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S E C T I O N 3 3 . 5 • The RLC Series Circuit 1045 (b) ∆V max φ ∆V L – ∆V C ∆V R (a) ω ∆V R I max φ ∆V L ∆V C ∆V max Active Figure 33.15 (a) Phasor diagram for the series RLC circuit shown in Figure 33.13a. The phasor !V R is in phase with the current phasor I max , the phasor !V L leads I max by 90°, and the phasor !V C lags I max by 90°. The total voltage !V max makes an angle - with I max . (b) Simplified version of the phasor diagram shown in part (a). At the Active Figures link at http://www.pse6.com, you can adjust the resistance, the inductance, and the capacitance of the circuit in Figure 33.13a. The results can be studied with the graphs in Figure 33.13b and the phasor diagram in this figure. in Figure 33.15b, we see that (33.24) Therefore, we can express the maximum current as Once again, this has the same mathematical form as Equation 27.8. The denominator of the fraction plays the role of resistance and is called the impedance Z of the circuit: (33.25) where impedance also has units of ohms. Therefore, we can write Equation 33.24 in (33.26) We can regard Equation 33.26 as the AC equivalent of Equation 27.8. Note that the By removing the common factor I max from each phasor in Figure 33.15a, we can construct the impedance triangle shown in Figure 33.16. From this phasor diagram we (33.27) Also, from Figure 33.16, we see that cos - " R/Z. When X L ' X C (which occurs at high frequencies), the phase angle is positive, signifying that the current lags behind the L . X C , the phase angle is negative, signifying that the current leads the applied voltage, and the circuit is more capacitive than inductive. L " X C , the phase angle is zero and the circuit is purely resistive. Table 33.1 gives impedance values and phase angles for various series circuits containing different combinations of elements. - " tan * 1 " X L * X C R # ∆V
max " I
max
Z Z $ √ R
2 $ (X L * X C ) 2 I
max " ∆V
max √ R
2 $ (X L * X C ) 2 ∆V
max " I
max
√ R 2 $ (X L * X C ) 2 ∆V
max " √ ∆V
2 R $ ( ∆V L * ∆V C ) 2 " √ (I
max R) 2 $ (I
max X L * I
max X
C ) 2 Maximum current in an RLC circuit Impedance Phase angle Figure 33.16 An impedance triangle for a series RLC circuit gives the relationship √ R 2 $ (X L * X C ) 2 . X L – X C Z φ R |