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If an AC circuit consists of a source and an inductor, the current lags behind the voltage by 90°. That is, the voltage reaches its maximum value one quarter of a period If an AC circuit consists of a source and a capacitor, the current leads the voltage by 90°. That is, the current reaches its maximum value one quarter of a period before the In AC circuits that contain inductors and capacitors, it is useful to define the inductive reactance X L and the capacitive reactance X C as X L $ #L (33.10) (33.18) where # is the angular frequency of the AC source. The SI unit of reactance is The impedance Z of an RLC series AC circuit is (33.25) This expression illustrates that we cannot simply add the resistance and phase angle - between the current and voltage being (33.27) The sign of - can be positive or negative, depending on whether X L is greater or less than X C . The phase angle is zero when X L " X C . The average power delivered by the source in an RLC circuit is (33.31) An equivalent expression for the average power is (33.32) The average power delivered by the source results in increasing internal energy in the The rms current in a series RLC circuit is (33.34) A series RLC circuit is in resonance when the inductive reactance equals the capacitive resonance frequency # 0 of the circuit is (33.35) The current in a series RLC circuit reaches its maximum value when the frequency of the source equals # 0 —that is, when the “driving” frequency matches the resonance frequency. Transformers allow for easy changes in alternating voltage. Because energy (and therefore power) are conserved, we can write (33.42) to relate the currents and voltages in the primary and secondary windings of a I
1 ∆V
1 " I
2 ∆V
2 #
0 " 1 √ LC I
rms " ∆V
rms √ R
2 $ (X L * X C ) 2 ! av " I 2 rms R ! av " I
rms
∆V
rms cos - - " tan * 1
" X L * X C R # Z $ √ R
2 $ (X L * X C ) 2 X C
$
1 # C Summary 1057 |