Download Chapter 21

Chapter 21
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Alternating Current Circuits
and Electromagnetic Waves
Alternating Current
Resistor in an AC circuit
Capacitor in an AC circuit
Inductor in an AC circuit
RLC series circuit
Resonance
Transformer
Electromagnetic waves
Properties of electromagnetic waves
Doppler effect and electromagnetic waves
AC Circuit
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What is alternating current?
What is an AC circuit?
Resistor in an AC Circuit
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What is it?
The voltage source
The concept of phase
Behavior of the
resistor
What is the power
dissipated by the
resistor?
What is rms current?
Why do we need it?
What is the average
power?
What about ohm’s
law?
Example – rms Current
1.
For the circuit show, calculate the maximum
current and voltage, rms current and
voltage and rms power.
Capacitors in an AC Circuit
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Behavior of the
capacitor
What is the Voltage
and Current
relationship?
What is capacitive
reactance?
How do I calculate
it?
How do I use ohm’s
law?
Inductors in an AC Circuit
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Behavior of the
inductor
Voltage and current
relationship
What is inductive
reactance?
How do I calculate
it?
How do I apply
ohm’s law?
Example - Inductor
1.
In a purely inductive ac circuit as
shown, Vmax=100V.
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2.
If the maximum current is 7.5A at 50 Hz,
calculate the inductance L.
At what angular frequency ω is the
maximum current 2.5 A?
The RLC Series Circuit
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What is the
current in the
circuit?
What about
voltage and
current
relationship?
Phasor Diagrams
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What is a
phasor?
diagram?
How do I use
it?
Phasor Diagram for RLC
series circuit
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What is the
maximum Voltage?
What is the phase?
What about rms
values?
Impedance of a Circuit
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What is
impedance?
How do I
calculate it?
How do I apply
ohms’ law?
Example - RLC Circuit
1.
A coil of resistance 35 Ω and inductance
20.5 H is in series with a capacitor and a
200-Vrms, 100-Hz source. The rms current
in the circuit is 4.0A.
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Calculate the capacitance in the circuit.
What is Vrms across the coil?
Summary of Circuit Elements,
Impedance and Phase Angles
Power in an AC Circuit
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How do I calculate the average
power?
What is a power factor?
Power losses in a capacitor and an
inductor
Example - RLC Circuit
1.
In a certain series RLC circuit, Irms=9.0A,
Vrms=180V, and the current leads the
voltage by 37°.
1.
2.
What is the total resistance in the circuit?
Calculate the magnitude of the reactance of the
circuit (XL-XC).
Resonance in an AC Circuit
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What is resonance in
an AC circuit?
When does is occur?
How do I calculate
resonance frequency?
Example of devices
that use resonance
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Example - Resonance in an
AC Circuit
1.
Calculate the resonant frequency of a
circuit of negligible resistnace containing
an inductance of 40mH and a capacitance
of 600pF.
Transformers
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What is a
transformer?
How does it
work?
How do I use it?
What about
power?
Example - Transformers
1.
An AC adapter for telephone-answering unit
uses a transformer to reduce the line
voltage of 120Vrms to a voltage of 9.0 V.
The rms current delivered to the answering
system is 400mA.
1.
2.
If the primary (input) coil in the transformer in
the adapter has 240 turns, how many turns are
there on the secondary (output) coil?
What is the rms power delivered to the
transformar? Assume an ideal transformer.
Nikola Tesla
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1865 – 1943
Inventor
Key figure in
development of
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AC electricity
High-voltage
transformers
Transport of electrical
power via AC
transmission lines
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Beat Edison’s idea of
DC transmission lines
Why do we use AC and not
DC
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What would you want to conserve?
Why do we have high voltage
transmission lines when our homes
only have 120V?
James Clerk Maxwell
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1831 – 1879
Electricity and
magnetism were
originally thought to be
unrelated
in 1865, James Clerk
Maxwell provided a
mathematical theory
that showed a close
relationship between all
electric and magnetic
phenomena
More of Maxwell’s
Contributions
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Electromagnetic theory of light
Kinetic theory of gases
Nature of Saturn’s rings
Color vision
Electromagnetic field interpretation
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Led to Maxwell’s Equations
Maxwell’s Predictions
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Hertz’s Confirmation of
Maxwell’s Predictions
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1857 – 1894
First to generate and
detect
electromagnetic
waves in a
laboratory setting
Showed radio waves
could be reflected,
refracted and
diffracted
The unit Hz is named
for him
Hertz’s Basic Apparatus
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How does it work?
What about resonance
frequency?
What is the source for
energy transfer?
What are
electromagnetic waves?
How do I measure the
speed of the waves?
Electromagnetic Waves
Produced by an Antenna
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Acceleration of charge
Antenna
Charges and Fields,
Summary
Electromagnetic Waves,
Summary
Electromagnetic Waves
are Transverse Waves
Properties of EM Waves
The EM
Spectrum
Doppler Effect and EM Waves
The light of a
moving source is
blue/red shifted by
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Dl/l0 = vr/c
l0 = actual
wavelength
emitted by the
source
Blue Shift (to higher
frequencies)
vr
Red Shift (to lower
frequencies)
Dl = Wavelength
change due to
Doppler effect
vr = radial
velocity
Example – EM Waves
1.
What are the wavelength ranges
in
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2.
AM radio band (540-1600 kHz) and
The FM radio band (88-108 MHz)?
Example – EM Waves
1.
A speeder tries to explain to the
police that the yellow warning
lights on the side of the road
looked green to her because of
the Doppler shift. How fast would
she have been traveling if yellow
light of wavelength of 580 nm
had been shifted to green with a
wavelength of 560 nm?