My Science School

You're walking down the street when you suddenly hear your favourite song start to play. But where is the music coming from? As you look around, you notice that the person walking ahead of you is searching through his pockets. Of course the song is his special signal, letting him know that he has a call or a message on his mobile phone.

A mobile phone rings, vibrates, or plays music when it receives a signal. A display shows the phone number of the caller and possibly a message, too.

Some people carry mobile phones in their purses, briefcases, or pockets. Mobile phones use radio signals to carry their messages.

The service area for mobile phones is divided into groups called cells. Each cell has a radio transmitter. Signals are sent to the cell where the person using the mobile phone is. People who move around a lot still get their signals. The signals bounce from the transmitter in one cell to the transmitter in the next cell. This happens very quickly. Even if people are on a train while they are talking on a mobile phone, their conversation is not interrupted.

 

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When you call your friend on the phone, how does the sound of your voice reach them? You speak into the mouthpiece of your phone. The sound waves push on a thin sheet of metal called a diaphragm.

The diaphragm then vibrates. It presses against a small cup filled with carbon grains.

Electricity passes through the carbon on its way through the telephone wire. When the carbon grains are squeezed together, the electric current gets through easily. But when the grains are spread apart, only a little current gets through. So the vibrating diaphragm causes strong or weak pushes of electricity to travel through the telephone wire.

Your friend hears what you say through the earpiece on her phone. Inside it is an electromagnet a long coil of wire wound around an iron core. The strong or weak pushes of electricity reach the electromagnet. They cause it to make strong or weak pulls on another diaphragm. This diaphragm vibrates and makes sounds just like the ones your voice made. So your friend hears a copy of your voice-a copy made by electricity in a wire.

The first telephone was not like our phones today. You could speak into it but you could not hear the other person on it. Alexander Graham Bell first used it in March 1876. But by October of that year, Bell had the first two-way conversation on his invention.

 

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Pick up your telephone and call a friend. As you punch the buttons, signals travel on the phone line to your local telephone centre. That centre sends the signals to the phone centre nearest your friend.

While you are calling your friend, thousands of other people are making calls, too! And some people are using modems. Modems are machines that send information from one computer to another over phone lines. Other people use fax machines or e-mail to send letters over phone lines. Today, people are sending out more messages than ever, and phone lines help us to communicate across the world!

Microwave links and optical fibres can handle many thousands to millions of phone calls at the same time. Microwave links send signals as a radio beam. At the receiving station, the microwave signals are decoded. Optical fibres are long strands of coated glass. Cables made of these strands use lasers to turn messages into pulses of light. The light pulses are then decoded at the other end of the optical fibre. These cables can carry millions of telephone conversations and faxes, as well as computer information. They can also be used to carry television programmes.

 

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A shop owner is shutting up shop for the night. Before leaving, the owner sets the security alarm by punching a code into a keypad.

Many people use security systems to stay safe. Security systems use signals that tell them when to sound the alarm. Codes are entered in a keypad near the door. This turns the system on and off. If you forget to turn off the alarm when you return, the alarm will sound.

Many security systems use a motion alarm. A small transmitter inside the house sends radar waves into the room. The waves scatter and bounce around the room. They set up an invisible pattern. When someone enters the room, the pattern is upset, and the alarm goes off. A motion alarm will not work well if you have pets. When the pets move, they set off the alarm.

Another type of security system uses metal strips on the windows. When the window is closed, the metal strips touch each other. When the window is opened, this connection is broken and an alarm goes off.

Security signals keep us safe in many ways. Airports and courthouses use metal detectors to detect weapons. Many look like doorways. A transmitter on each side of the doorway sends out radio waves. Metal absorbs some of these waves and reflects them, or sends them back. The detector's transmitter then beeps, or signals that metal is there.

Libraries and stores prevent theft by placing coded targets on items. The target sets off an alarm at the exit if it has not gone through the check-out process.

 

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Aeroplanes get into traffic jams just in the same way cars do-especially around busy airports. But people called air traffic controllers know where each plane is located. They use radar to help them direct air traffic.

Radar allows the controllers to find planes that are too far away to see. And radar does this at night and in rain, fog, or snow.

A radar set sends out radio waves. When the radio waves hit a flying plane, or even a raindrop, they bounce back to the radar set. This makes spots of light appear on the tracking screen.

The moving spots of light tell a controller where the object is. They know how far away it is, how high it is, how fast it is moving, and which way it is going. Then the controllers can direct the air traffic, much as police officers direct road traffic. They make sure each plane follows a safe path when flying, taking off, or landing.

The planes have radar sets, too. The pilot can look at the radar to make sure that no violent storms or other planes are dangerously close.

 

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Have you ever tried to switch channels on TV when someone stood in the way? Nothing happened. Why not? The transmission of the signal was blocked. The signal from the remote control hit someone's body instead of the TV.

Remote means "far away". When you use the remote control, you are controlling the TV from a distance. The remote control uses an invisible type of light called infrared light to send a signal to a receiver on the TV.

The buttons on your remote control send different codes to the TV. The code consists of long and short flashes of infrared light. When you press a button, the remote control sends the code for that button to the receiver in the TV. The TV "sees" the signal and carries out the command.

Some toy cars use a radio remote control to guide their movements. Turning knobs or moving levers sends a signal to the car to go forwards or backwards, or turn left or right. A garage door opener uses a radio wave to send its signal. Different openers have different frequencies so that you won't open your neighbour's garage door by mistake.

 

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Did you know that the pictures on TV are a jumble of red, green, and blue dots? When you sit across the room, the dots blend into the images you see.

At the television station, a camera records the picture and sound from the scene you are watching. Mirrors in the camera split light from the scene into red, green, and blue. A tube in the camera changes the light to radio signals. The television station broadcasts the programme to your home.

TV antenna, cable, or satellite dish A receives many broadcast signals at once. The television tuner is used to select the signal for the TV channel you want to see. The tuner passes this signal to the amplifier. The amplifier separates the sound from the pictures.

The sound goes to the speakers. The picture signal is sent to a decoder. The decoder sends the signal to the electron guns. There is one gun for each colour red, blue, and green. The electron guns zip across the screen in weak or strong bursts of light. These bursts form the picture you see on the screen.

 

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Radio signals are heard almost everywhere. But how does a radio work? The radio transmitter (where the sounds come from) changes sound waves into electrical signals. It sends them through the air as radio waves. The radio waves that leave the transmitter have different frequencies for each station. Frequencies are the number of times the waves vibrate per second.

You tune your radio by choosing a number on the controls. Each number represents a frequency. So if you always tune in the same frequency, you always pick up the same station.

When you tune in a station, signals are picked up by the radio. These signals are sent to an electromagnet in the radio speaker. The electromagnet makes a cone on the speaker vibrate. These vibrations are the sounds you hear on your radio. They sound exactly like the sounds made at the transmitter-voices, music, or even the squeak of a mouse.

An Italian inventor, Guglielmo Marconi, realized that coded messages could be sent over long distances without using wires. Because the transmitter and receivers did not need wires, the process was called wireless telegraphy. This discovery led to present-day radio.

 

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Many people work M together to design a new car.

First, drawings are made with a computer. This is called computer aided design, or CAD. Computers are used to create, test, and change the plans. This saves time and money. Next, artists may make a clay model of the car. The clay is coated with shiny film. It looks like a real car.

Other artists create the inside of the car. They design the seats. They decide where the controls will be. They plan everything from turning signals and seat belts to airbags and drink holders.

Finally, a fibreglass model of the car is built. It has real tyres, glass windows, and trim. This final model looks exactly like the new car will look.

Product engineers plan how each part of the car will be made. They use a computer that traces every line and curve on the final model. Factories make the parts and a completed car is made and tested.

Next, parts are shipped to several factories to be assembled. Each worker in the assembly line adds a different part. At the end, a complete car rolls off the line.

 

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A jet fighter plane is very powerful, but it can't fly into space. Why not? There is no oxygen in space to power the engine!

All fuel needs oxygen to burn, but a rocket engine does not need air It carries its own supply of oxygen.

Planes get oxygen from the air. But rockets get oxygen from a substance called an oxidizer. Some space rockets use solid fuels with solid oxidizers. They work in the same way as a fireworks rocket-a fireworks rocket as big as a 10 storey building!

Other space rockets use liquid fuels and oxidizers, so that the engines can be switched on or off.

The liquids are pumped into a special part of the rocket called the combustion chamber. Here the fuel burns violently to thrust the rocket upwards further away from the earth.

The Saturn V moon rocket burned over 2,120,000 litres of fuel during its first 105 seconds of flight. This pushed the rocket off the launching pad with a huge amount of force. If you want to get to the moon and back again you have to think big-really big!

Fireworks rockets are displayed at many festivals. Fuel in a fireworks rocket burns like fuel in a rocket engine. In fireworks, the fuel is charcoal and sulphur. The oxygen is supplied by a solid oxidizer called saltpetre. This mixture burns very hot. The gases given off push in all directions against the inside of the rocket. The gases that push against the top of the rocket make the rocket go! The fireworks rocket has a stick that keeps it pointed in the right direction. 

 

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A turbine blade is similar to the blades on an electric fan. The angled blades draw air from behind and pull it forwards. This activity will show you how the angled blades help the turbine spin.

You Will Need:

• scissors


• a piece of cardboard


• a pencil

What to do:

1. Cut a 15-cm circle out of the cardboard. Using a pencil or the point of the scissors, carefully poke a 1/2-cm hole in the centre.

2. Draw a circle around the hole about 1/2 cm outside it.

3. Make 8 slits, evenly spaced around the circle. Cut from the edge to the 1/2-cm mark. Do not cut all the way through. These are the blades.

4. Bend one side of each blade in the same direction.

5. Put the pencil through the hole.

6. Holding each end of the pencil, place the turbine about 20 cm from your body.

7. Blow on the turbine. How fast can you make it spin?

 

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Aeroplanes are usually pushed through the air either by propellers or by jet engines. A propeller looks like two thin, pointed wings joined end to end. As it spins around, it forces air backward. This pushes the aeroplane forward.

The engine of a propeller plane uses the power of burning fuel to turn the propeller. But a jet engine uses burning fuel to make the wheels of a turbine spin. The gases and air from the spinning turbines shoot out from the back of the engine and push the plane forward. An aeroplane with a jet engine can fly much faster than a propeller aeroplane.

Large passenger aeroplanes are usually powered by jet engines. Some jumbo jets can carry more than 400 people. They fly at a speed of 800 to 970 kilometres an hour.

 

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The helicopter pilot starts the engine. Its big blades spin-slowly at first, then faster and faster. But the helicopter is still on the ground. The pilot twists the big blades at an angle. This forces air down. Then the helicopter rises off the ground. The pilot tilts the blades back. The helicopter stops rising and moves forwards. A small propeller at the back of the helicopter keeps it from spinning while the big blades are turning. It also keeps the helicopter from spinning around when it is in the air.

Helicopters are very useful. Since they rise straight into the air, they can land and take off in very small spaces. They can also hover, or stay in one place. So they are often used to help people who are trapped on a mountain, at sea, or on top of a high building.

 

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Submarines are vessels that can travel underwater. They can also float on the surface of the sea and move like any other ship.

Many submarines are powered by a nuclear reactor. The nuclear reactor creates extreme heat used to turn water into steam. The steam drives an engine that turns the propeller. The propeller pushes the submarine through the water.

Tanks on the sides of the submarine are filled with air or water. They allow the submarine to dive under water or surface on the water. Doors, called vents, on the top and bottom of each tank, open and close, letting in water or air.

While the submarine is on the surface, the tanks are filled with air. To dive, the vents are opened.

To make the submarine surface, the bottom vents are opened. The top vents remain shut. Air is pumped into the tanks to blow out the water. When the submarine reaches the surface again, all the vents are shut.

 

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A ship is a big boat Ships are designed to travel the ocean. They usually have several decks, or floors, below the water level.

Some ships, like cruise liners, are out at sea for a long time. They have places for sleeping, eating, working, relaxing, and bathing. A luxury cruise liner even has a hospital, hair salon, shops, and swimming pools.

Most ships are working ships. They are used to carry goods or cargo.

Freighters carry bananas, cotton, coffee, plastics, and cloth. The cargo travels in big metal boxes. Tankers carry bulk cargo, like crude oil or wheat. The cargo is poured right into the hull.

Some fishing ships are like floating factories. They process and freeze the fish on board the ship. Sometimes these ships are at sea for months.

Some Navy ships have large upper decks. Fighter jets land and take off from this deck. Some Navy ships carry researchers, soldiers, tanks, and even helicopters.

 

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