What is Alternating Current?

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Jonathan James Fri, Jan 21, 2022 04:24 PM UTC

AC circuits, Phasor diagram, Power, Inductors, Capacitors, RLC circuit, Transformers

1. Chapter 33
Alternating Current Circuits

2. Alternating Current Circuits
Electrical appliances in the house use alternating current (AC) circuits.
If an AC source applies an alternating voltage to a series circuit containing
resistor, inductor, and capacitor, what are the amplitude and time characteristics
of the alternating current.
Other devices will be discussed
 Transformers
 Power transmission
 Electrical filters
Introduction

3. AC Circuits
An AC circuit consists of a combination of circuit elements and a power source.
The power source provides an alternating voltage, v.
Notation note:
 Lower case symbols will indicate instantaneous values.
 Capital letters will indicate fixed values.
Section 33.1

4. AC Voltage
The output of an AC power source is sinusoidal and varies with time according to
the following equation:
 Δv = ΔVmax sin ωt
 Δv is the instantaneous voltage.
 ΔVmax is the maximum output voltage of the source.
 Also called the voltage amplitude
 ω is the angular frequency of the AC voltage.
Section 33.1

5. AC Voltage, cont.
The angular frequency is
2∂
ù = 2∂ ƒ =
T
 ƒ is the frequency of the source.
 T is the period of the source.
The voltage is positive during one half
of the cycle and negative during the
other half.
Section 33.1

6. AC Voltage, final
The current in any circuit driven by an AC source is an alternating current that
varies sinusoidally with time.
Commercial electric power plants in the US use a frequency of 60 Hz.
 This corresponds with an angular frequency of 377 rad/s.
Section 33.1

7. Resistors in an AC Circuit
Consider a circuit consisting of an AC
source and a resistor.
The AC source is symbolized by
ΔvR = Vmax= Vmax sin t
ΔvR is the instantaneous voltage across
the resistor.
Section 33.2

8. Resistors in an AC Circuit, cont.
The instantaneous current in the resistor is
Δv R ΔVmax
iR = = sin ù t = I max sin ù t
R R
The instantaneous voltage across the resistor is also given as
ΔvR = Imax R sin ωt
Section 33.2

9. Resistors in an AC Circuit, final
The graph shows the current through
and the voltage across the resistor.
The current and the voltage reach their
maximum values at the same time.
The current and the voltage are said to
be in phase.
For a sinusoidal applied voltage, the
current in a resistor is always in phase
with the voltage across the resistor.
The direction of the current has no
effect on the behavior of the resistor.
Resistors behave essentially the same
way in both DC and AC circuits.
Section 33.2

10. Phasor Diagram
To simplify the analysis of AC circuits, a
graphical constructor called a phasor
diagram can be used.
A phasor is a vector whose length is
proportional to the maximum value of
the variable it represents.
The vector rotates counterclockwise at
an angular speed equal to the angular
frequency associated with the variable.
The projection of the phasor onto the
vertical axis represents the
instantaneous value of the quantity it
Section 33.2

11. A Phasor is Like a Graph
An alternating voltage can be presented in different representations.
One graphical representation is using rectangular coordinates.
 The voltage is on the vertical axis.
 Time is on the horizontal axis.
The phase space in which the phasor is drawn is similar to polar coordinate graph
 The radial coordinate represents the amplitude of the voltage.
 The angular coordinate is the phase angle.
 The vertical axis coordinate of the tip of the phasor represents the
instantaneous value of the voltage.
 The horizontal coordinate does not represent anything.
Alternating currents can also be represented by phasors.
Section 33.2

12. rms Current and Voltage
The average current in one cycle is zero.
Resistors experience a temperature increase which depends on the magnitude of
the current, but not the direction of the current.
The power is related to the square of the current.
The rms current is the average of importance in an AC circuit.
 rms stands for root mean square
Imax
Irms = = 0.707 Imax
2
Alternating voltages can also be discussed in terms of rms values.
ΔVmax
ΔVrm s = = 0.707 ΔVmax
2
Section 33.2

13. The rate at which electrical energy is delivered to a resistor in the circuit is given
 P=i2R
 i is the instantaneous current.
 The heating effect produced by an AC current with a maximum value of Imax is not
the same as that of a DC current of the same value.
 The maximum current occurs for a small amount of time.
 The average power delivered to a resistor that carries an alternating current
is
2
Pav = Irms R
Section 33.2

14. Notes About rms Values
rms values are used when discussing alternating currents and voltages because
 AC ammeters and voltmeters are designed to read rms values.
 Many of the equations that will be used have the same form as their DC
counterparts.
Section 33.2

15. Inductors in an AC Circuit
Kirchhoff’s loop rule can be applied and
Δv + Δv L = 0 , or
di
Δv −L = 0
dt
di
Δv = L = ΔVmax sin ù t
dt
Section 33.3

16. Current in an Inductor
The equation obtained from Kirchhoff's loop rule can be solved for the current
ΔVmax ΔVmax
iL = sin ù t dt = − cos ù t
L ∫ ùL
ΔVmax ⎛ ∂ ⎞ ΔVmax
iL = sin ⎜ù t − ⎟ I max =
ùL ⎝ 2⎠ ùL
This shows that the instantaneous current iL in the inductor and the instantaneous
voltage ΔvL across the inductor are out of phase by (/2) rad = 90o.
Section 33.3

17. Phase Relationship of Inductors in an AC Circuit
The current is a maximum when the
voltage across the inductor is zero.
 The current is momentarily not
changing
For a sinusoidal applied voltage, the
current in an inductor always lags
behind the voltage across the inductor
by 90° (π/2).
Section 33.3

18. Phasor Diagram for an Inductor
The phasors are at 90o with respect to
each other.
This represents the phase difference
between the current and voltage.
Specifically, the current lags behind the
voltage by 90o.
Section 33.3

19. Inductive Reactance
The factor ωL has the same units as resistance and is related to current and
voltage in the same way as resistance.
Because ωL depends on the frequency, it reacts differently, in terms of offering
resistance to current, for different frequencies.
The factor is the inductive reactance and is given by:
 XL = ωL
Section 33.3

20. Inductive Reactance, cont.
Current can be expressed in terms of the inductive reactance:
ΔVmax ΔVrms
Imax = or Irms =
XL XL
As the frequency increases, the inductive reactance increases
 This is consistent with Faraday’s Law:
 The larger the rate of change of the current in the inductor, the larger the back emf,
giving an increase in the reactance and a decrease in the current.
Section 33.3

21. Voltage Across the Inductor
The instantaneous voltage across the inductor is
di
Δv L = −L
dt
= −ΔVmax sin ù t
= −Imax X L sin ù t
Section 33.3

22. Capacitors in an AC Circuit
The circuit contains a capacitor and an
AC source.
Kirchhoff’s loop rule gives:
Δv + Δvc = 0 and so
Δv = ΔvC = ΔVmax sin ωt
 Δvc is the instantaneous voltage
across the capacitor.
Section 33.4

23. Capacitors in an AC Circuit, cont.
The charge is q = CΔVmax sin ωt
The instantaneous current is given by
dq
iC = = ù C ΔVmax cos ù t
dt
⎛ ∂ ⎞
or iC = ù C ΔVmax sin ⎜ù t + ⎟
⎝ 2⎠
The current is /2 rad = 90o out of phase with the voltage
Section 33.4

24. More About Capacitors in an AC Circuit
The current reaches its maximum value
one quarter of a cycle sooner than the
voltage reaches its maximum value.
The current leads the voltage by 90o.
Section 33.4

25. Phasor Diagram for Capacitor
The phasor diagram shows that for a
sinusoidally applied voltage, the current
always leads the voltage across a
capacitor by 90o.
Section 33.4

26. Capacitive Reactance
The maximum current in the circuit occurs at cos ωt = 1 which gives
ΔVmax
Imax = ù C ΔVmax =
(1 / ù C )
The impeding effect of a capacitor on the current in an AC circuit is called the
capacitive reactance and is given by
1 ΔVmax
XC ≡ which gives Imax =
ùC XC
Section 33.4

27. Voltage Across a Capacitor
The instantaneous voltage across the capacitor can be written as ΔvC = ΔVmax sin
ωt = Imax XC sin ωt.
As the frequency of the voltage source increases, the capacitive reactance
decreases and the maximum current increases.
As the frequency approaches zero, XC approaches infinity and the current
approaches zero.
 This would act like a DC voltage and the capacitor would act as an open
circuit.
Section 33.4

28. The RLC Series Circuit
The resistor, inductor, and capacitor
can be combined in a circuit.
The current and the voltage in the
circuit vary sinusoidally with time.
Section 33.5

29. The RLC Series Circuit, cont.
The instantaneous voltage would be given by Δv = ΔVmax sin ωt.
The instantaneous current would be given by i = Imax sin (ωt - φ).
 φ is the phase angle between the current and the applied voltage.
Since the elements are in series, the current at all points in the circuit has the
same amplitude and phase.
Section 33.5

30. i and v Phase Relationships – Graphical View
The instantaneous voltage across the
resistor is in phase with the current.
The instantaneous voltage across the
inductor leads the current by 90°.
The instantaneous voltage across the
capacitor lags the current by 90°.
Section 33.5

31. i and v Phase Relationships – Equations
The instantaneous voltage across each of the three circuit elements can be
expressed as
Δv R = Imax R sin ù t = ΔVR sin ù t
⎛ ∂ ⎞
Δv L = Imax X L sin ⎜ù t + ⎟ = ΔVL cos ù t
⎝ 2⎠
⎛ ∂ ⎞
Δv C = Imax X C sin ⎜ù t − ⎟ = −ΔVC cos ù t
⎝ 2⎠
Section 33.5

32. More About Voltage in RLC Circuits
ΔVR is the maximum voltage across the resistor and ΔVR = ImaxR.
ΔVL is the maximum voltage across the inductor and ΔVL = ImaxXL.
ΔVC is the maximum voltage across the capacitor and ΔVC = ImaxXC.
The sum of these voltages must equal the voltage from the AC source.
Because of the different phase relationships with the current, they cannot be
added directly.
Section 33.5

33. Phasor Diagrams
To account for the different phases of the voltage drops, vector techniques are
Remember the phasors are rotating vectors
The phasors for the individual elements are shown.
Section 33.5

34. Resulting Phasor Diagram
The individual phasor diagrams can be
Here a single phasor Imax is used to
represent the current in each element.
 In series, the current is the same in
each element.
Section 33.5

35. Vector Addition of the Phasor Diagram
Vector addition is used to combine the
voltage phasors.
ΔVL and ΔVC are in opposite directions,
so they can be combined.
Their resultant is perpendicular to ΔVR.
The resultant of all the individual
voltages across the individual elements
is Δvmax.
 This resultant makes an angle of φ
with the current phasor Imax.
Section 33.5

36. Total Voltage in RLC Circuits
From the vector diagram, ΔVmax can be calculated
2
(
ΔVmax = ΔVR2 + ΔVL − ΔVC )
2
= ( Imax R )2 + (Imax X L − Imax X C )
2
ΔVmax = Imax R 2 + ( X L − XC )
Section 33.5

37. The current in an RLC circuit is
ΔVmax ΔVmax
Imax = =
R 2 + ( X L − XC )
2 Z
Z is called the impedance of the circuit and it plays the role of resistance in the
circuit, where
2
Z ≡ R 2 + (X L − X C )
 Impedance has units of ohms
Section 33.5

38. Phase Angle
The right triangle in the phasor diagram can be used to find the phase angle, φ.
⎛ X −X C ⎞
ö = tan−1 ⎜ L ⎟
⎝ R ⎠
The phase angle can be positive or negative and determines the nature of the
Section 33.5

39. Determining the Nature of the Circuit
If  is positive
 XL> XC (which occurs at high frequencies)
 The current lags the applied voltage.
 The circuit is more inductive than capacitive.
If  is negative
 XL< XC (which occurs at low frequencies)
 The current leads the applied voltage.
 The circuit is more capacitive than inductive.
If  is zero
 XL= XC
 The circuit is purely resistive.
Section 33.5

40. Power in an AC Circuit
The average power delivered by the AC source is converted to internal energy in
the resistor.
 Pavg = ½ Imax ΔVmax cos  = IrmsΔVrms cos 
 cos  is called the power factor of the circuit
We can also find the average power in terms of R.
 Pavg = I2rmsR
When the load is purely resistive, and cos  = 1
 Pavg = Irms ΔVrms
Section 33.6

41. Power in an AC Circuit, cont.
The average power delivered by the source is converted to internal energy in the
No power losses are associated with pure capacitors and pure inductors in an AC
 In a capacitor, during one-half of a cycle, energy is stored and during the
other half the energy is returned to the circuit and no power losses occur in
the capacitor.
 In an inductor, the source does work against the back emf of the inductor
and energy is stored in the inductor, but when the current begins to decrease
in the circuit, the energy is returned to the circuit.
The power delivered by an AC circuit depends on the phase.
Some applications include using capacitors to shift the phase to heavy motors or
other inductive loads so that excessively high voltages are not needed.
Section 33.6

42. Resonance in an AC Circuit
Resonance occurs at the frequency ωo where the current has its maximum value.
 To achieve maximum current, the impedance must have a minimum value.
 This occurs when XL = XC
 Solving for the frequency gives
ùo = 1
LC
The resonance frequency also corresponds to the natural frequency of oscillation
of an LC circuit.
The rms current has a maximum value when the frequency of the applied voltage
matches the natural oscillator frequency.
At the resonance frequency, the current is in phase with the applied voltage.
Section 33.7

43. Resonance, cont.
Resonance occurs at the same
frequency regardless of the value of R.
As R decreases, the curve becomes
narrower and taller.
Theoretically, if R = 0 the current would
be infinite at resonance.
 Real circuits always have some
resistance.
Section 33.7

44. Power as a Function of Frequency
Power can be expressed as a
function of frequency in an RLC
2
Pav =
(ΔVrms ) Rù 2
2
2 2 2
R ù +L ù −ù ( 2 2
o )
This shows that at resonance,
the average power is a
Section 33.7

45. Quality Factor
The sharpness of the resonance curve is usually described by a dimensionless
parameter known as the quality factor, Q.
Q = ωo / Δω = (ωoL) / R
 Δω is the width of the curve, measured between the two values of ω for
which Pavg has half its maximum value.
 These points are called the half-power points.
A high-Q circuit responds only to a narrow range of frequencies.
 Narrow peak
A low-Q circuit can detect a much broader range of frequencies.
A radio’s receiving circuit is an important application of a resonant circuit.
Section 33.7

46. An AC transformer consists of two
coils of wire wound around a core of
The side connected to the input AC
voltage source is called the primary and
has N1 turns.
The other side, called the secondary, is
connected to a resistor and has N2
The core is used to increase the
magnetic flux and to provide a medium
for the flux to pass from one coil to the
Section 33.8

47. Transformers, cont.
Eddy-current losses are minimized by using a laminated core.
Assume an ideal transformer
One in which the energy losses in the windings and the core are zero.
 Typical transformers have power efficiencies of 90% to 99%.
In the primary,
d ΦB
Δv1 = −N1
dt
The rate of change of the flux is the same for both coils.
The voltage across the secondary is
d ΦB
Δv 2 = −N2
dt
Section 33.8

48. Transformers – Step-up and Step-down
The voltages are related by
N2
Δv 2 = Δv1
N1
When N2 > N1, the transformer is referred to as a step-up transformer.
When N2 < N1, the transformer is referred to as a step-down transformer.
The power input into the primary equals the power output at the secondary.
 I1ΔV1 = I2ΔV2
The equivalent resistance of the load resistance when viewed from the primary is
2
⎛ N1 ⎞
Req = ⎜ ⎟ RL
⎝ N2 ⎠
Section 33.8

49. Transformers, final
A transformer may be used to match resistances between the primary circuit and
the load.
This way, maximum power transfer can be achieved between a given power
source and the load resistance.
 In stereo terminology, this technique is called impedance matching.
Section 33.8

50. Nikola Tesla
1856 – 1943
American physicist / inventor
Key figure in development of
 Alternating-current electricity
 High-voltage transformers
 Transport of electric power using
AC transmission lines
Section 33.8

51. The process of converting alternating current to direct current is called
A rectifier is the converting device.
The most important element in a rectifier circuit is the diode.
 A diode is a circuit element that conducts current in one direction but not the
other.
Section 33.9

52. Rectifier Circuit
The arrow on the diode ( ) indicates the direction of the current in the diode.
 The diode has low resistance to current flow in this direction.
 It has high resistance to current flow in the opposite direction.
Because of the diode, the alternating current in the load resistor is reduced to the
positive portion of the cycle.
The transformer reduces the 120 V AC to the voltage needed by the device.
 Typically 6 V or 9 V
Section 33.9

53. Half-Wave Rectifier
The solid line in the graph is the result
through the resistor.
It is called a half-wave rectifier because
current is present in the circuit during
only half of each cycle.
Section 33.9

54. Half-Wave Rectifier, Modification
A capacitor can be added to the circuit.
The circuit is now a simple DC power supply.
The time variation in the circuit is close to zero.
 It is determined by the RC time constant of the circuit.
 This is represented by the dotted lines in the graph shown in fig. 33.21 b.
Section 33.9

55. Filter Circuit, Example
A filter circuit is one used to smooth out or eliminate a time-varying signa.l
After rectification, a signal may still contain a small AC component.
 This component is often called a ripple.
By filtering, the ripple can be reduced.
Filters can also be built to respond differently to different frequencies.
Section 33.9

56. High-Pass Filter
The circuit shown is one example of a
high-pass filter.
A high-pass filter is designed to
preferentially pass signals of higher
frequency and block lower frequency
Section 33.9

57. High-Pass Filter, cont
At low frequencies, ΔVout is much
smaller than Δvin.
 At low frequencies, the capacitor
has high reactance and much of
the applied voltage appears across
the capacitor.
At high frequencies, the two voltages
are equal.
 At high frequencies, the capacitive
reactance is small and the voltage
appears across the resistor.
Section 33.9

58. Low-Pass Filter
At low frequencies, the reactance and voltage across the capacitor are high.
As the frequency increases, the reactance and voltage decrease.
This is an example of a low-pass filter.
Section 33.9

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