My Science School

 

                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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How do we measure temperature?

               Temperature is a measurement of the amount of heat that is stored in an object. It is measured against a scale in either degrees Celsius (°C) or degrees Fahrenheit (°F) on a thermometer. The Celsius scale starts at the freezing point of water (0°C) and measures the boiling point of water at 100°C. The centre of the Sun probably reaches an incredible temperature of 150 million °C.

               The absolute minimum temperature possible is —273°C, or absolute zero, where it is so cold that molecules and atoms stop moving altogether. More ordinary temperatures are measured using thermometers that contain a liquid such as mercury or coloured alcohol. Other thermometers use electronic sensors or the expansion of a metal coil to turn an indicating needle.

 

 

 

 

 

 

 

 

 

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               There is a basic law of physics that says that energy is never created or destroyed; it is simply transferred from one place to another or from one form of energy to another. So all the energy that exists today has been around since the formation of the Universe.

               The Earth’s energy comes from heat trapped inside the Earth when it was originally formed, or from energy radiated out from the Sun. We cannot make energy, but we can extract it from coal, oil, fast-flowing rivers, nuclear fuel and various other sources. Even the wind can supply us with a source of energy.

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What is light?

               Light is a form of electromagnetic energy. It is produced by electrons that have gained extra energy from another source. This energy can be given off as heat or light. Light travels in waves of energy that consist of very tiny particles called photons. Atoms emit (give off) photons when heated to a high temperature. The extreme heat causes the atoms to collide with each other, and the extra energy is given off as light. The amount of energy released determines the colour of the light. The hotter an object, the more high-energy blue light is produced. As the object cools, the lower energy produces red light. When metal is heated to white hot, it gradually becomes red as it cools.

 

How fast does light travel?

               The speed of light is the fastest speed known — light travels through the vacuum of space at 300,000 km per second. Nothing else can travel at this speed, and the theory of relativity, conceived by Albert Einstein, says that nothing can even approach this speed. This means that travel to the stars will not be possible, because it would take hundreds of years to reach them.

 

Why does my breath ‘steam’ in cold weather?

               When water boils the steam it produces is not visible while the water remains at boiling point. As the steam cools it forms tiny droplets of water, making it look cloudy. This is called water vapour, which is what you see when your breath ‘steams’ in very cold weather. Explorers in the Arctic and Antarctic find that water vapour condenses and freezes to form ice around their nostrils and mouth.

 

 

 

What happens when iron rusts?

               The reddish powdery rust that forms on unprotected iron and steel is the result of a process called oxidation. It takes place when the metal reacts with oxygen from the air and water. Both air and water are needed for rusting to take place. This form of rusting eats into the metal until it collapses into scales and dust of iron oxide.

               When aluminium is cut it oxidizes very quickly, but the newly formed layer of aluminium oxide prevents exposure of the metal to more oxygen. The aluminium oxide stops the process of oxidation, so the metal remains bright and shiny.

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What happens when a liquid boils?

               When a liquid is heated, at a certain point it begins to change to a gas, or vapour. This happens because at high temperatures the molecules in the liquid move faster, until they escape into the air. Light molecules escape more easily than heavy a molecule, which means that heavy, thick liquids only boil at very high temperatures.

               The boiling point of a liquid depends on the air pressure. The pressure becomes lower at altitude, so high up on a mountain slope; water boils at a much lower temperature than normal. Water boils at 100°C at sea level, but at only 72°C at an altitude of 3,048 m.

 

What is evaporation?

               Evaporation happens when a liquid or a solid changes to a gas. It is a similar process to boiling, because it involves the molecules of a liquid passing into the air. The process of evaporation is much slower when the air above the liquid is already full of molecules of vapour. For example, water will evaporate only very slowly on a warm, damp day when the air is already saturated with water vapour.

               As a liquid evaporates it loses heat energy, making it cooler. This is the principle on which refrigerators and air conditioners work. Evaporation of water from the seas and land produces water vapour in the form of clouds, which eventually drop water back onto the Earth’s surface as rain.

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