Download Greenock Academy Physics Department

Rosshall Academy Physics Department
Standard Grade Physics
Unit 1: Telecommunications
Notes
Standard Grade Physics
Telecommunications
Notes
1
Section 1: Communication Using Waves
G1.
Give an example which illustrates that the speed of sound in air is
less than the speed of light in air, e.g., thunder and lightning.
G2.
Describe a method of measuring the speed of sound in air
(using the relationship between distance, time and speed).
G3.
Carry out calculations involving the relationship between distance,
time and speed in problems on sound transmission.
G4.
State that waves are one way of transmitting signals.
G5.
Use the following terms correctly in context: wave, frequency, wavelength, speed, energy
(transfer), amplitude.
G6.
Carry out calculations involving the relationship between distance, time and speed in problems
on water waves.
G7.
Carry out calculations involving the relationship between speed, wavelength and frequency for
water and sound waves.
C8.
Explain the equivalence of f x  and d/t.
Speed
The speed of an object is a measure of the ___________ _________________ in 1 second.
Mathematically this can be written as
Distance = speed x time
d = v x t
Units: Speed is measured in metres per second (_______ or _____).
Light and sound have different speeds through air.
Consider an electrical storm. The lightning bolt and thunder clap are produced at
exactly the same time.
Which are we aware of first – the thunder or the lightning? ________________________
Which travels faster – light or sound? _______________________
The speed of light in air (technically the speed of light in a vacuum) is ___________________ ms-1.
This can be written in another, neater form: _________________________ ms-1.
The speed of sound in air can vary; it depends on factors like the air temperature and the weather
conditions. For the purpose of questions in this course the speed of sound should be taken as
_________ ms-1.
[The exact value of the speed of sound that should be used in exam questions should be taken from the
data sheet.]
Standard Grade Physics
Telecommunications
Notes
2
Speed of Sound and Light – Worked Examples
1.
A boy is riding his bike when he hears a thunder clap 7 s after seeing a
flash of lightning.
How far would he have to cycle in order to reach a tree that had been struck by the
lightning bolt?
2.
A spectator standing on the finishing line of a 100 m race hears the bang from the
starting pistol 0.29 s after seeing the flash from the gun. What is the speed of sound on
this day?
3.
If the speed of sound on a particular day is 340 ms-1 how long would it take the bang from
a firework to reach a spectator standing 600 m below the explosion.
Standard Grade Physics
Telecommunications
Notes
3
Measurement of the Speed of Sound
Explain how the apparatus shown below could be used to measure the speed of sound.
Microphones attached to a computer
Metre stick
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
Standard Grade Physics
Telecommunications
Notes
4
Waves
A wave is a method of transferring ________________ from one place to another.
Waves are used in telecommunications to transmit ____________.
A Wave Profile
_____________
____________ 
Line of Zero Disturbance
______________
___________
Crest
-
this is the _______________ point on the wave.
Trough
-
this is the _______________ point on the wave.
Amplitude
-
this is the _________ of the wave. It is measured between the line of
______________________ and either a ________ or a ____________.
The amplitude of a wave is a measure of the ___________ carried by the wave –
the larger the amplitude the ______________ the amount of ____________
carried by the wave.
Units: Amplitude is measured in _________ (___).
Wavelength
-
this is the distance between successive ____________. It is the minimum
distance required for the wave pattern to be repeated.
Units: Wavelength is measured in __________ (____).
Frequency
-
this is the ___________ of waves passing a point per ______________.
Units: Frequency is measured in _______ (____). 1 _______ = 1 wave per second.
Wave Speed
-
this is the distance travelled by a wave in 1 second.
Units: Wave speed is measured in ___________________ (ms-1 or m/s).
Standard Grade Physics
Telecommunications
Notes
5
Waves – Worked Example
If a wave travels a distance of 800 m in 20 s, what is its speed?
The Wave Equation
The wavelength of a wave is the distance covered by one wave and the frequency of a wave is the
number of waves per second. We can use both pieces of information to calculate the speed of a wave.
Wave Speed
=
v =
Frequency x Wavelength
f
The Wave Equation – Worked Example
A wave generator in a swimming pool generates 20 waves in 4 seconds.
If the speed of the waves is 2 ms-1, what is the wavelength of these waves?
Standard Grade Physics
Telecommunications
Notes
6
Section 2: Communication Using Cables
G1.
Describe a method of sending a message using code (Morse or similar).
G2.
State that coded messages or signals are sent out by a transmitter and
are picked up by a receiver.
G3.
State that the telephone is an example of long range communication
between transmitter and receiver.
G4.
State the energy changes
(a) in a microphone (sound  electrical)
(b) in a loudspeaker (electrical  sound)
G5.
State that the mouthpiece of a telephone (transmitter) contains a microphone and
the earpiece (receiver) contains an earphone (loudspeaker).
G6.
State that electrical signals can be transmitted along wires during a telephone communication.
G7.
State that an electrical signal is transmitted along a wire at a speed >> speed of sound (almost
300 000 000 ms-1).
G8.
Describe the effect on the signal pattern displayed in an oscilloscope due to a change in:
(a) loudness of sound
(b) frequency of sound.
G9.
Describe, with examples, how the following terms relate to sound: frequency and amplitude.
C16. Explain the electrical signal pattern in telephone wires in terms of loudness and frequency
changes in the sound signal.
Telecommunication
Telecommunication means communicating over large distances. Nowadays we can do this in many ways –
telephones, both mobile and fixed line, radio and television and the internet to name just a few.
It has not always been so easy to communicate over large distances. Give three examples of methods
used to send coded messages before electricity.
1.
______________________________________________________________________
2.
______________________________________________________________________
3.
______________________________________________________________________
Standard Grade Physics
Telecommunications
Notes
7
Morse Code Telegraph
The Morse code telegraph revolutionised long distance communication. Messages which previously
would have taken days to be sent by mail could now be transmitted in a matter of minutes.
Explain how you would use the following apparatus to send a message in Morse code. You should include
a circuit diagram in your description.
2 batteries, 1 bulb, a tap-key and a number of leads
Diagram:
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
The Morse code telegraph had the advantage of speed over its predecessors but it also had some
disadvantages.
Give three disadvantages of the Morse code telegraph.
1.
____________________________________________________________________
2.
____________________________________________________________________
3.
____________________________________________________________________
Standard Grade Physics
Telecommunications
Notes
8
Transmitters and Receivers
Modern communications systems require a great deal of electronic components but all have two crucial
elements – a transmitter and a receiver.
A transmitter is needed to _________ ______ signals and the receiver __________ ___ the
transmitted signals.
Telephones
A telephone is a device that can be used to communicate over large distances.
A telephone contains both a transmitter and a receiver.
We talk into the mouthpiece of the telephone handset. The mouthpiece
contains an electronic device known as a ___________________. This changes
________________ energy into ___________________ energy.
We listen to the earpiece of the handset. This contains another electronic device, known as a
__________________. This changes _________________ into _______________ energy.
Transmitter/Receiver
Electronic Device
Energy Change
Mouthpiece
Earpiece
A telephone communication is carried by _____________________ signals. These signals can be
transmitted along a wire at a speed of almost _______________________ ms -1.
It is possible to ‘look’ at the signals being transmitted through the wires using a device known as an
oscilloscope.
Before we ‘look’ at these signals we must look at two terms used to describe sound – pitch and
loudness.
Sound is transmitted as waves and the energy in a wave is measured by its ____________________.
The loudness of a sound is a measure of the energy carried by the sound and so loudness is measured
by the __________________ of the wave.
The pitch of a sound is a measure of how __________ or ____________ the sound is. In physics we
describe this as the ______________________ of the sound.
______________________ = loudness
______________________ = pitch
Standard Grade Physics
Telecommunications
Notes
9
Oscilloscope Traces
Complete the diagrams below to show how the traces would change if the sound changed as described.
Same frequency but louder.
Same loudness but higher frequency.
Standard Grade Physics
Telecommunications
Notes
10
G10.
Describe what is meant by an optical fibre.
G11.
Describe one practical example of telecommunication which
uses optical fibres.
G12.
State that electrical cables and optical fibres are used in some
telecommunication systems.
G13.
State that light can be reflected.
G14.
Describe the direction of the reflected light ray from a plane ‘mirror’.
G15.
State that signal transmission along an optical fibre takes place at very high speed.
C17.
Compare some of the properties of electrical cables and optical fibres, e.g., size, cost, weight,
signal capacity, signal quality, signal reduction per km.
C18.
State the principle of reversibility of ray paths.
C19.
Describe the principle of operation of an optical fibre transmission system.
C20.
Carry out calculations involving the relationship between distance, time and speed in problems
on light transmission.
Optical Fibres
Optical fibres are very fine threads of optically pure glass along which light signals can be
transmitted.
Modern telecommunications systems are making more and more use of optical fibres – they have many
advantages over traditional electrical transmission cables.
Property
How optical fibres compare with electrical cable
Size
Cost – raw material
Cost – technology
Mass
Signal Capacity
Signal Quality
Signal Reduction per km
Speed
Standard Grade Physics
Telecommunications
Notes
11
Optical Fibre Transmission System
Optical fibres transmit light signals but the output from the microphone in a telephone is an electrical
signal. This means that to use optical fibres in a telecommunications system the electrical signals have
to be converted into light signals – and the light signals then have to be converted back to an electrical
signal before being fed into the loudspeaker.
Transmitter
{
Sound
Fibre optic cable
___________
Encoder
_________________
___________
Receiver
{
Sound
________ source
(laser)
Decoder
_________________
Photosensor (light
detector)
Sound signals are picked up by the ___________________ which converts them into ___________
signals. The encoder and _________ then take these electrical signals and change them to
___________ signals that are transmitted through the ___________ ____________ cables.
The photosensor detects these ________ signals and the decoder converts them to _____________
signals that are fed into the __________________ which converts them into ______________
signals.
Note that in this transmission system both electrical cables and fibre optic cables are used.
Standard Grade Physics
Telecommunications
Notes
12
Reflection of Light
Reflection is a property of light.
The Law of Reflection
When light is reflected from a mirror it behaves in a predictable way.
i
r
______________ ray
______________ ray
___________
In optics all angles are measured between the ray and a line called the _____________. The normal is
drawn at __________ ____________ to the surface at the point the ray strikes the surface.
The Law of Reflection states that:
the angle of _________________ equals the angle of ______________
Reflection has no effect on the wavelength, frequency or speed of a wave.
Principle of Reversibility
Consider the above diagram and think of directing a ray along the path of the reflected ray on that
diagram – on the diagram below draw the path of this reflected ray.
i
These two diagrams show a principle known as the principle of reversibility of ray paths.
Standard Grade Physics
Telecommunications
Notes
13
Total Internal Reflection
Complete the diagram below to show what happens when light strikes the inside of a glass block at
different angles. (Coloured pencils may help you tell one ray from another.)
20
Critical Angle
50
For angles of incidence __________ the critical angle the majority of the light is transmitted
through the glass. A small quantity of light is reflected back into the glass.
For light striking at the critical angle the light is bent at 90 to the normal.
For angles of incidence ________ the critical angle the light undergoes ________
______________ __________________. During this process the law of reflection in obeyed.
Total Internal Reflection and Optical Fibres
Light is transmitted through an optical fibre using total internal reflection.
Optical Fibres – Worked Example
If the speed of light through glass is 2 x 108 ms-1 calculate how long
it would take to transmit a signal along an optic fibre cable 700 km long.
Standard Grade Physics
Telecommunications
Notes
14
Section 3: Radio and Television
G1. State that the main parts of a radio receiver are:
Aerial, tuner, decoder, amplifier, loudspeaker, electricity supply;
and identify these parts on a block diagram.
G2. Describe in a radio receiver the function of the aerial, tuner, decoder,
amplifier, loudspeaker and electricity supply.
C7. Describe the general principle of radio transmission in terms of transmitter, carrier wave, amplitude
modulation and receiver.
Radio Transmission
Each radio broadcast is transmitted from a _______________ aerial using radio waves. Radio waves
belong to a large family of waves known as the _________________ spectrum. Each member of the
electromagnetic spectrum travels through air at a speed of ____________________ ms -1.
Each radio transmitter sends out waves of a particular frequency – this is what lets you tune in to
different radio stations - each station has its own frequency.
When you listen to a radio broadcast you are not listening to radio waves – you are listening to sound
waves. The wave transmitted from the radio station is a mixture of the ____________ signal, known
as a carrier wave, and the __________ signal, known as the audio wave. The sound wave is added to
the carrier wave in a process known as _______________ modulation.
Radio (____________) Wave [________ Frequency]
Sound (__________) Wave [________ Frequency]
Complete the diagram below to show the amplitude modulated wave that results from adding the above
carrier and audio wave.
Amplitude Modulated Wave
Standard Grade Physics
Telecommunications
Notes
15
The Radio Receiver
There are five main parts to a radio receiver: aerial, tuner, decoder, amplifier (+ electricity supply)
and loudspeaker.
Complete the block diagram below to show these parts in the correct order.
__________
__________
__________
__________
__________
_____________
_____________
Each of these parts has a job to do.
Part
What it does
Aerial
Tuner
Decoder
Amplifier
Loudspeaker
The wave shown below is the output from the tuner of a radio.
Tuner
What part of the radio tuner have the following waves passed through?
Standard Grade Physics
Telecommunications
Notes
16
G3.
State that the main parts of a television receiver are:
Aerial, tuner, decoders, amplifiers, tube, loudspeaker,
electricity supply and identify these parts on a
block diagram of a television receiver.
G4.
Describe in a television receiver the function of:
aerial, tuner, decoders, amplifiers, tube, loudspeaker, electricity supply.
G5.
Describe how a picture is produced on a TV screen in terms of line build-up.
G6.
State that mixing red, green and blue lights produces all colours seen on a colour television
screen.
C8.
Describe the general principle of television transmission in terms of:
Transmitter, carrier wave , modulation and video and audio receivers.
C9.
Describe how a moving picture is seen on a television screen in terms of :
Line build-up
Image retention
Brightness variation
C10. Describe the effect of colour mixing lights (red, green and blue).
Television Transmission
Each television station has a transmitter that sends out a television wave of a particular
______________. In a similar manner to radio transmission, the process of ___________
modulation adds an audio signal to the television (carrier) wave but unlike radio transmission the TV
wave also carries a video signal (TV signals carry both pictures and sound).
The Television Receiver
Again this is similar to a radio receiver but the video signal also has to be decoded.
_________
_________
_________
_________
_________
_________
_________
_________
_________
_________
_________
_________
_________
_________
Note: Remember both amplifiers need to be connected to a __________________ ___________.
Standard Grade Physics
Telecommunications
Notes
17
Picture Formation
There are three main features in the formation of a moving picture on a TV screen:
Line build-up
Image retention
Brightness variation
1.
Line build-up
The picture tube in a television contains an electron gun that fires electrons at a screen that is coated
with a special phosphorescent paint. This paint converts the kinetic energy of the electrons into light
energy when the electrons strike the screen.
Inside of the screen is given a
phosphorescent coating that
glows when hit by electrons.
Electron beam
Electron gun
Electron Beam
Magnetic coils
Fly Back
Sets of electromagnets allow the beam to be moved around the screen. The beam sweeps out a series
of lines – 625 lines make up one picture on a British TV screen.
2.
Image Retention
Step 1 allows a single, stationary picture to be shown on a TV screen – but a TV shows a moving image!
Each second of a television transmission contains _______ pictures, each one slightly different from
the previous picture.
The human brain retains an image for approximately 1/10 th of a second. If the next image is received
before the previous one has been lost then the brain does not detect any gaps between the pictures
and a moving image is obtained.
3.
Brightness Variation
Each TV picture has areas that are bright and others that are dark. To achieve this brightness
variation the intensity of the electron beam is changed. The intensity of the beam is a measure of the
___________ of electrons arriving at the screen per ___________.
To make the screen ______________ the intensity of the beam should be increased i.e. more
electrons should strike the screen per second.
To make the screen darker the intensity of the beam should be ___________ i.e. _________
electrons should strike the screen per second.
Standard Grade Physics
Telecommunications
Notes
18
Colour Mixing
All colours can be created by mixing the primary colours. For light these primary colours are
__________, _____________ and __________.
Complete the diagram below to show what colours are made when the primary colours are mixed as
shown.
Red
Green
Blue
When two primary colours are added together the resultant colour is known as a _____________
colour.
Red + Green
=
_____________________
Red + Blue
=
_____________________
Blue + Green
=
_____________________
When all three primary colours are added together the resultant colour is ________________.
[Note: Black is not a colour but is an absence of light.]
Different colours can be created by mixing different strengths of the primary colours.
For example:
Strong red + weak blue =
________________________
Weak red + strong blue =
________________________
Colour Television
A colour television has _____________ electron guns and ________ types of paint. Each electron
gun fires electrons at one type of paint and it will glow either _________, _________ or ________.
By mixing the primary colours all other colours can be obtained as shown above.
To change the strength of the primary colours the intensity of the electron beam is altered – a high
intensity beam (lots of electrons per second) gives a _________ primary colour. A low intensity beam
(fewer electrons per second) gives a ________ primary colour.
Standard Grade Physics
Telecommunications
Notes
19
Section 4: Transmission of Radio Waves
G1.
State that mobile telephones, radio and televisions are
examples of long range communication which do not need cables
(between transmitter and receiver).
G2.
State that microwaves, television and radio signals are waves which
transfer energy.
G3.
State that microwaves, television and radio signals are transmitted at very high speed.
G4.
State that microwaves, television and radio signals are transmitted through air at
300 000 000 ms-1.
G5.
State that a radio transmitter can be identified by wavelength or frequency values.
C13. Carry out calculations involving the relationship between distance, time and speed in problems
on microwaves, television and radio waves.
C14. Carry out calculations involving the relationship between speed, wavelength and frequency for
microwaves, television and radio waves.
C15. Explain some of the differences in properties of radio bands in terms of source strength,
reflection etc.
C16. Explain in terms of diffraction how wavelength affects radio and television reception.
Transmission of Radio Waves
Waves can be used to transmit __________ from one place to another.
In the modern world it is possible to communicate over large distances with no need for cables
between the transmitter and receiver. ____________ telephones, ____________ and television are
examples of long range communication which don’t require cables.
Mobile telephones transmit signals using __________________ or radio waves, radio and television
signals are carried by _________ and __________________ waves. All of these waves travel at
very high speed - _______________________ ms-1 through air.
Radio and Microwave Transmission – Worked Examples
1.
A girl in Greenock wants to phone her grandmother who lives in New York.
If the distance between Greenock and New York is 4800 km and
the girl uses her mobile phone how long will her signal take to reach New York?
Standard Grade Physics
Telecommunications
Notes
20
GOAL!!
2.
Radio 5 Live broadcasts radio waves that have a wavelength of 909 m.
What frequency does Radio 5 Live broadcast on?
Wave Bands
Radio 5 Live broadcasts in what is known as the medium waveband. There are various different
wavebands that are used in the UK and each has a different set of properties and uses. For example
the ultra high frequency (UHF) band is used for ground to air communication and terrestrial television
channels.
Waveband
Frequency
Range
30 – 3kHz
Properties
Very Low
Frequency
(VLF)
3 – 30kHz
Can be reflected off the lower
layers of the atmosphere.
Low Frequency
(long wave)
30 – 300kHz
Can be reflected off the lower
layers of the atmosphere.
This increases the range of the
signal.
Medium
Frequency
(medium wave)
300k – 3MHZ
Can be reflected off the lower
layers of the atmosphere.
This increases the range of the
signal.
High
Frequency
(short wave)
3 – 30 MHz
Can be reflected off the lower
layers of the atmosphere.
This increases the range of the
signal.
Very High
Frequency
(VHF)
30 – 300MHz
Travel in straight lines. Require
line of sight transmission.
Extra Low
Frequency
(ELF)
Uses
Capable of passing deep into the
oceans.
Standard Grade Physics
Telecommunications
Notes
21
Diffraction
Diffraction is a property of all waves – it is the _________________ of waves round a gap or
obstacle.
The amount of diffraction depends on the _______________ of the waves - __________ are
diffracted more than ____________ waves.
Short Waves e.g. TV waves
Long Waves e.g. radio waves
Diffraction has no effect on the wavelength, frequency or speed of a wave.
TV waves have a _______________ wavelength than radio waves and therefore are diffracted
___________ than radio waves. This means that in hilly areas the TV signal in the area behind the
hills will be ____________ than radio signals in the same area.
Standard Grade Physics
Telecommunications
Notes
22
G6.
State that curved reflectors on certain aerials or receivers make
the received signal stronger.
G7.
Explain why curved reflectors on certain aerials make
the received signal stronger.
G8.
Describe an application of curved reflectors used in telecommunication
e.g. satellite TV, TV link, boosters, repeaters or satellite communication.
G9.
State that the period of a satellite orbit depends on its height above the Earth.
G10. State that a geostationary satellite stays above the same point on the Earth’s surface.
G11. Describe the principle of transmission and reception of satellite television broadcasting using
geostationary satellites and dish aerials.
C17. Explain the action of curved reflectors on certain transmitters.
Dish Aerials (Curved Reflectors)
We have all seen dish aerials on the roofs of buildings. Why do
satellite TV signals require dish aerials rather than the
conventional aerials that used to be more common?
Receiving Aerials
Dish aerials make use of curved reflectors. Curved reflectors make the received signal _________
The dish aerial brings the signal to a _______________ __________. If the receiver is placed at
this point then the received signal is at its maximum possible strength.
Standard Grade Physics
Telecommunications
Notes
23
Transmitting Aerials
It is possible to use dish aerials as transmtting aerials.
F
The transmitter is placed at the ____________ ______________. The signal emerges as a
____________ beam. This means that the beam remains more concentrated and can be transmitted
in a particular direction.
Modern telecommunication systems can use a series of dish aerials – both transmitting and receiving.
Communication satellites tend to transmit microwaves but a disadvantage of using these waves is that
they are unable to diffract. This means that the transmitting and receiving aerials must be in what is
known as line of sight. Microwaves also have a fairly short range and they require a require a series of
repeating stations placed at regular intervals along the transmission path.
Standard Grade Physics
Telecommunications
Notes
24
Satellite Communication
Satellites are objects that orbit planets. The Earth has one natural satellite,the
_____________ and thousands of man-made satellites. Man-made satellites have
made large scale global telecommunications possible.
World-wide communication is possible with as few as ___________ satellites placed in orbit around
the Earth.
Satellites orbit above the Earth, the higher the orbit the ______________ it takes the satellite to
complete an orbit. The time taken to complete one orbit is known as the _______________ of the
satellite.
As a satellite orbits the Earth, the Earth is also spinning on its
axis. The Earth rotates about its axis once every __________
hours and a satellite in a low Earth orbit can orbit in about 90
minutes.
This means that satellites in low orbits are only above a particular part of the Earth for a short period
of time during each orbit. This makes continuous communication impossible using these low Earth orbit
satellites.
The ____________ the orbit of a satellite the longer it takes to orbit the Earth and the longer it
stays above a particular part of the Earth (the satellite is said to be on station). At a height of
_____________ km above the Earth’s surface the orbital period of a satellite is 24 hours. This
means that it orbits the Earth in the same time as the Earth rotates around its axis. Such a satellite
is said to be in a ________________________ orbit. This means that it appears to be stationary
above one point on the Earth’s surface. (Of course the satellite isn’t actually stationary – it is actually
travelling at approximately 3 kms-1!)
Geostationary satellites mean that we can have continuous world-wide communication because we don’t
have to wait until the satellite is in the correct position before transmitting a signal to it.
Standard Grade Physics
Telecommunications
Notes
25
Satellite Television Broadcasting
Geostationary satellites can be used to transmit television signals around the world.
For example: To transmit a signal from the USA to the UK, a _____________ station in the US
would use a dish aerial to transmit a signal to a satellite which would then _________________ the
signal before re-transmitting the signal to a _____________ aerial at a ground station in the UK.
The Finishing Line
Standard Grade Physics
Telecommunications
Notes
26