Telephone and Fax

Telephone work by turning the sounds of the human voice into electrical signals. For 'land-line' phones, these electrical signals are sent along cables, and then turned back into sound at the other end. Mobile phones do the same thing using radio waves, while faxes send a stream of electrical signals, to be decoded at the other end by another fax machine.

How does a fax work?
A fax machine scans a page by dividing it into thousands of tiny squares and checking whether each square is black or white. It then sends a stream of yes/no signals that follow the pattern of black/white squares on the original, which the receiving fax then prints out. A fax does not recognise text, but sees it as a pattern of black marks.

Why must you not use a mobile phone on an aeroplane or in a hospital?
Mobile phones are small radio transmitters. They create an electrical field when they are used, and this can disturb sensitive equipment nearby, such as an aeroplane's navigation and control systems. Hospital equipment - heart monitors and kidney dialysis machines, for example - can also be affected. Since they send and receive signal even in 'standby' mode, mobile phone should always be switched off completely in aircraft and hospitals.

Making a telephone call
When you dial the phone number of a friend in another town, your call goes first to your local telephone exchange passes it on to the nearest trunk (long-distance) exchange, which sends it to the trunk exchange closest to your friend's town. This directs the call to a local exchange, which sends it straight to your friend's home. This all happens almost instantly - as your finger lifts off the last number dialled, your friend's phone is already ringing. International calls go straight from your nearest trunk exchange to an international exchange. This sends the call to its destination by undersea cable or satellite.

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Electromagnets
Some magnets work all the time, but electromagnets produce magnetism only when they are supplied with an electric current. This is useful, because it means that they can be switched off when not needed. Electromagnets operate doorbells, microphones and loudspeakers, and they are inside telephones, computer disc drives and televisions. They are also used for lifting heavy metal objects-like wrecked cars.

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How do magnets work?
In an ordinary piece of iron, the electrons spin in different directions. But in an iron magnet, they all spin in the same direction. This creates a magnetic field, an invisible region where magnetic forces are at work. Put some iron fillings on a piece of thin card, and a magnet underneath. If you tap the card, the fillings line up in the direction of the field.

Magnetic field
Magnets have two poles. The magnetic field spreads around the magnet from one pole to the other.

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Static experience
Static electricity, the kind that makes clothes crackle, is produced when particles are rubbed off one object and left on another. Touching a machine called a Van de Graaff generator, which is specially designed to make static electricity. It is quite harmless, but can produce some hair-raising effects.

How the Van de Graaff generator works?
Charging up : The Van de Graaff generator makes static electricity by creating charged particles called ions. These are atoms that have extra electrons, which give them a negative charge. The negative ions are created on needles in the base of the generator, and are carried to the base of the generator, and are carried to the top by a rubber belt. They are collected by another set of needles, and transferred to a metal domes, which takes on their negative charge.
Lift of : When touches the dome, the negative ions spread into body, to the tips of hair. Each hair becomes negative, which means that it repels every other hair, and they all stand on end.

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How do batteries store electricity?
Unlike heat, electricity cannot be stored until it is needed. Instead, it has to be made. Batteries make electricity by using chemical reactions. In a battery, the reaction creates a charge at its negative terminal, so that it is ready to produce a current. As soon as you switch on, the battery drives electrons around the circuit. It can keep the electrons moving until its supply of chemicals is used up, and it goes flat.

Harmless shocks
Static electricity builds up in houses, cars and clothes, and can sometimes give you a shock. These shocks can measure up to 50 000 volts, but they involve few electrons and are not nearly as dangerous as ones from a household electricity supply.

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ELECTRICITY AND MAGNETISM

If you ever hear you cloth crackle when you take them of, you will have made your own electricity. Electricity is a form of energy, carried by tiny particles of matter. When these particles are on the move, their energy can do lots of useful things. Closely linked with electricity is magnetism. This is an invisible force that makes things attract or repel each other, and can be strong enough to lift cars off the ground.

How does electricity move through a wire?
Electric currents are carried by particles called electrons, which are part of atoms. When a current flows through a wire, electron are jolted from atom to atom, carrying energy with them. Unlike static electricity, electric currents can only move through conductors, such as electric wires, and they always travel in a circuit. Conductor are metals and similar substances which have electrons that are easily dislodged.

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How does an echo work?
Sounds waves can bounce off solid objects, just as light waves can bounce off off mirrors. But because sound travels much more slowly than light, it can take several seconds to reach our ears. When it arrives, we hear it as an echo. Bats and dolphins use high-pitched echoes to find their food. Echoes are also used in sonar, a way of detecting submarines and mapping the seabed.

Calling back
If you shout at a wall, the sound will bounce back as an echo. To produce a good echo, the wall needs to be big and at least 25 m (80 ft) away.

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Overtaking sound
If the plane flies faster than the speed of sound, air begins to pile up in front of it. When this air slips out of the way, it suddenly expands, making a loud noise called a sonic boom. At one time, scientists thought it would be dangerous for planes to fly 'supersonically', but now planes like Concorde break the sound barrier every day. They usually do it over the sea, so the deafening sonic boom is not heard on land.

hearing faint sounds
You may not be able to hear a pin drop, but your ear can pick up some amazingly faint sounds. The faintest sound that most people can hear is about 10 billion times quieter than the sound of a live pop concert.

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Sound on the move
When sound comes from aboard a slow-moving boat, the sound waves spread out evenly through the water, where they move even more quickly than in the air. When the sound comes from a plane, the pattern of sound waves is quite different. This is because the plane moves much more quickly, 'catching up' the sound waves that it makes.

Sound in the air : Sound travels through the air at abut 330 m (1082 ft) a second. It move slightly faster on a warm day than on a cold one.

Underwater sound : Sound travels nearly four times faster in water than it does in air. This means that a fish will hear the boat before a person standing nearby it on land.

Chasing sound : Most passenger jets move at about two-thirds of the speed of sound. At this speed, a plane keeps catching up the sound waves in front of it, and race away from the one it leaves behind. These trailing waves make a low-pitched rumble as the plane flies away.

Fading out : As the sound waves travel outwards, their energy is slowly soaked up by the air. Eventually, they become too weak to be heard. This is why it is hard to make yourself heard a long way off, even if you shout.

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Things that makes sound

Rocket liftoff : 150-190 dB

Jet takeoff : 120-140 dB

Motorbike : 70-90 dB

Orchestra : 50-70 dB

Whispering : 20-30 dB

Rustling leaves : 10 dB

What's in a wave?
Closely spaced waves produce a high-pitched sound, widely spaced waves a deeper one. The sound is soft if the waves are shallow, and loud if they are deep.

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THE MAKING WAVES OF SOUND

Sound is caused by waves of pressure that spread out though the air, making the air molecules squeeze together and then push apart. These waves are started by anything that is vibrating, from a car engines to the strings of violin. Unlike light, sounds always has to travel through something, so if you were floating in empty space, you would hear nothing but total silence.

How loud do sounds get?
Scientists normally measure the loudness of a sound by using units called decibels (dB). The softest sound that the human ear can hear has a decibel rating of about 4, while the loudest ones that it can cope with have a rating of over 150. The decibel scale works logarithmically. This means that if two sounds differ by 10 decibels, one of them is 10 times as loud as the other.

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Making shadows
Transparent object let light shine through them. Opaque ones, like book, block the light and cast a shadow. Because light travels in straight lines, the shadow is the same shape as the book.

Reflecting light
When light hits something shiny, like a metal can, it bounces off it. This called reflection. Shiny flat surfaces are the best at reflecting light. Thing that do not reflect light look black, because they soak up the light that falls on them.

Bending light
When light travels from on substance to another, for example from air into glass, or from glass into water, it changes direction. This is called refraction. It explains why a drinking straw in a glass looks bent.

Focusing light
Lenses are designed to bend light in a particular way. The lenses in glasses help to bend light so that it makes an image on the back of the eye. Instead of being fuzzy, the image is sharp or 'focused'.

Starlight
Light from stars can take millions of years to reach us.

Making colours
A beam of sunlight contains all the colours of the rainbow. When it shines onto something with lots of narrow lines, like a CD, the different colours are separated, so that they can be seen. This is called diffraction. Some birds and butterflies get their colour in this way.

What makes a rainbow?
Rainbows appear when the Sun is shinning and there is rain in the air. The falling drops split the Sun's light into seven different colours, and reflect it towards your eyes. The rainbow you see is always a 'private' one, even if someone is standing close to you, they see a rainbow made by a different set of raindrops. No matter how hard you try, you can never see a rainbow from the side or from behind, and you can never get to the rainbow's end. If you move, the rainbow moves too.

Home light show
Scientists investigate light with complicated pieces of equipment, but you can make a start with the everyday objects shown here. If you put them on a tabletop on a sunny day, you will be able to see how light it can be broken up into different colours and how it casts shadows.

How fast does light travel?
Nothing moves faster than light, 299 792 km (186 287 miles) a second. It travels from the Sun to the Earth in around eight minutes, and it can cross the average room in about one hundred-millionth of a second. When light shines through transparent matter such as glass, it slows down by up to a third, but even then, it is still faster than anything else.

Why can't you see around corners?
You can hear around corners because sound spreads in all directions, but you cannot see around them because light nearly always travels in straight lines. One way of getting around this is to use a mirror. This reflects the light around the corner, so you can see what, or who, is hiding behind it.

Light you cannot see
If the beam of light shone past your nose in completely empty outer space, you would not be able to see it. The reason for this is that you can see light only when it shines into your eyes. On Earth, you can see beams from the side because dust and air scatter some of the light your way.

THE VISIBLE ENERGY OF LIGHT

Light is the only kind of energy that we can see. It moves about a million times faster than sound, and it can travel through empty space. Some things, like the Sun, make light. But most of the things we see do not. Instead, they reflect some of the light that falls on them. Light normally travels in straight lines, but it can be made to change course, and it can be split up into different colours.

Why is light made of?
Light is pure energy. You cannot touch it or pick it up, but whenever it shines into your eyes, you can see that it is there.Light energy travels in pulse called waves, and the distance between the wave decides its colour. In blue light, for example, the wave are about 0.0005 mm (0.00002 in) apart, roughly 200 times narrower than a human hair. Light belongs to a whole family of waves that includes X-rays, microwaves and radio waves.

Demolition job
One way to demolish a tower block is to knock it down bit by bit. A much speedier way is to use explosives to release a burst of energy, bringing the building crashing to the ground. Below is how potential energy turns into other kinds of energy, and the dramatic results of the changes.

1. Potential energy is stored energy. Before the bang, there is potential, energy in the explosives, and in the tower block, because it contains heavy materials perched high above the ground

2. When the explosion takes place, the explosives potential energy is changed into heat, movement and sound. The tower block's potential energy also changes into movement and sound, as thousands of tons of concrete and steel come crashing down.

3. When the dust settles after the explosion, the potential energy in the explosives and the tower block has been changed. Most of it has been converted into different kinds of energy, which have escaped into the air.

What happen to energy after it is used?
No matter how many times energy is changed, it can never be destroyed. But whenever it changes, some of it always escapes. For example, a car engine turns much of the potential energy in petrol into movement. But it also turns some of it into noise and heat, and this escapes as the car moves along. No matter how well machines are designed, they always waste some of the energy that they release.

A waste of energy
As a workman digs a hole with a pneumatic drill, energy escapes in the form of noise and heat, this energy cannot be used.

Energy in a raindrop
The amount of energy in a falling raindrop is about 175 thousand billion times less than the amount of energy of a cruising jumbo jet. But added together at any one time, the energy in all the raindrops falling on Earth is much greater than the energy of all the planes in the air.

How hot or cold can things become?
There is no limit to how hot things can become, but if something becomes extremely cold, its atoms stop moving. It cannot get any colder. Scientists have come close to reaching this temperature, called absolute zero, at -273.16 Celsius (-459.69 Fahrenheit). Near absolute zero, matter starts to behave in strange ways. Some gases turn into metals, and some metals become 'superconductors' This means that electricity can flow through them forever, without needing any energy from outside.

What happens when things get hot?
Heat is one of the most important forms of energy. When something warms up, the atoms inside it start to move more quickly, and they also spread farther apart. The hotter something is, the faster its atoms move. When atoms become really hot, they start to give out light. First they glow dull red, but if they get even hotter they turn bright red, yellow, white and finally blue. Heat is a bit like water, because it always flows 'downhill'. That is, it moves from warm things to cool things, but never the other way round. Things feel hot when some of their heat energy travels into your skin, and they feel cold when heat energy flows from you into them.