My Science School

         

     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.

 

               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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                Electrical currents normally flow in one direction through an electrical conductor. This kind of electricity, which is produced by batteries for example, is called direct current. When the electrical current is made to reverse its direction rapidly, it is known as alternating current. This kind of electrical current is used in domestic wiring, and it usually switches direction and back again about 50 to 60 times per second. This is the cause of the hum that you can sometimes hear near electrical equipment such as fluorescent lights.

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            A good conductor such as copper has only very low electrical resistance. It does not get very hot when electricity flows through it, making copper suitable for household wiring.

            Other metals such as iron and nickel have much greater resistance to the passage of electricity, so they become hot. A very thin wire has more resistance than a thick one, and a long wire has more resistance than a short wire. In an electric fire, coils of thin, high-resistance wire glow and produce heat when an electrical current passes though them.

            In a light bulb, coils of an extremely thin conductor ensure that heat cannot be radiated away quickly enough and some of the energy is converted to light.

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               Batteries produce electricity by means of chemical action. A battery contains two different conductors, or electrodes. Usually, one of these is the metal case of the battery. The electrodes are separated by a conducting liquid or paste, called the electrolyte. The substances in the battery react chemically with each other to produce an electrical current. As a result of chemical activity a positive charge builds up at one electrode, and this can flow through a conductor such as a wire to the other (negative) electrode. Eventually the chemical energy runs out and the battery becomes exhausted.

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               Energy cannot be created. An electrical generator is simply a means of converting mechanical energy into electrical energy. In its simplest form, a generator spins coils of wire in a magnetic field, causing the flow of electrical current in the conducting wire. The power to spin the generator comes from other forms of energy. This energy might be stored energy in fossil fuels such as coal or oil, hydroelectric power from dams, wind power from turning huge windmills, or nuclear power produced by the radioactive decay of elements. In all these cases, one form of energy is simply converted into electrical energy.

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               Some materials do not have the loosely attached electrons that are needed to conduct electricity — these substances are called insulators. Rubber, most plastics, ceramics and glass are examples of good insulators. They do not allow the passage of electrical current, and so they are used to cover electrical wiring or to prevent electrical current leaking away. The more free electrons that are present in a conductor, the better it will be at conducting electrical current. Metals and many liquids are very good conductors of electricity, and some gases conduct electricity when they are very hot.

               Electricity powers our lights, heating, electronic appliances such as computers and television, and a host of other essential services that we take for granted. However, electricity has much more important aspects because it is a fundamental feature of all matter. Electricity is the force that holds together the molecules and atoms of all substances.

               The type of electricity that is most familiar to us is electrical current. This is the flow of electrical charges through a substance called a conductor, such as a metal wire. This flow happens because some of the negatively charged electrons circling the nuclei of the conductor’s atoms are held loosely. The electrons can move from one atom to the next, producing an electrical current.

               Gravity is the force that holds us on the Earth. It is also the force that keeps the Earth from flying off into space as it orbits the Sun. The larger the mass of any object the more it attracts other objects with the force of its gravity. The Sun’s gravity helps to pull the Earth’s oceans, causing tides.

               Astronauts in space experience zero gravity, because they orbit the Earth so quickly that the effects of gravity are neutralized. The Moon is much smaller than the Earth, and so it produces less gravity. Astronauts on the Moon weighed only one-sixth of their normal weight on the Earth, even though their body mass remained the same.

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               Rainbows are formed when sunlight falling on raindrops is split into the different colours of the spectrum (range) of light. Sunlight is really a mixture of all the colours of the spectrum: red, orange, yellow, green, blue, indigo and violet, but our eyes always see them as ordinary white light.

               When a beam of light passes at an angle through a curved transparent surface, such as a raindrop, the beam is bent when it emerges. The different colours of light are bent by different amounts, so the white light is split into the colours of the spectrum. This effect can also be seen when light passes through a piece of glass cut at different angles, such as a prism.

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               Infrared radiation, or heat, is a form of electromagnetic energy that is not visible to our eyes. The wavelength of infrared radiation is greater than that of visible light, but much shorter than the wavelength of microwaves. All forms of heat are based on infrared radiation. Infrared photography allows objects to be seen in the dark. It uses cameras that are sensitive to infrared radiation and so they can detect warm objects in the dark.

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               Microwaves are a form of radiation. They can pass through things that would block ordinary radio waves, such as rain and fog. Microwaves can also be focused and sent in a narrow beam, making them very useful for transmitting radio messages over long distances. Microwaves are widely used to transmit television signals to and from orbiting satellites in space, and they are also the basis on which modern mobile telephones work.

               In radar, a beam of microwave energy scans the target area and bounces back signals, producing a picture of the object. Microwaves penetrate the food in a microwave oven, causing the molecules to move about very rapidly. This rapid movement generates the heat that cooks the food.

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