Category Science & Technology

HEAT MOVES

Heat can move around and between objects in three main ways. One is conduction, when heat energy passes between two objects in physical contact. When you touch an object to see how warm it is, you receive some of its heat by conduction. A second way is by convection. This only happens in liquids and gases. As some of the atoms or molecules receive heat energy and become warm they spread out more. The heated part of the liquid or solid is now less dense than its cooler surroundings so it rises or floats. As it rises, it carries its heat energy in the form of convection current. You can feel this as warm air rising from a central heating radiator.

The third way that heat moves is by radiation. It is in the form of infrared waves which are part of a whole range of waves, including radio waves, light and X-rays, known as the electromagnetic spectrum. Conduction and convection both need matter to transfer heat. Radiation does not. Infrared waves can pass through space, which is how the Sun’s heat reaches Earth.

Like light waves, infrared waves reflect from light-coloured or shiny surfaces. On a hot day, light-coloured clothes reflect the Sun’s warmth and keep you cooler than dark clothing, which absorbs the warmth. Substances that slow down conduction and convection, such as wood, plastic and glass fibre, are called thermal insulators. Layers of fat, or blubber in a whale, are good insulators.

The faster an aircraft goes, the greater the heat from friction with air. Very fast planes like the X-15 rocket have special heat-radiating paint that gives out heat as fast as possible, to prevent the metal skin of the plane melting at high speed.

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HEAT

How warm is the weather today? It may be cold and wintry or hot and summery. Heat is a vital part of our lives. We need to keep our bodies comfortably warm with clothing, especially in cold conditions. If body temperature falls from its normal 37°C to below about 30°C, fatal hypothermia may set in.

We cook our food with heat using gas or electricity. Countless machines and industrial processes use heat, from making pottery or a photocopy to a steelworks or power station. Heat is also given off as a waste form of energy by many processes. In a power station most of the heat is used to generate electricity, but some is released as clouds of steam from huge cooling towers.

Heat is a type of energy – the vibrations of atoms and molecules. The more an atom moves or vibrates, the more heat or thermal energy it has. In a solid, the atoms have fixed central positions but each atom vibrates slightly about its central position, like a ball tied to a nail by elastic. Heat the solid and the atoms vibrate more. When they have enough vibrations, the atoms break from their fixed positions (the “elastic” snaps), and they move about at random. The solid has melted into a liquid. Heat it more and the atoms fly further apart. The liquid becomes a gas.

 TEMPERATURE

Cold is not the presence of something that opposes heat, but simply the lack of heat. Temperature is not the same as heat. Heat is a form of energy, while temperature is a measure of how much heat energy a substance or object contains. A slice of apple pie at 40°C contains more heat energy than a same-sized slice of the same pie at 30°C. We can judge its temperature quite accurately when we touch the slice with our skin, and especially with our fingertips or lips. But this judgement is only safe within a certain range. Temperatures greater than about 50°C or lower than about -10°C cause pain and may damage the skin. We measure temperatures accurately using devices called thermometers.

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CONSERVING ENERGY

Energy can be changed or converted from one form to another. But it is never destroyed or created, lost or gained. It is conserved – the amount stays the same. At the end of a process or event, the total amount of energy is the same as at the beginning. For example, the chemical energy in a car’s petrol is converted into the same amount of energy as the car’s motion, heat and sound. The principle of energy conservation means the total amount of energy in the Universe is always the same.

Another form of energy is matter itself. Matter can be converted into energy and energy can be changed into matter. This conversion is used in nuclear power stations. A nuclear particle called a neutron smashes into the nucleus of a uranium atom (1). The nucleus breaks into two parts (2). This releases large amounts of heat and other energy and also two more fast-moving neutrons (3). These smash into more uranium nuclei and so on in a chain reaction (4). Splitting of nuclei is known as nuclear fission. During the process bits of matter cease to exist and become vast quantities of energy instead.

A similar process of changing matter into energy happens naturally in the Sun. The Sun is made mainly of hydrogen. Tremendous temperatures and pressures at its centre squeeze or fuse together the nuclei of the atoms (1) to form the nucleus of a helium atom (2).Vast amounts of energy are given off (3) which emerge from the Sun mainly as light and heat. A neutron may also be given off to continue the reaction (4). Since the nuclei join or fuse, this is called nuclear fusion. Compared to fission used in our nuclear power stations, fusion power would cause less radioactive wastes and pollution. Fusion power may be the energy source of the future.

            Geothermal energy from hot rocks deep in the Earth causes geysers, jets of hot water and steam. This form of energy will last millions of years.

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ENERGY

Energy is the ability to make things happen, cause changes and carry out work. Any change anywhere in the Universe, from a tiny meteorite hitting a planet to an exploding star, means that energy is at work. In daily life, energy is all around us in many different forms. Light and sound energy travel through the air as waves. Heat is a form known as thermal energy. Movement or motion is, too, and is called kinetic energy. Objects even have energy because of their place or position. This is called potential energy. A boulder on a hilltop has potential energy because gravity tries to pull it down. As the boulder begins to roll its potential energy changes into kinetic energy.

Energy can cause changes and it can change itself. It can convert between one form and another. The boulder rolls down the hill, converting some of its potential energy to kinetic energy. Water also flows downhill with kinetic energy. We can harness this kinetic energy in a hydro-electric power station and convert it into electrical energy, yet another form of energy. Electricity is very useful in our modern world. It can be transported long distances along wires. It can be converted to other forms of energy, like light from a light bulb, heat in an electric kettle and sound from a loudspeaker.

Matter contains chemical energy, in the links or bonds between atoms. The bonds need energy to form and they release this energy when they are broken. We make use of chemical energy in fuels such as petrol. The bonds break as the fuel burns and releases heat.

Energy from the Sun bathes our world. It is in two main forms, light and heat. It takes more than 8 minutes to travel nearly 150 million kilometres through space to Earth.

Energy is all around, present in different forms and changing from one form to another. Without energy our world would be completely dark, cold, still and silent.

The human body needs energy to drive its life processes like heartbeat, breathing and movement. The energy is present in chemical form as the nutrients in our food. We digest the food to obtain the energy and store it as body starches and sugars.

Chemical energy in the body in the form of blood sugar is taken to muscles. The muscles convert it into the energy of motion so we can move about.

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