# Physical Science Physics Course - Topic 1 - TELECOMMUNICATIONS

Discussion in 'Physical Science' started by Icewolf, Mar 1, 2005.

Section 1 - Communicating through Air and Wires.

Ways of Communicating

Telecommunication means communicating at a distance, often using coded information. Drums use sound to send coded messages. Smoke signals and semaphore use light to transmit information. Radio and television use electromagnetic waves which travel at the same speed as light.

The Speed of Sound

Light travels very much faster than sound. So lightning is seen before thunder is heard, and an athlete sees a puff of smoke from the starter gun before hearing the bang.

To Find the Speed of Sound

Apparatus

Method

Use a metre stick to measure the distance (d) from the metal block to the microphone. Hit the block sharply. This starts the timer. When the sound reaches the microphone, the timer is stopped, showing the time (t). Use the formula (below) to calculate the speed.

v = d/t
v = 3.3/0.01
v = 330 m/s

Result

The speed of sound is about 330 metres per second, (m/s)

Examples on the Formula

1, How far does sound travel in 4s?

d = vt
d = 330 * 4
d = 1320 m

2, How long does it take sound to travel 990m?

t = d/v
t = 990/330
t = 3s

NOTE

v is always a small v, it stands for velocity and is usually measured in metres per second (m/s)
t is always a small t, it stands for time and is usually measured in seconds (s)
d is always a small d, it stands for distance and is usually measured in metres (m)

Section 2 - Transmitters and Receivers

In all telecommunication systems, the signals are sent out by a transmitter and picked up by a receiver.

Advantages of Using Wires to Transmit Information

1, Messages can be sent over long distances.

2, Messages travel very quickly - almost at the speed of light.

3, A good degree of privacy can be maintained.

The Morse Code Telegraph

A telegraph sends coded messages along wires using pulses of electricity. These pulses are produced by opening and closing a switch in a circuit like this.

The Telephone

The telephone also sends messages along wires. The mouthpiece contains a microphone. This is the transmitter. It converts sound energy to electrical energy. The earpiece contains a loudspeaker. This is the receiver. The loudspeaker converts electrical energy back to sound.

The electrical signals travel along the wires very quickly - at almost the speed of light.

Looking at Wave Patterns

An oscilloscope can be used to look at sound wave patterns and the patterns of electrical waves in wires. If an oscilloscope is connected to a loudspeaker and signal generator, the shape of the waves is seen.

Section 3 - Telecommunication Through the Phone

The Phone

To investigate the waves in a telephone circuit, the oscilloscope is connected into the circuit using a connector box.

Loudness and Frequency

An Oscilloscope shows whether a sound wave is loud or quiet and whether it has a high or low pitch (frequency)

The greater the height if the wave pattern, the louder the sound.
The greater the number of waves, the greater the pitch or frequency.

Wave Diagrams

Section 4 - Waves

Measuring Waves

Wavelength: the distance between any point on one wave and the corresponding point on the next wave. Wavelength is measured in metres (m).

The symbol for wavelength is - (lambda)

Speed or velocity: the distance a wave crest travels in one second. Speed or velocity is measured in metres per second (m/s) (or ms^-1)

The symbol for Speed (velocity) is v

Frequency: the number of waves which passes a point in one second. Frequency is measured in hertz (Hz). For example, 6Hz means 6 waves per second. The higher the frequency, the higher the pitch.

The symbol for frequency is f.

Amplitude: the height of the wave. This is measured from the centre line to the top of the crest, or the bottom of the trough. The bigger the amplitude, the more energy the wave carries. Amplitude is measured in metres (m). the greater the height of the amplitude, the louder the sound.

The Wave Equation

The next equation you need to learn in this topic would be the wave equation.

v = f

(speed = frequency * wavelength)

And also

distance = wavelength * no of waves

Using the Wave Equation

The waves shown in the diagram (4.1) are produced with a frequency of 20Hz.

Find
(a) their wavelength
(b) their peed

(a) Wavelength = distance/no of waves
= 100 / 2.5
= 40cm (0.4m)

(b) Speed = f
= 20 * 0.4
= 8 m/s

Section 5 - Radio and Television

Radio (and television) signals travel through space as waves. They carry energy and travel at the same speed as light. This means that their speed is 300 000 000 metres per second.

(N.B. their actual speed is 299 792 458 m/s, but for the sake of simplicity this figure is normally rounded up to 300 000 000 m/s)

Examples of TV and Radio calculations

1. Radio 2 has a wavelength of 330 m. Calculate the frequency of the radio wave.

f = v/
= 300 000 000/330
= 909 000 Hz (909 kHz)

2. How long does it take for a TV signal to travel 140 km from the transmitter to a house?

t = d/v
=140 000/ 300 000 000
= 0.00047 s

1. Radio signals can be sent over very long distances.
2. Radio stations can transmit different programmes at the same time on different wavelengths.

Anyone with a radio receiver may be able to tune into any radio communication so that private communication is difficult.

Aerial
Tuner
Decoder
Amplifier
Loudspeaker
Electrical supply

Every radio receiver needs energy to make it work. This energy is supplied by the electricity supply.

The aerial is made of metal, and is used to detect radio waves coming from the transmitter.

Each radio transmitter transmits waves of a different frequency and wavelength. The tuner is used to pick out one particular radio frequency.

The transmitted information (speed or music) needs to be separated from the rest of the radio wave. This is done by the decoder.

The energy of the transmitted signal is very small. The amplifier increases the size or amplitude of the signal.

The electrical signals in the radio are finally changed into sound by the loudspeaker.

These processes are shown here;

Amplitude Modulation

We hear sounds with frequencies between 20Hz and 20 000Hz.
Radio waves have frequencies of millions of hertz. In a radio transmitter, a low frequency sound wave is carried by a high frequency radio wave. Its amplitude is altered to take on the shape of the sound wave. This is called amplitude modulation, (a.m). We say that the high frequency radio wave is the carrier wave for the low frequency sound or audio wave.

Frequency Modulation

In some types of transmitter, the low frequency sound waves are carried by very high frequency (vhf) radio waves.

The frequency (not the amplitude) of this carrier wave is altered by the sound pattern.
This is called FREQUENCY MODULATION.

FM radio produces better quality sound than AM radio and so FM is used for stereo broadcasting.

Section 8 - Diffraction

Ripple Tank Diffraction

Hilly Regions

These diagrams show that waves of long wavelength bend round obstacles more than waves of short wavelength,

This means that, in a hilly region, it will be more difficult for short radio waves to bend round the hills.

Short wave reception will therefore be more difficult than long wave reception.

Section 9 - Television

Television

Television signals are sent out from a transmitter and are collected by the aerial of a receiver. The high frequency TV carrier wave is modulated by the transmitter so that it carries both sound and picture information. The receiver, therefore, must decode the sound information and the picture information to provide a complete sound and visual picture.

The Parts of a Television.

Section 10 - The Picture Tube

The Picture Tube

TV pictures are made by using a beam of electrons, which are fired across the evacuated tube from an electron gun. The screen is coated with a paint which glows when hit by electrons. This results in a small spot of light being produced. The electron beam can be moved to any point on the screen and so the whole-screen pictures can be built up. Electron beams are sometimes called cathode rays. For this reason, the TV tube is often called a cathode ray tube.

The electron beam may be deflected in two ways. One uses the electrical effect of deflecting plates (electrostatic deflection)

The other uses the magnetic effect of deflection coils (magnetic deflection)

The X - plates move the electron beam from side to side.
The Y - plates move the electron beam up and down.

The vertical coils move the electron beam from side to side.
The horizontal coils move the electron beam up and down.

Building up a Black and White picture

The picture is built up by moving the electron beam backwards and forwards across the screen in a pattern of lines. In British TV there are now 625 lines in each picture. The spot moves very rapidly. Light and dark parts of a picture are made by changing the spots brightness (intensity of electron beam) continuously as it moves across the screen.

Making the Picture Move

Twenty-five different pictures are produced every second. This means there is a gap of 1/25s between each picture. However, the human eye retains an image for a short time after the picture disappears. This means the picture seems to move smoothly, rather than in jerks.

This is known as Persistence of Vision.

Section 11 - The Colour Television Tube

The Colour Television Tube

All the colours we see are produced by combining different brightness’s of red, blue and green. These are known as the primary colours.

Producing Colour

To produce colour TV pictures, three electron guns are used, and the screen is made up from three different paints - one which glows red, one which glows blue, and the third which glows green. Each electron beam is arranged so that it hits only one colour of paint. This is done by placing a metal screen with holes in it. in front of the screen. As the guns are turned on and off by the TV signal, the primary colours also are turned on and off. Any colour, therefore, can be produced on the screen. The eye cannot distinguish between the very small differently coloured dots, when viewed from a distance. Full colour pictures can be produced in this way.

Colour Mixing of Light

Red + Blue = Magenta
Red + Green = Yellow
Blue + Green = Cyan

Diagram of Colour TV

Section 12 - Optical Fibres

An optical fibre is a very thin glass fibre, through which optical pulses can be transmitted at very high speeds. Optical fibres are usually grouped together into thicker optical cables which can carry a great many messages at the same time.

Reflection

When light is reflected from a mirror, the angle of reflection is always equal to the angle of incidence.

Total Internal Reflection

When light travels from glass into air, its direction changes. This is called refraction. However, if the light ray strikes-the glass-air surface at an angle greater than about 42 degrees, the light is totally reflected. This process is called total internal reflection. The smallest angle of incidence at which this process occurs is called the critical angle.

The Speed of Light

The speed of light in air is 300,000,000 m/s
The speed of light in an optical fibre is about 200,000,000 m/s

Advantages of using Optical Fibres in Communications

1. Optical signals can be transmitted at a very high speed over long distances (but not as fast as in wires.)
2. There is little loss of energy and so only a small number of booster stations are needed along the route.
3. Optical signals are free from interference from high voltage electric cables.
4. Cables made from optical fibres are lighter than metallic cables.
5. They cost less.
6. They can carry many more signals at the same time than is possible with metal cables
7. The signal quality at the end of the cable is much better.

Optical fibres in Communication Systems

Optical fibres can be used instead of wires in communication systems.
For example, many underground telephone wires are being replaced by optical
fibres.
The changing electrical signal from the microphone of the telephone is applied
to a simple light-emitting diode (LED). This makes it flicker.

The variations in light level are transmitted along the fibre very quickly
At the other end, a light detector converts the light back to an electrical signal.

Section 13 - Satellites and Dish Aerials

Communication Via Satellites

World wide communication over a long distance is difficult because of the curvature of the Earth. Signals from a transmitter can be sent much further if the transmitter is on top of a hill. If a satellite is put into orbit high above the Earth, it can be used to send signals very long distances. Three satellites - suitably positioned - can relay signals right round the world.

Messages could not be sent directly from Britain to America because they could not pass round the curve of the Earth.

Satellite Orbits

The period of a satellite is the time it takes to go round the Earth once. The period depends on how high the satellite is - the higher the satellite, the longer it takes to go round. For example, a weather satellite 300 km above the Earth's surface will have a period of about 90 minutes, while a TV broadcasting satellite which is 36,000 km above the Earth's surface will have a period of 24 hours.

Geostationary Orbits

A satellite with a period of 24 hours will go round the Earth once every day. But the Earth also spins round itself once every day. So if the satellite is put above the equator, it will always stay above the same point on the Earth. This kind of satellite is called a geostationary satellite. Geostationary satellites are always 36,000 km above the Earth's surface. Looking up from the Earth, they seem to stay at the same place in the sky. This means that aerials which point at them do not need to move. The aerials can be fixed to houses, for example, to pick up satellite TV.

Dish Aerials

Because satellites are so far away, the signals that pass between them and Earth lose a lot of energy, and become very weak. Special dish aerials are used to receive the signals.
The dish collects the signals from a large area and concentrates them. It does this by reflecting them and focussing them onto a detector in the middle.
(The detector is said to be at the focus of the aerial.)

Transmitting with Dish Aerials

As well as receiving (picking up) signals, dish aerials are used to transmit them (send them out). By putting the transmitter at the focus point, the signals are reflected from the dish into a parallel beam. This makes the signal much stronger, and the beam can be aimed accurately.

The same procedure is used with light in torches, car headlights, etc. The bulb is put at the focus, and a parallel beam of light is given out.

Repeaters

Microwaves are also used to transmit telecommunication information around the country. Near the Earth's surface, microwaves can travel only about 40 kilometres. Repeater stations are used to boost the signals at regular intervals. Often several booster stations are required.

Congratulations you have now finished;
Topic 1 - Telecommunication