11/30/2015
Radios and radiowaves
Physics 1010: Dr. Eleanor Hodby
Day 27:
Finish radio waves
Start fluorescent lighting
Radiowaves Summary
Reminders:
HW 11 due Monday (12/7)
Reading quiz
Final: Monday Dec 14 at 1.30-4pm
• History of radio waves
• Creating and receiving a radio wave (and other EM waves)
- Electric charges surrounded by an electric field
- F = qE
- EM waves created by accelerating charge
- The oscillation frequency of the charge matches the frequency of the EM wave produced
• Optimizing transmission/reception of a radio wave
- Polarization
- Power
- Antenna length
• Tuning your radio: Tank circuits
• Carrying information with a radio wave
- Modulation schemes (AM and FM)
- Bandwidth
• Dangers of radio waves?
Radio frequencies and channels
Radio frequencies and channels
• Each radio station broadcasts at a particular ‘carrier’ frequency.
• AM stations 530 to 1600 kHz
• FM stations 88 to 108 MHz
• Each radio station broadcasts at a particular ‘carrier’ frequency.
• AM stations 530 to 1600 kHz
• FM stations 88 to 108 MHz
Tuning your radio
Radio frequencies and channels
What are you doing when you tune your radio?
You are getting it to selectively detect the broadcast frequency of your favorite channel.
1490 AM
Current in tank circuit
Inside the radio is a ‘resonant’ circuit often called a ‘tank circuit’
A circuit that is designed to respond wildly to radio waves at a specific frequency and ignore
others
Frequency of incoming radio
wave
Other resonant systems:
Only respond to a specific driving frequency
• Each radio station broadcasts at a particular ‘carrier’ frequency.
• AM stations 530 to 1600 kHz
• FM stations 88 to 108 MHz
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11/30/2015
Tuning your radio: Tank circuits
What are you doing when you tune your radio?
You are getting it to selectively detect the broadcast frequency of your favorite channel.
Inside the radio is a ‘resonant’ circuit often called a ‘tank circuit’
A circuit that is designed to respond wildly to radio waves at a specific frequency and ignore
others
receiving
antenna
• No tank circuit,
• Electrons go through once and gone.
• Same small response for all
frequencies.
I
R
• The weak incoming radio wave gently pushes on electrons in the antenna and attached
resonant circuit at a regular frequency fw
• If fw matches the resonant frequency of the tank circuit (f0), a large oscillating current builds up.
• This current produces a LARGE oscillating voltage across the capacitor
• If fw  f0, then no current builds up.
• The tank circuit selectively strengthens signals at a specific frequency, f0, that are fed to radio electronics
• The tuning knob changes the electronic components which determine f0
I
Inductor
Current in tank circuit
Tuning your radio
Capacitor
V
V
radio electronics,
converts V to sound
radio electronics,
converts V to sound
.
Big oscillating current
f0
Frequency of incoming radio
wave
Same approach used in reverse on
broadcast to get big
current in antenna at particular
frequency.
Tuning your radio
This system of transmitting information at high frequency between resonant circuits has
2 advantages
a) Multiple radio stations can operate simultaneously at different frequencies
The receiving circuit responds to just one of the incoming frequencies
b) Random electric fields in the environment do not affect the signal because they do
not push resonantly on the tank circuit.
Other resonant systems
You find resonant systems all over physics:
Like pushing a child on a swing, or driving oscillations in this rope.
How do we send the sound of a voice over the radio?
Imagine we send out a steady radio wave at 100 MHz.
E
Distance
3m
Is this wave carrying information about the human voice, song etc?
a) Yes
b) No
c) Might be
Motor
Variable mass
• Motor pushes gently on rope at a fixed frequency fM
• Resonant frequency of rope (f0) can be ‘tuned’ by adjusting tension (f0 = (T/m)1/2/2L)
• fM = f0  big response/oscillation
• fM  f0  no response/oscillation
Carrying information on a radio wave
Signal
.
Radio wave
High frequency
1 E8 oscillations/sec
Carrying information on a radio wave
. Signals that make a radiowave plotted as a function of distance
Sound wave
Low frequency
1 E3 oscillations/second
Time
S
Sound information (should the loudspeaker move forward or backward at this instant)
can be carried as a slow modulation on the amplitude or frequency of the radio wave.
e.g. Increase in amplitude/frequency: Move speaker forward
Decrease in amplitude/frequency: Move the speaker backward.
Note: Changes in frequency are SMALL. Modulated wave is still detected by the resonant
circuit in the receiver
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Bandwidth
• To carry sound information, radio stations are not transmitting at an exact single
frequency
- They are transmitting over a narrow range of frequencies
• The frequency range of 2 different stations must not overlap, otherwise your radio will
output sounds from both at once (like when the tuner is between 2 channels).
• Each station is allocated a BANDWIDTH: a range of frequencies centered on the ‘carrier
frequency’ that they can use.
• AM bandwidth: 10 kHz
Are radio waves dangerous?
There are 2 dangers associated with EM radiation
a) Heating: The EM wave carries energy and if absorbed by your body it will be
converted into heat. Extreme heating could be harmful
b) Breaking a chemical bond in the body.
- Break a chemical bond in a gene molecule that controls cell
reproduction
- The cell goes mad and starts to reproduce uncontrollably
- You have cancer
• FM bandwidth: 200 kHz
Can radio waves break chemical bonds in your body?
To transmit a 4kHz note requires 8 kHz of bandwidth: 4kHz above and 4kHz below the
carrier frequency.
Can an AM station transmit all of the audible range of sounds/frequencies?
EM radiation can be thought of as packets of energy called photons
Each photon carries energy E = hc/l
 Shorter wavelength, more energy in photon.
 Longer wavelength, less energy in photon
a. Yes
b. No
c. Maybe
• There is a minimum photon energy needed to break a chemical bond
• UV photon or higher (X rays or gamma rays)
• Radio wave photons do not have enough energy to break a chemical bond
and cause cancer
The electromagnetic spectrum
Smart meters
Photon energy
http://emfsafetynetwork.or
g/smart-meters/smartmeter-health-complaints
Summary of dangers……not much to worry about
Fluorescent lights
• Radio waves are around us all the time and have been for nearly a century
• Cell phones have been in widespread use for 25 years.
• No clear link to any medical conditions to date at these low power levels…..
…and lots of people have been looking into it.
• The body is very good at repairing small amounts of damage safely.
 Low level damage from low power levels over a long period much better
than a sudden large dose
• However the human body, especially the brain is VERY complicated and not
fully understood.
• There might be a mechanism, yet to be understood, by which damage could
be caused…..so be sensible
- Use an earpiece if you talk for hours on a cell phone
- Don’t buy a house at the base of a radio tower etc etc
- But likely nothing to worry about.
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11/30/2015
Recall: Incandescent lights
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- -- -- --
How to do better:
- atomic discharge lamps (neon signs, yellow streetlights)
- fluorescent lamps.
Power supply provides energy to push electrons around circuit.
In the filament electrons bump into filament atoms and heat it up.
Hot electrons in the filament shake at many frequencies, producing EM
radiation at many frequencies (wavelengths).
Where does most of the electrical energy end up?
a. as visible light
b. making wires and filament hot
c. as infrared light
d. as ultraviolet light
Discharge lamp basics
Fluorescent lighting summary
I.
Large V
Atomic discharge lamps
- How atoms work
- How atoms can emit light
- Problem: single colors/wavelengths - not white.
II.
-
How fluorescent bulb produces “white” light.
+
- Phosphors
- How the eye perceives color and white light (review)
Evacuated tube with low pressure vapor of Ne, H, etc
1.
2.
3.
4.
Electrons accelerate between electrodes gaining KE
Bash into atoms, giving up KE
Internal electrons absorb and store energy: Atom becomes ‘excited’
Internal electrons give up energy as light of certain wavelength:
atom returns to ground state
Look at color of Ne, H, Na discharge lamps
Why does an atom emit light of a particular color?
Why not white light like the sun?
Nucleus
++
++
Electron
Electron
-
• Electrons in the sun are free.
• Electrons in atoms are bound to
the nucleus
Nucleus
++ --
• Electrons in atom organised
into orbitals or energy levels
• Only certain energy levels are
allowed
When electron moves to an orbital located further from the nucleus,
a. Energy of electron decreases because energy is released as positive and
negative charges are separated.
b. Energy of electron increases because it takes energy input to separate
positive and negative charges.
• Max. 2 electrons per energy level
Allowed orbital/electron energy level
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11/30/2015
Why does an atom emit light of a particular color?
Why not white light like the sun?
Example: the wavelength of red light is about 650 nm. How much
energy is in a single photon of this light?
• When the electron “jumps”
from one orbit (or energy level)
to a lower one, it releases a
certain amount of energy – the
difference in energy between
the 2 levels
a)
b)
c)
d)
650 J
3.1 X 10-19 J
4.3 X 10-31 J
1.98 X 10-25 J
Ephoton = hc/l = hf
h = Plank’s constant
= 6.626 x 10-34 J s
• The energy difference is released as a single photon (or energy packet) of light
• Photon energy directly related to wavelength or color of light
 Ephoton = hf = hc/l
• Each atom has just a few energy jumps that produce visible photons so produces
just a couple of different colored photons. Not the full spectrum of white light.
• Different types of atoms  Different orbital structures
 Different electron energy jumps
 Different energy/color photons emitted
Neon lamp emits a strong red line. Sodium emits a strong yellow line.
What accounts for this difference?
a.
b.
c.
d.
The electrons in the discharge hit the neon atoms with more speed than
the electrons hit the sodium atoms
The electrons in the discharge hit the neon atoms with less speed than
the electrons hit the sodium atoms
The 2 atoms have different energy level structures. The pair of energy
levels involved in producing the red neon light are closer together than
the pair of energy levels involved in producing the yellow sodium light.
The 2 atoms have different energy level structures. The pair of energy
levels involved in producing the red neon light are further apart than the
pair of energy levels involved in producing the yellow sodium light.
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