Alternating Current Circuits And
Electromagnetic Waves
Chapter 21
Introduction
•
Alternating current
circuits play an
important part in our lives.
–
Electrical power production and
transmission
–
Communication systems
–
Entertainment
–
Medicine
224, 229
•
We will study
–
Circuits containing a source of emf
and one or more of the following in
series:
•
Resistors
•
Capacitors
•
Inductors
•
We will study
–
Electromagnetic waves
•
Composed of fluctuating electrical and magnetic waves
–
Radio frequencies (RF)
–
Radar
–
Microwaves
–
Infrared
–
Visible light
–
Ultraviolet
–
X

rays
–
Gamma Rays
AC Generators
•
The output of an ac generator
–
ac generator symbol
–
Formula for instantaneous voltage
(
D
v)
21.1/21.2
Resistors In An ac Circuit
•
Current and voltage are
in step
with each other.
(in phase)
•
The average value
of the current
over one cycle is zero.
•
Power formula:
rms Current
•
rms current
(root mean square)
–
It is the direct current that would
dissipate the same amount of energy
in a resistor as an ac current of that
value.
Rms Voltage
•
Rms voltage
–
120 V rms in our homes
Ohm’s Law In An ac Circuit
•
Ohm’s Law
is valid for resistors in
ac circuits.
Review
•
Capacitors in a dc circuit
–
Charging
•
As the voltage across the capacitor
increases, the current decreases.
–
RC time constant
21.4
Capacitors In An ac Circuit
•
In a
capacitive
ac circuit, the
current
always
leads
the
voltage by 90
o.
21.5
Capacitive Reactance
•
Capacitive reactance (ohms)
–
Similar to resistance in a dc circuit
•
Decreases with frequency
–
Opposes a change in ac voltage
–
Formulas:
fC
2
1
X
C
Inductive Reactance
•
Inductive reactance (ohms)
–
Similar to resistance in a dc circuit
•
Increases with frequency
–
Opposes a change in ac current
–
Formulas:
21.6/21.7
fL
2
X
L
•
In an
inductive
ac circuit, the
voltage
always
leads
the current by
90
o.
E
L
I
the
I
C
E
man
21.7
E
L
I
the
I
C
E
man
•
Three
important relationships:
–
The instantaneous voltage across the
resistor is in phase with the instantaneous
current.
–
The instantaneous voltage across the
inductor leads the instantaneous current by
90
o.
–
The instantaneous voltage across the
capacitor lags the instantaneous current by
90
o.
The RLC Series Circuit
•
Impedance (Z)
–
The total resistance (ohms) in an ac circuit
–
Formula:
221
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
x
2
2
)
(
C
L
X
X
R
x
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
2
)
(
C
L
X
X
R
2
C
L
2
)
X
(X
R
Phasors
•
Phasors
–
Rotating vectors
which are used to represent the
voltage across each component.
•
Separated by 90
o
–
Phase angle
(f)
222
Phase Angle
•
Phase angle
Schematic Symbols
•
Review of circuit symbols
–
Table 21.2 (pg. 705)
–
Electronic Symbol Handout
23
Power In An ac Circuit
•
No power is lost in capacitors or
in
pure
inductors.
–
A capacitor in an ac circuit does not
dissipate energy.
–
A pure inductor in a ac circuit does
not dissipate energy.
•
Power Formulas:
NOTE:
cos
f
is called the power factor
Review
•
Mechanical Resonance
91, 95
Resonance In A Series RLC Circuit
•
Resonant frequency (f
o
)
•
The frequency at which
X
L
= X
C
•
Important in tuning circuits
•
Radios, televisions, cell phones
•
Airport metal detectors
•
Formula:
223
QUESTIONS
1

4, 6, 7
Pg. 726
Transformers
•
What is a transformer?
218
Transformer Voltage
•
A transformer
steps up
or
steps down
ac voltages
•
Primary and secondary coils
218
Transformer Power
•
The power input to the primary
equals the power output at the
secondary.
Transformer Efficiency
•
Efficiencies are between 90% and
99%.
The Transformer Equation
N
p
/N
s
is the turns ratio
The Transformer
•
Applications:
•
AC/DC electronic equipment
•
Power distribution
•
Automobile ignition systems
219, 220
Maxwell’s Theory
•
Electric field lines originate on positive charges
and terminate on negative charges.
•
Magnetic field lines always form closed loops.
•
A varying magnetic field induces an emf and
hence an electric field.
•
Magnetic fields are generated by moving
charges (or currents).
21.18
Maxwell’s Hypothesis
•
A changing electric field should
produce a magnetic field.
•
This could not be proven
experimentally because the electric
fields were too weak to detect at the
time.
Maxwell’s Prediction
•
Maxwell came up with the concept
of electromagnetic waves and
predicted that they would travel at
the speed of light.
21.20
Maxwell’s Conclusion
•
He concluded that light waves are
electromagnetic in nature.
Hertz’s Discovery
•
Hertz was the first to generate and
detect electromagnetic waves in
the laboratory.
21.17
Hertz’s Apparatus
EM Resonance
•
Hertz achieved resonance and
transferred electromagnetic energy
between two coils which were
several meters apart.
•
Resonant frequency
(f
o
)
formula
Properties EM Radiation
•
Hertz showed that electromagnetic radiation
exhibited wave properties.
•
Reflection
•
Refraction
•
Diffraction
•
Interference
•
Polarization
231, 232
The Speed of EM Waves
•
Hertz showed that electromagnetic
radiation traveled at the speed of
light.
Hertz’s Contribution
•
Hertz’s experiments provided the
first proof of Maxwell’s theories.
Production Of EM Waves By An
Antenna
•
Energy stored in an LC circuit is
constantly transferred between the
electric and magnetic fields.
•
Electromagnetic waves are generated if
this happens rapidly.
•
Antennas are used to produce electromagnetic
waves.
177, 178
EM Waves
228
Properties Of EM Waves
•
Electromagnetic waves are
transverse waves.
•
They travel at the speed of light.
•
Light is an electromagnetic wave
•
They can travel through a vacuum.
A Special Relationship
•
The ratio of the electric field
strength
(E)
to the magnetic field
strength
(B)
equals the speed of
light.
Energy and Momentum
•
Electromagnetic waves carry both
energy and momentum.
The Speed of EM Waves
•
Important formula:
Visible Light
•
Range of wavelengths for visible light:
•
400 nm
to
700 nm
69
The Spectrum Of EM Waves
•
Types of waves
(increasing in frequency and energy)
•
Radio waves
•
Microwaves
•
Infrared waves
(heat waves)
•
Visible light
•
Ultraviolet light
(UV)
•
X

rays
•
Gamma rays
21.23
Review

The Doppler Effect
•
Sound waves exhibit the
Doppler effect
•
Trains
•
Race cars
The Doppler Effect For EM Waves
•
Astronomy
•
The expanding universe
•
Red shift
•
Blue shift
•
Rotation of galaxies
The Red Shift
QUESTIONS
9

15
Pg. 726
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