Download Lecture 26

11/17/2015
Radios and radiowaves
Midterm 1 topics - Motion
Physics 1010: Dr. Eleanor Hodby
Reminders:
HW10 due Monday Nov 30th at 10pm.
Regular help session schedule this week
Final: Monday Dec 14 at 1.30-4pm
Day 26:
Radio waves
•
Position, velocity and acceleration
– Definitions, Units
– Scalars and vectors
– Graphs of x, v, a vs time and relationships between graphs
– Equations of motion and how to use them:
Constant velocity:
x = x0 + vt
Constant acceleration: v = v0 +at
x = x0 + v0t + ½ at2
•
Forces
– Definition, units, vector
– Fgravity = mg downwards
– Ffriction  0.3 × weight in direction opposing motion
– Fspring = -kx in direction opposing extension/compression
– Fnet = ma  If a = 0, Fnet = 0
– Free body diagrams and finding Fnet
Midterm 2 summary
• Conservation of energy
Wext - Wfriction = DPE + DKE
- Work done by a force = F × d//
- Looked at work done by external forces and by friction
- GPE = mgh, KE = ½ mv2, PPE = PV, SPE= ½ kx2, Thermal energy = constant × T
- Ramps, roller coasters, balls…….
- Power = energy/s
• Bernoulli’s equation
Etpv = P + ½ rv2 + rgh
- Conservation of energy for an incompressible fluid
• Sound
- Wave basics: f, A, T, l, v (and relationships)
- How to get different notes from a violin
- Harmonics on a violin string
Review Topics – midterm 3
Blackbody spectrum
- Introduction to EM waves and the EM spectrum
- Kelvin temperature scale
- Stefan-Boltzman law
- Shape of BB spectrum at different temperatures –
 why the sun produces visible light efficiently and incandescent lightbulbs don’t.
- Green house effect
Static electricity
- Coulomb’s law for force between point charges
- Voltage and electric potential energy (EPE)
Electric circuits
- Ohm’s Law
- Power dissipation law
- Batteries in series
- Bulbs in series and Parallel
Creating radiowaves
Radio waves so far
• Electric charges surrounded by an electric field
• F = qE
• EM waves created by accelerating charges
• The oscillation frequency of the charge matches the frequency of the EM wave produced
Radiowave Summary
• History of radio waves
• Creating and receiving a radio wave (and other EM waves)
• Electric fields and forces on charged particles
• 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?
A
60 m
The blue circles are electrons. What’s the direction of the force on the “test”
electron at A?
a.
b.
c.
d.
e.
What is the direction of the electric field at A?
a.
b.
c.
d.
e.
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Understanding radiowaves
Creating radiowaves
Now the electron on the left moves suddenly down:
several meters
-
A
60 m
c = 3×108 m/s
+
-
What’s the direction of the force on the electron at A. ?
a.
b.
c.
d.
• When the electron in the antenna is oscillating, the electric field at a distance is
also constantly changing in time.
• The value of the field (and hence force on a charged particle) outside the antenna
at any instant depends on
e. can’t
tell
e. Can’t tell unless I know when I measure the force at A.
• It takes time for the field (and hence force) at point A to know electron moved!
• This information travels the speed of light.
• It is an electromagnetic wave
a) Space - How far it is from the antenna
b) Time - At what point in its oscillation the antenna electron is at
Understanding radiowaves
several meters
-
3×108
m/s
A
c = 3×108 m/s
+
-
A
B
-
This picture shows the electric field of a radio wave at a certain instant in time.
A very short time later, the strength of the field at A will be
a. More downward,
b. The same,
c. Weaker in magnitude (down but smaller),
d. Zero,
e. Large upward
This picture shows the electric field of a radio wave at a certain instant in time.
What is the force on an electron at B at this instant?
a. Zero
b. Upwards but small
c. Upwards and large
d. Downwards and small
e. Downwards and large
Understanding radiowaves
Understanding radiowaves
several meters
-
A
several meters
-
c=
+
several meters
-
c=
+
Understanding radiowaves
3×108
m/s
B
-
This picture shows the electric field of a radio wave at a certain instant in time.
What will the force be on the electron at B a fraction of a second later?
a. Zero
b. Down
c. Up
c = 3×108 m/s
+
A
B
-
If the wavelength of the EM wave is 200m, at what frequency is the electron
oscillating in the transmitting antenna?
a. 3 × 108 Hz
b. 1.5 × 108 Hz
c. 1.5 × 106 Hz
d. 6 × 1010 Hz
e. 1.7 × 10-11 Hz
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Understanding radiowaves
several meters
-
c = 3×108 m/s
+
A
B
-
If the wavelength of the EM wave is 200m, at what frequency does the force on the
electron at B oscillate?
a. 3 × 108 Hz
b. 1.5 × 108 Hz
c. 1.5 × 106 Hz
d. 6 × 1010 Hz
e. 1.7 × 10-11 Hz
Receiving radio waves
So how do we receive a radio wave?
• What if we stick another metal spike (antenna) in the path of the wave.
• Oscillating E field in the wave will exert an oscillating force on free electrons in
the antenna.
• They will be pushed up and down the antenna en mass
• Moving electrons are an electric current!
• We can detect this oscillating current and hence detect the incoming radio wave!
• Can be used to drive a loudspeaker
Phet
broadcast
antenna
c = 3×108 m/s
Understanding radiowaves
If the frequency of the transmitting antenna is increased from 530 Hz to 1060 Hz so that
the electrons oscillate up and down transmitting antenna more times per second, the
electrons in the receiving antenna will:
a. be unaffected by the change and continue to oscillate up and down at the same
frequency as before the change
b. oscillate up and down at a higher frequency than before the change.
c. oscillate up and down in a full cycle 1060 times per second.
d. oscillate up and down at a lower frequency than before the change.
e. b and c.
What do the green arrows represent?
a. The velocity of the electrons that are at each of those points, moving due to the
electromagnetic wave.
b. The positions of evenly spaced electrons moving up and down between the two
antennae.
c. The strength and direction of the electric field that is emitted by the antenna
d. The force resulting from electrons moving off of the transmitting antenna towards the
receiving antenna following the curved path.
Understanding radiowaves
Understanding radiowaves
a
b
Where do these fields (and forces) come from again?
a) Charged particles are surrounded by an electric field
b) When an electron in the transmitting antenna is oscillating up and down its
surrounding field is constantly changing
c) It takes time for these changes to radiate outwards creating an EM wave that changes
in time and space
d) EM wave travels outwards at speed of light (c).
e) E field in EM wave exerts oscillating force on electron in receiving antenna
The speed of the wave (signal) is measured as…
a. how fast the peak moves towards antenna.
b. how fast the peak moves up and down.
c. both a and b
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Transmitting radio waves: Power
Optimizing transmission and reception
- Power
- Polarization
- Antenna length
Set up radio transmitter.
Hook a flashlight bulb between two halves of receiving antenna (wires)
A
B
The bulb will
a. light up if the signal strong enough,
b. not light up because there is no current through it,
c. not light up because the current oscillates up and down so fast.
Will the light bulb be:
Transmitter
a. Brighter at A than at B
b. Just as bright at A and B
c. Dimmer at A than at B
d. no way to tell
Transmitting radio waves: Power
r
A
B
Cell phones emit high frequency radio wave/microwaves (1000 MHz) to communicate
with the cell phone tower
When I hold my cell phone to my ear, it is 1cm away from my head.
When I put it in my pocket (and use an earpiece on a wire) it is 0.5m from my head.
By what factor do I reduce the EM power near my head when I use the earpiece?
a.
b.
c.
d.
e.
0.2
0.04
0.002
0.0004
None of the above
• Power in EM wave spread over
surface of sphere, area 4r2.
• Power per area = P0/4r2
• So signal gets weaker as
1/(distance from transmitter)2
Optimal transmitting antenna size
Antenna orientation and polarization
Consider a vertical broadcast antenna and a receiving antenna which we can orientate
either parallel to broadcast antenna (vertically) or perpendicular to broadcast antenna
(horizontally).
How does the signal strength in the 2 receiving antennae compare:
a. parallel to broadcast antenna has stronger signal than perpendicular
b. parallel to broadcast antenna has same signal strength as perpendicular
c. parallel to broadcast antenna has weaker signal than perpendicular
We find electrons oscillate up and down a transmitting antenna at a frequency of
890 kHz. What is the wavelength of the radiowave?
(Speed of light = 3 x 108 m/s)
a. 330,000 m
b. 337 m
c. 2670 m
d. 3.0 m e. 522 m
do experiment to check
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Antenna size and wavelength
Amplitude question
• Optimum antenna length for broadcast is (l/4) = 84 m.
• AM broadcast antennas are tall!
This is similar to the violin string, where the note (frequency) you
produce depends on the length of the string.
If we increase amplitude of motion in transmitter, the wave will get to the receiver
a) sooner than the small amplitude wave,
b) at the same time,
c) Later than the small amplitude wave
If we increase the amplitude of motion in the transmitter, then:
a) The electrons in the receiver move up and down with higher frequency.
b) The force that the radiowave exerts on receiving electron will increase,
c) The receiver electrons will move up and down with lower frequency,
d) a and b
e) b and c
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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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
• FM bandwidth: 200 kHz
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?
a. Yes
b. No
c. Maybe
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