Radio 1
Radio 2
Observations about Radio
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Radio
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It can transmit sound long distances wirelessly
It involve antennas
It apparently involves electricity and magnetism
Its reception depends on antenna positioning
Its reception weakens with distance
There are two styles of radio: AM and FM
Turn off all electronic devices
Radio 3
Radio 4
3 Questions about Radio
1.
2.
3.
How can a radio wave exist?
How is a radio wave emitted and received?
How can a radio wave represent sound?
Question 1
Q: How can a radio wave exist?
A: Electric and magnetic fields create one another.
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Radio waves are electromagnetic
g
waves:
structures made only of electric and magnetic fields
they are emitted or received by charge or pole
 they are selfself-sustaining while traveling, even in vacuum
 their electric and magnetic fields recreate one another
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Radio 5
Radio 6
Electromagnetism (Version 3)
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Magnetic fields are produced by
magnetic poles and subatomic particles,
 moving electric charges,
 and
nd changing
h n in electric
l tri fi
fields
fields.
ld .
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Structure of a Radio Wave
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Electric fields are produced by
electric charges and subatomic particles,
moving magnetic poles,
 and changing magnetic fields.
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Electric field is perpendicular to magnetic field
Changing electric field
creates magnetic field
Ch i magnetic
Changing
i field
fi ld
creates electric field
Polarization of the wave
is associated with the
wave’s electric field
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Radio 7
Radio 8
Question 2
Q: How is a radio wave emitted and received?
A: Accelerating charge ↔ electromagnetic wave
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A Tank Circuit
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Acceleratingg charge
g causes electromagnetic
g
wave
It makes an electric field that changes with time
It makes a magnetic field that changes with time
 and the two fields can form an electromagnetic wave
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Electromagnetic wave causes accelerating charge
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Its electric field pushes on the charge
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magnetic field in its inductor
electric field in its capacitor
Tank circuit can accumulate energy
Frequency set by capacitor & inductor
Radio 10
An Antenna is a Tank Circuit

It consists of a capacitor and an inductor
Ch
Charge
cycles
l through
h
h the
h circuit
i i
Tank’s energy alternates between
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Radio 9
For bigger wave, slosh charge in a tank
Tank circuit is a harmonic oscillator
An antenna is a straightened tank circuit!
Antenna’s frequency is set by its length
Resonant when it is ½ radio wavelength long
A conducting surface can act as half the antenna
Above a conducting
surface, antenna is
resonant when it is
¼ wavelength long
Radio 11
Emitting and Receiving Waves
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Radio 12
Question 3
Q: How can a radio wave represent sound?
A: Vary the wave to send sound information.
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AM or “Amplitude
p
modulation”
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FM or “Frequency modulation”
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A transmitter uses a tank circuit to slosh charge
up and down its antenna,
which acts as a second tank.
A receiver uses a tank circuit to
detect charge sloshing on its
tank--circuit antenna.
tank
Transmitter antenna charge
affects receiver antenna charge
Antenna orientations matter!
AM Modulation
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Fluctuating amplitude conveys sound information
Fluctuating frequency conveys sound information
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Information can be encoded as a fluctuating
amplitude of the radio wave
The air pressure variations
that are so
sound
nd ca
cause
se changes
in the amount of charge
moving on the antenna and
thus the intensity of the wave
The receiver detects these
changes in radio wave intensity.
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Radio 14
Radio 13
FM Modulation
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Information can be encoded as a fluctuating
frequency of the radio wave
The air pressure variations
that are so
sound
nd ca
cause
se slight
shifts in the frequency of
charge motion on the antenna
and the frequency of the wave
The receiver detects these
changes in radio wave frequency.
Radio 15
Summary about Radio
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Accelerating charges cause electromagnetic waves
Electromagnetic waves cause accelerating charges
Those waves are only electric and magnetic fields
Accelerating charge on a transmitting antenna
 produces a radio wave
 that causes charge to accelerate on a receiving antenna
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Radio waves can represent sound information
Radio 16
Observations About Microwaves
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Microwave Ovens
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Microwave ovens cook food from inside out
They often cook foods unevenly
They don’t defrost foods well
You shouldn’t put metal inside them?!
Do they make food radioactive or toxic?
Turn off all electronic devices
Radio 17
1.
2.
3.
4.
4 Questions about
Microwave Ovens
Why do microwaves cook food?
How does metal respond to microwaves?
Why do microwave ovens tend to cook unevenly
How does the oven create its microwaves?
Radio 18
Question 1
Q: Why do microwaves cook food?
A: Water in the food responds to their electric fields.

Microwaves are a class of electromagnetic
g
waves
Long-wavelength EM waves: Radio & Microwave
LongMedium--wavelength: IR, Visible, UV light
Medium
 Short
Short--wavelength: XX-rays & GammaGamma-rays
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Microwaves have rapidly fluctuating electric fields.
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Radio 20
Radio 19
Water Molecules
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Water molecules are
unusually polar
An electric field tends to
orient water molec
molecules
les
A fluctuating electric field
causes water molecules to
fluctuate in orientation
Radio 21
Microwave Heating
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Radio 22
Question 2
Q: How does metal respond to microwaves?
A: Currents flow back and forth in the metal.
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Microwaves have alternating electric fields
Water molecules orient back and forth
Liquid water heats due to molecular “friction”
Ice doesn’t heat due to orientational stiffness
Steam doesn’t heat due to lack of “friction”
Food’s liquid water content heats the food
Non-conductors p
Nonpolarize in the microwaves
Conductors carry currents in the microwaves
Good, thick conductors reflect microwaves
 Poor or thin conductors experience resistive heating
 Sharp conductors initiate discharges in the air
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Radio 23
Introductory Question (revisited)
If you put a CD in a microwave oven, it will
A.
B.
C.
do nothing.
burn up the microwave oven.
burn up the CD.
Radio 24
Question 3
Q: Why do microwave ovens tend to cook unevenly?
A: Interference produces nonuniform electric fields.
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Interference is when the fields add or cancel
Adding fields are constructive interference
 Canceling fields are destructive interference
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Reflections lead to interference in a microwave
Most ovens “stir” the waves or move the food
Question 4
Q: How does the oven create its microwaves?
A: A magnetron tube radiates microwaves.
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Magnetrons
g
are vacuum tubes invented in WWII
Electrons travel through empty space
obtaining power from a strong electric field
bent by a strong magnetic field and the Lorentz force
 delivering power to an electromagnetic field
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4
Radio 25
Radio 26
Generating Microwaves
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Magnetron tube has tank circuits in it
Streams of electrons amplify tank oscillations
A loop of wire extracts energy from the tanks
A short ¼¼-wave antenna emits the microwaves
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Summary about
Microwave Ovens
They cook food because of its water content
Polar water molecules heat in microwave fields
Thin or sharp metals overheat or spark
The microwaves are produced by a magnetrons
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