Radar works by sending out a beam of high- frequency radio waves, which are reflected back when they strike a solid object. The radio waves can also be reflected by clouds or other weather features. The reflected waves are collected and used to produce an image. Usually the radar beam is emitted from a rotating scanner, so a complete 360° image is produced. Radar is important in the navigation of ships, aircraft, and weather forecasting.
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Any modern electrical device requires a huge number of connections to join together all the small components needed for it to work effectively. At one time these connections were made by wires that had to be soldered together. The wires have now been replaced by the printed circuit, which is effectively a picture of the wiring that works just as well.
A thin sheet of copper is attached to a special board made of insulating material. An image of the electrical circuit is then printed photographically onto the copper sheet. Chemicals are used to dissolve most of the copper, leaving behind a thin film of metal bands, to which all the necessary components can be attached. This process can be carried out automatically.
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Radio signals are transmitted using a carrier wave. A radio transmitter changes, or modulates, this radio wave in order to convey information. In AM radio the height of the carrier wave is altered according to the sound picked up by a microphone. In FM radio the frequency, or distance between the peaks in the radio wave, is changed. The radio receiver picks up these signals and amplifies and decodes them. If the signal is weak, AM radio sounds crackly. It is now being replaced by FM radio, which gives much clearer reception.
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Radio waves form part of the electromagnetic spectrum. They are not part of the visible spectrum of light, and they have even longer wavelengths than infrared radiation. Radio waves with the longest wavelengths are bounced off a layer high up in the Earth’s atmosphere, called the ionosphere. In this way radio messages can be bounced for very long distances. Radio waves with shorter wavelengths penetrate the Earth’s atmosphere completely and can be used to communicate with spacecraft.
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Many animals migrate for very long distances, but they arrive back at their birthplace in order to breed. This behaviour has puzzled scientists for many years, but they have now found that many of these creatures contain tiny particles of magnetite in special organs, allowing them to orientate themselves accurately.
Magnetic navigation alone is not enough to account for the amazing ability of these animals to find their way over vast distances. It is thought that they also use other navigation aids, such as the position of the Sun, to help them.
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Some experimental trains have been made to float a few centimeters above the ground by using a system of electromagnetic coils embedded in the track. Electromagnetic force can also be made to propel these trains. They have no wheels, so there is very little friction to waste energy. This magnetic propulsion system, which is completely silent, is called magnetic levitation.
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Electromagnets only produce magnetism when an electrical current passes through them. They usually consist of a metal core made of iron or a similar material, around which are wrapped many coils of thin insulated wire. The metal core becomes magnetized when an electric current passes through the wire coil, but the effect disappears immediately the current is shut off. Very powerful electromagnets can be created by using many coils and strong electrical currents.
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Permanent magnets are usually made from mixtures of iron, cobalt and nickel, but others contain far more complicated mixtures of metals. Magnetite is a natural magnetic substance, which used to be called lodestone. It is a material that contains iron, and it was originally used to make the first primitive compasses.
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One of the most important uses of magnets is electric motors, which are used to power machines ranging from small toys to enormous railway engines. Video and audio tape recorders also depend on magnetism. Their tapes contain tiny metal particles that are magnetized by the recording heads. When played back, another part of the head decodes the magnetic patterns. The hard drive and floppy disks in a computer work in a similar way.
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The magnetized needle of a compass tries to align itself towards the Earth’s field of magnetic force. The magnetized needle balances on a pointed pin or floats on oil to allow it to rotate freely and to reduce friction. A compass needs to be used with care, because if it is near any object that can distort the Earth’s magnetic field it will give a false reading. A nearby object made of iron or steel, or any source of electrical power, will cause the compass needle to swing wildly.
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Iron, steel and nickel are magnetic substances, that is, they can be attracted by another magnet. Once these materials are in a magnetic field, they act as magnets themselves because the electrons in their atoms become aligned along the magnetic lines of force. This means that you can pick up a whole string of paperclips attached to a single magnet. Once the first paperclip is separated from the magnet, the whole string will collapse because the paperclips are only temporarily magnetized.
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An ordinary magnet has two poles, which are usually referred to as the North Pole and South Pole. Similar (like) poles repel each other, while unlike poles attract each other. You can easily see this happening if you hold two magnets close together. The north pole of one magnet will stick firmly to the south pole of the other magnet, while any two similar poles will be forced apart.
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The area around a magnet where its magnetic force can be detected is called the magnetic field. You can see how this works if you lay a piece of paper over a magnet and sprinkle iron filings on it. The filings immediately arrange themselves in curved lines. You can see how these lines of magnetic force align themselves between the two poles, or ends, of the magnet.
Is the Earth a magnet?
The Earth is actually a huge magnet, with a magnetic north and a magnetic south pole. These poles are not quite the same as the true geographical poles, and they wander about slightly. There is geological evidence that the magnetic north and south poles sometimes switch their positions completely. The reasons for the Earth’s magnetism are not really understood, but it is thought to be due to the movement of electrical charges around the Earth’s core, which is probably mostly made up of iron.
The Earth’s magnetic field extends out into space. The Sun and the other planets in the Solar System also have magnetic fields. Some distant stars are known to have exceptionally powerful magnetic fields.
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Electrical currents are able to influence other electrical currents, and this force is called magnetism. Permanent magnets are materials in which this magnetic effect occurs because of the natural movement of electrons. Magnetism can also be caused by the flow of an electrical current through wires. The magnetic force of a magnet can repel (push) or attract (pull) another magnet or magnetic material.
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When electrons flow through a conductor they collide with atoms and electrical energy is gradually lost in the form of heat. This process is called electrical resistance. However, when some substances become very cold, all electrical resistance is lost and the current flows freely without loss of energy. These substances are called superconductors. Their use allows some devices to work faster and more efficiently than ever before. Because of the difficulty in keeping superconductors cold, their use is limited. In the future, superconductors may be used to make efficient computers.
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