating against signals of different frequencies. If the response of the circuit is more narrowly peaked around the chosen frequency, we say that the circuit has higher "selectivity". A "quality factor" Q, as described below, is a measure of that selectivity, and we speak of a circuit having a "high Q" if it is more narrowly selective.
Since that width turns out to be Δω =R/L, the value of Q can also be expressed as 
Index AC Circuits Reference Serway & Beichner Ch 33  

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Power in a Series Resonant CircuitThe average power dissipated in a series resonant circuit can be expressed in terms of the rms voltage and current as follows: Using the forms of the inductive reactance and capacitive reactance, the term involving them can be expressed in terms of the frequency.

Index AC Circuits Reference Serway & Beichner Ch 33  

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