Transcript Electromagnetic Waves: The Radio & TV

Are Electricity & Magnetism really that Different ?
What produces electric fields ?
- the presence of electric charges
What produces magnetism ?
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v
- the presence of moving electric charges (current!)
If someone runs carrying a charge rod, what would you see?
What would the runner be seeing ?
In the frame of the runner, he sees Electric Fields due to
the stationary charges.
In your frame, you see Magnetism due to the moving charges!
Electricity & Magnetism – two sides of the same coin;
intimately related
Now what happens if you accelerate charges ?
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-v
+v
• produce a mixture of changing electric and magnetic fields
• these ‘fluctuations of electric, magnetic fields are
periodic and can travel in empty space
• these fluctuations have a frequency identical to the
frequency at which you jiggle the charges.
• fluctuations are called ‘radio waves’
Can these be useful ?
What if you pass an AC Current thru a metal rod ?
AC
Current I
This can be an ‘antenna’ !
Quick Observations about Radios
• Ability to transmit sound wirelessly over long distances,
even thru vacuum!
• Involves transmitter, receiver and antennas
• Reception depends on distance, antenna orientation
• Appears to involve electricity and magnetism as
basic driving principles
• Two typical modes; AM, FM
Antennas and Tank Circuits
Antenna
Transmitter Tank
Antenna
Tank Receiver
• Electric charges sloshing up/down transmitting antenna
propagate thru space and causes identical motion of charges
on the receiver antenna.
•
What’s inside a Tank Circuit ?
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Capacitor – two parallel plates
w/ opposite charges. Stores
energy in its electric field.
Inductor – solenoid, stores
energy in its magnetic field
• Energy sloshes back and forth between the capacitor
(as electric field) and inductor (as magnetic field).
• This ‘sloshing’ occurs at a frequency characteristic of
of capacitor, inductor used. The larger they are, the more
energy then can store and the lower the frequency of
sloshing.
• Tanks are ‘resonant devices’ that build up energy
at a particular frequency.
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Tank Circuit Half-Cycle
current
+ +
--
-++
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Emitting Radio Waves
How are Radio Waves generated, transmitted and received ?
Changing Magnetic Fields
Changing current
Changing Electric Fields
Radio Waves – fluctuations in electric fields and magnetic fields,
that can travel in vacuum at the speed of light!
Antenna
Antenna
I
Transmitter Tank
I’
Tank Receiver
What does a Radio Wave Look like ?
Oscillating
charges
Direction of
propagation
Electric Field above is vertically-polarized.
• Electric, magnetic fields are mutually perpendicular
• The polarization of the electric field – depend on how the
charges are oscillating.
• Maxima, minima of E fields coincide with maxima, minima
of magnetic fields, respectively.
• Fluctuations travel in vacuum (require no elastic medium!)
at the speed of light (c=3 x 108m/s)
• Radio waves propagated at right angles to oscillation axis (never
along the axis)
Optimizing Reception of Radio Waves
Antenna
Antenna
I
TransmitterTank
I’
Tank Receiver
• For maximum reception, receiving tank circuit must be
resonant at the radio wave’s frequency.
• For resonant transfer of energy, antenna length = ¼
the wavelength of the tank’s broadcast frequency.
• Receiving antenna must be perpendicular to direction
of radio wave propagation (or parallel to polarization)
To ensure good reception, some radio stations transmit
a circularly polarized wave – a superposition of vertical,
horizontal polarizations.
Embedding Sound(info) in AM/FM Radio Waves
Amplitude Modulation (AM
• information encoded in the
fluctuating amplitude or envelope
of modulation signal
• pressure variations causes
Changes in intensity of carrier wave
Frequency Modulation (FM)
• information encoded in the
Exact frequency of the charge motion
• presure variations causes shifts in
Frequency of charge motion