My Science School

               Television pictures, radio and telephone communications are bounced off satellites to cover the greatest possible area of the world. Satellites orbiting the Earth must travel at high speed to escape being brought down by the Earth’s gravity. As the Earth itself is spinning rapidly, there is a point above the Earth’s surface where the orbiting speed of the satellite can be matched with the rotational speed of the Earth. At this point —35,900 kilometres above the Earth — the satellite appears to stand still and is said to be in a geostationary orbit.

Geostationary satellites can be positioned right over the areas where they are needed. They can also be used as spy satellites, because they remain constantly over a region of interest.

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               Television cameras break a picture into electrical signals, separating them into three colours (red, blue and green) and turning them into coded messages. Sound is recorded and coded at the same time. The coded pictures and sounds are transmitted by radio waves, electrical cables or optical fibres to the receiver. Inside a television receiver the signals travel to three electron guns — one for each colour. The electron guns emit streams of electrons, which are directed at a fluorescent screen. Magnets bend the electron streams so that they scan back and forth from top to bottom, exciting the phosphors in the screen and producing a colour image. They scan so quickly that our eyes see the images as a continuous picture.

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           Telecommunications is the passing of messages and information over long distances by means of electrical signals. These signals may be carried along wires, like telephone, telegraph and fax messages, or by radio waves.

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               Telephones transmit speech messages along wires by means of electrical signals. Telephones were invented as long ago as 1876.The handset of a telephone includes a loudspeaker and a very small microphone, which contains granules of carbon. When you talk into the microphone the sound waves of your voice cause a metal diaphragm to vibrate, and it presses against the carbon granules. The vibrations vary depending on the sounds. They change the very small amount of current flowing out along the wires to the receiver of another telephone.

               When the electric current carrying the signals reaches the receiver handset, the same variations in the current run through an electromagnet. This causes another diaphragm to vibrate in the earpiece, accurate reproducing the sound of the speaker’s voice.

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               We exchange ideas and messages by communication. Humans, as well as other animals, communicate visually and by sounds. Some animals also communicate by means of smells that convey messages. Humans differ from all other living things because we can communicate by means of symbols. These are compressed pieces of information, such as written letters or numbers that actually communicate very complex ideas.

 

 

 

 

 

 

 

 

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               Helicopters are lifted into the air by their large rotating propellers, or rotors. These work like long narrow wings, generating lift as they spin rapidly through the air. The tail rotor generates enough thrust to stop the fuselage spinning.

               A helicopter turns by increasing or decreasing the thrust of its tail rotor. It climbs by increasing the angle of attack of the rotor blades. It moves forwards by increasing the angle of the blade moving back on every rotation so that it pushes against the air.

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               The hovercraft is an ingenious machine that rides on a cushion of air. It looks like a flat-bottomed ship, and is usually powered with huge propellers like those of an aeroplane. Engines draw in large amounts of air, which is pumped out under the hovercraft and kept in by flexible rubber skirts. The craft rises up, supported on the column of air, and is then able to travel quickly over the water or over land. There is little friction because the bottom of the hovercraft does not actually touch the water.

               Hovercrafts have a number of uses, but have never replaced ordinary ships because they are expensive to run, very noisy and are unable to operate in stormy seas.

 

 

               A ship, or any other floating object, displaces (pushes away) water. You can see this for yourself if you fill a bowl with water right up to the brim, and then float a smaller dish inside it. Water will spill over, and the amount spilled will be the same as the volume of the smaller dish. A vessel will float if it is lighter than the amount of water it displaces.

               A very small boat built of steel would probably sink like a stone. However, a very large ship built of steel would be so buoyant that it could also carry huge amounts of oil or other cargo. The capacity of a cargo ship is measured by the weight of water that it displaces.

            Engines that burn fuel inside them are called internal combustion engines. A petrol engine injects fuel into a cylinder and ignites it with a spark. The mixture of fuel and air explodes and drives a piston down the cylinder. The piston is connected to a crankshaft, which turns as the piston moves down. As the crankshaft continues to revolve, it pushes the piston up again.

            A diesel engine works in the same way, except that a spark is not needed to ignite the fuel. Instead, the engine is spun over by a starter. As the piston comes up the cylinder it compresses the mixture of diesel oil and air. Compressing the mixture heats it up and it explodes, driving the piston down.

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            Like rockets, jet engines propel an aeroplane with a stream of hot gases. Unlike rockets, which produce hot gas as a result of burning an explosive mixture, jet engines burn fuel in the air that they draw in at the front of the engine?

            Air enters the front of the engine and is compressor wheel. Fuel is injected into the compressed air, burns fiercely and expands. The hot gas rushes out of the jet pipe at the rear of the engine. The hot gas passes through a turbine, which drives the compressor at the front of the engine.

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               As an aeroplane moves though the air, the air passes over the surface of the wings. These are shaped with a curved top surface and a flatter lower surface, which means that air passing over the top of the wing, has to travel a little faster than that below the wing. This lowers the pressure above the wing, while the air pressure below pushes up, and the end result is the lift that keeps the aeroplane in the air.

               The tail surfaces keep the wing at the proper angle to provide the right amount of lift. The power to propel the aeroplane can come from the engines or, in the case of gliders, from rising air currents.

 

  

               Infinity is a number, or value, which is so huge that it cannot be measured. For instance, the distance to the end of the Universe is called infinity, because if there is an end, it is so far away that it could never be measured.

               Sometimes infinity can be the result of a mathematical calculation. For example, the formula for calculating the distance around the outside of a circle is pi times the diameter. Pi is a Greek letter, and it represents a value of approximately 3.14159. Pi can never be fully calculated, because you would finish up with a string of numbers extending to infinity.

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                    Geometry is a part of mathematics that deals with the shape, position and size of things of geometric forms such as squares, triangles, cubes and cones. Its name comes from the Greek words meaning ‘earth measuring’, because it was probably originally invented as a means of surveying and measuring land.

                   The ancient Egyptians also used geometry when constructing buildings and tombs. Nowadays, geometry is important to engineers and architects. It is also essential in navigation because geometry is used to follow charts and maps.

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               Our numbers are called Arabic numerals, although they probably appeared first in India around AD600. By the 800s the Arab numbering system was use throughout Europe because it was much easier to use than the old Roman system. At first the numbers varied in the way they were written, but with the invention of printing the numbers became standardized. The basic Arab numbers are 0 to 9, and can be used to write any combination of numbers.

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