OpenStax Staging4 module: m10416
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RLC Series Circuits with AC
∗
Words Numbers
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Creative Commons Attribution License 4.0†
Abstract
By the end of the section, you will be able to:
• Describe how the current varies in a resistor, a capacitor, and an inductor while in series with an
ac power source
• Use phasors to understand the phase angle of a resistor, capacitor, and inductor ac circuit and to
understand what that phase angle means
• Calculate the impedance of a circuit
The ac circuit shown in Figure 1, called an
RLC
series circuit,
is a series combination of a resistor,
capacitor, and inductor connected across an ac source. It produces an emf of
v (t) = V0 sin ωt.
Figure 1: (a) An RLC series circuit. (b) A comparison of the generator output voltage and the current.
The value of the phase dierence ϕ depends on the values of R, C, and L.
∗ Version
1.1: Aug 16, 2016 10:10 am -0500
† http://creativecommons.org/licenses/by/4.0/
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Since the elements are in series, the same current ows through each element at all points in time.
The relative phase between the current and the emf is not obvious when all three elements are present.
Consequently, we represent the current by the general expression
i (t) = I0 sin (ωt − ϕ) ,
where
I0
is the current amplitude and
ϕ
is the
(2)
phase angle between the current and the applied voltage.
The phase angle is thus the amount by which the voltage and current are out of phase with each other in a
I0 and ϕ.
i (t) , vR (t) , vC (t) , and vL (t) is helpful for analyzing the circuit. As shown
in Figure 2, the phasor representing vR (t) points in the same direction as the phasor for i (t) ; its amplitude
is VR = I0 R. The vC (t) phasor lags the i (t ) phasor by π /2 rad and has the amplitude VC = I0 XC . The
phasor for vL (t) leads the i (t ) phasor by π /2 rad and has the amplitude VL = I0 XL .
circuit. Our task is to nd
A phasor diagram involving
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Figure 2: The phasor diagram for the RLC series circuit of Figure 1.
At any instant, the voltage across the
RLC combination is vR (t) + vL (t) + vC (t) = v (t) , the emf of the
source. Since a component of a sum of vectors is the sum of the components of the individual vectorsfor
example,
(A + B)y = Ay + By
the projection of the vector sum of phasors onto the vertical axis is the sum
of the vertical projections of the individual phasors. Hence, if we add vectorially the phasors representing
vR (t) , vL (t) ,
and
vC (t)
and then nd the projection of the resultant onto the vertical axis, we obtain
vR (t) + vL (t) + vC (t) = v (t) = V0 sin ωt.
The vector sum of the phasors is shown in Figure 3.
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The resultant phasor has an amplitude
(3)
V0
and is
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directed at an angle
ϕ
4
with respect to the
onto the vertical axis is
v (t) = V0 sin ωt.
vR (t) ,
i (t), phasor.
or
The projection of this resultant phasor
We can easily determine the unknown quantities
I0
and
ϕ
from the
geometry of the phasor diagram. For the phase angle,
ϕ = tan−1
and after cancellation of
I0 ,
VL − VC
I0 X L − I0 XC
= tan−1
,
VR
I0 R
(4)
this becomes
note:
ϕ = tan−1
XL − XC
.
R
(5)
Furthermore, from the Pythagorean theorem,
q
V0 =
VR
2
q
q
2
2
2
+ (VL − VC ) = (I0 R) + (I0 XL − I0 XC ) = I0 R2 + (XL − XC ) .
2
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Figure 3: The resultant of the phasors for vL (t), vC (t), and vR (t) is equal to the phasor for v (t) =
V0 sin ωt. The i (t) phasor (not shown) is aligned with the vR (t) phasor.
The current amplitude is therefore the ac version of Ohm's law:
note:
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I0 = q
V0
R2 + (XL − XC )
=
2
V0
,
Z
(7)
where
note:
q
Z=
is known as the
impedance
2
R2 + (XL − XC )
(8)
of the circuit. Its unit is the ohm, and it is the ac analog to resistance in a
dc circuit, which measures the combined eect of resistance, capacitive reactance, and inductive reactance
(Figure 4).
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Figure 4:
Power capacitors are used to balance the impedance of the eective inductance in
transmission lines.
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The
RLC
8
circuit is analogous to the wheel of a car driven over a corrugated road (Figure 5).
The
regularly spaced bumps in the road drive the wheel up and down; in the same way, a voltage source increases
and decreases. The shock absorber acts like the resistance of the
amplitude of the oscillation.
RLC
circuit, damping and limiting the
Energy within the wheel system goes back and forth between kinetic and
potential energy stored in the car spring, analogous to the shift between a maximum current, with energy
stored in an inductor, and no current, with energy stored in the electric eld of a capacitor. The amplitude
of the wheel's motion is at a maximum if the bumps in the road are hit at the resonant frequency, which we
describe in more detail in Resonance in an AC Circuit.
Figure 5: On a car, the shock absorber damps motion and dissipates energy. This is much like the
resistance in an RLC circuit. The mass and spring determine the resonant frequency.
note:
To analyze an ac circuit containing resistors, capacitors, and inductors, it is helpful to think
of each device's reactance and nd the equivalent reactance using the rules we used for equivalent
resistance in the past. Phasors are a great method to determine whether the emf of the circuit has
positive or negative phase (namely, leads or lags other values). A mnemonic device of ELI the ICE
man is sometimes used to remember that the emf (E) leads the current (I) in an inductor (L) and
the current (I) leads the emf (E) in a capacitor (C).
Use the following steps to determine the emf of the circuit by phasors:
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1.Draw the phasors for voltage across each device: resistor, capacitor, and inductor, including
the phase angle in the circuit.
2.If there is both a capacitor and an inductor, nd the net voltage from these two phasors, since
they are antiparallel.
3.Find the equivalent phasor from the phasor in step 2 and the resistor's phasor using trigonometry or components of the phasors. The equivalent phasor found is the emf of the circuit.
Example 1
An RLC Series Circuit
The output of an ac generator connected to an
Hz and an amplitude of 0.100 V. If
RLC
series combination has a frequency of 200
R = 4.00Ω,L = 3.00 × 10−3 H,
and
C = 8.00 × 10−4 F,
what
are (a) the capacitive reactance, (b) the inductive reactance, (c) the impedance, (d) the current
amplitude, and (e) the phase dierence between the current and the emf of the generator?
Strategy
The reactances and impedance in (a)(c) are found by substitutions into , , and (8), respectively.
The current amplitude is calculated from the peak voltage and the impedance. The phase dierence
between the current and the emf is calculated by the inverse tangent of the dierence between the
reactances divided by the resistance.
Solution
a. From , the capacitive reactance is
XC =
1
1
= 0.995Ω.
=
ωC
2π (200 Hz) 8.00 × 10−4 F
(9)
b. From , the inductive reactance is
XL = ωL = 2π (200 Hz) 3.00 × 10−3 H = 3.77Ω.
c. Substituting the values of
(10)
R, XC , and XL into (8), we obtain for the impedance
Z=
q
2
2
(4.00Ω) + (3.77Ω − 0.995Ω) = 4.87Ω.
(11)
d. The current amplitude is
I0 =
V0
0.100 V
=
= 2.05 × 10−2 A.
Z
4.87Ω
(12)
e. From (5), the phase dierence between the current and the emf is
ϕ = tan−1
XL − XC
2.77Ω
= tan−1
= 0.607 rad.
R
4.00Ω
Signicance
The phase angle is positive because the reactance of the inductor is larger than the reactance of
the capacitor.
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Exercise 1
(Solution on p. 12.)
Check Your Understanding Find the voltages across the resistor, the capacitor, and
note:
the inductor in the circuit of Figure 1 using
v (t) = V0 sin ωt
as the output of the ac
generator.
1 Summary
ˆ
ˆ
ˆ
An
RLC series circuit is a resistor, capacitor, and inductor series combination across an ac source.
RLC series circuit at all points in time.
The same current ows through each element of an
The counterpart of resistance in a dc circuit is impedance, which measures the combined eect of
resistors, capacitors, and inductors. The maximum current is dened by the ac version of Ohm's law.
ˆ
Impedance has units of ohms and is found using the resistance, the capacitive reactance, and the
inductive reactance.
2 Conceptual Questions
Exercise 2
In an
(Solution on p. 12.)
RLC series circuit, can the voltage measured across the capacitor be greater than the voltage
of the source? Answer the same question for the voltage across the inductor.
3 Problems
Exercise 3
(Solution on p. 12.)
What is the impedance of a series combination of a
50-Ω
resistor, a
5.0-µF
capacitor, and a
10-µF
capacitor at a frequency of 2.0 kHz?
Exercise 4
(Solution on p. 12.)
A resistor and capacitor are connected in series across an ac generator. The emf of the generator is
given by
v (t) = V0 cos ωt, where V0 = 120 V, ω = 120π rad/s, R = 400Ω, and C = 4.0µF.
(a) What
is the impedance of the circuit? (b) What is the amplitude of the current through the resistor?
(c) Write an expression for the current through the resistor. (d) Write expressions representing the
voltages across the resistor and across the capacitor.
Exercise 5
(Solution on p. 12.)
A resistor and inductor are connected in series across an ac generator. The emf of the generator
is given by
v (t) = V0 cos ωt,
where
V0 = 120 V
(a) What is the impedance of the circuit?
and
ω = 120π rad/s;
also,
R = 400Ω
and
L = 1.5 H.
(b) What is the amplitude of the current through
the resistor? (c) Write an expression for the current through the resistor. (d) Write expressions
representing the voltages across the resistor and across the inductor.
Exercise 6
In an
(Solution on p. 12.)
RLC series circuit, the voltage amplitude and frequency of the source are 100 V and 500 Hz,
respectively, an
R = 500Ω,L = 0.20 H,
and
C = 2.0µF.
(a) What is the impedance of the circuit?
(b) What is the amplitude of the current from the source? (c) If the emf of the source is given by
v (t) = (100 V)
sin
1000πt,
C changed to 0.20µF.
how does the current vary with time? (d) Repeat the calculations with
Exercise 7
An
(Solution on p. 12.)
RLC series circuit with R = 600Ω, L = 30 mH, and C = 0.050µF is driven by an ac source whose
frequency and voltage amplitude are 500 Hz and 50 V, respectively. (a) What is the impedance of
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the circuit? (b) What is the amplitude of the current in the circuit? (c) What is the phase angle
between the emf of the source and the current?
Exercise 8
(Solution on p. 12.)
For the circuit shown below, what are (a) the total impedance and (b) the phase angle between
the current and the emf ? (c) Write an expression for
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i (t) .
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Solutions to Exercises in this Module
Solution to Exercise (p. 10)
vR = (V0 R/Z)
sin
vL = (V0 XL /Z)
(ωt − ϕ) ; vC = (V0 XC /Z)
sin
sin
(ωt − ϕ + π /2) = − (V0 XC /Z) cos (ωt − ϕ) ;
(ωt − ϕ + π /2) = (V0 XL /Z) cos (ωt − ϕ)
Solution to Exercise (p. 10)
yes for both
Solution to Exercise (p. 10)
55 Ω
Solution to Exercise (p. 10)
a.
770 Ω;
b. 0.16 A; c.
a.
690Ω;
b. 0.3 A; c.
a.
690Ω; b.
a.
6300Ω;
a.
5.7Ω;
I = (0.16 A) cos (120πt);
Solution to Exercise (p. 10)
I = (0.3 A) cos (120πt);
Solution to Exercise (p. 10)
0.15 A; c.
I = (0.15A)
Solution to Exercise (p. 10)
b. 0.0079 A; c.
sin
d.
d.
vR = 120cos (120πt); vC = 120cos (120πt − π /2)
vR = 120cos (120πt), vL = 120cos (120πt + π /2)
(1000πt − 0.753); d. 1100Ω, 0.092 A, I = (0.092A)
sin
(1000πt + 1.09)
−84.5 ◦
Solution to Exercise (p. 11)
b.
29 ◦ ;
c.
I = (30. A) cos (120πt)
Glossary
Denition 1: impedance
ac analog to resistance in a dc circuit, which measures the combined eect of resistance, capacitive
reactance, and inductive reactance
Denition 2: phase angle
amount by which the voltage and current are out of phase with each other in a circuit
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