Our food supplies us with energy. Our transport and industries rely on energy from oil and other fuels. Without energy, nothing would ever get done. But what is energy? What happens to it when we have used it?
What is Energy?
Energy is what makes things move. Food has energy because your body can use it to make your muscles move. Fuels use it to make your muscles move. Fuels have energy because engines can use them to produce motion. Batteries have energy because they can make electric motors spin round.
Heat is a form of energy. This may seem puzzling because an electric fire does not appear to move when it gets hot. In fact, there is movement, but you cannot see it. The atoms in any solid are always vibrating. If the solid is heated up, this atoms vibrate faster. This means that billions of tiny forces are giving the atoms extra speed.
Scientists measure energy in Joules(J). A joule is about as much energy as you would need to lift this book about 10 cm. Lifting more books would require more energy. Lifting them over a greater distance would take more energy still.
Whenever a force moves something, scientists say that work is done. The bigger the force and the further it moves, the more work is done. This means that there is a link between energy and work. If something has energy, it can do work. Like energy, work is measured in joules. In lifting this book about 10 cm, you would be doing about one joule of work.
So, energy is not really a 'thing' at all. An energy value in joules is a 'blank balance' of work that can be 'spent' in the future. The work may produce motion which you can see, like an aircraft taking off. Or it may produce motion which you cannot see, as happens when an electric fire heats up and atoms move faster.
Forms of Energy
Energy can exist in many different forms, but whatever form it takes, it can be used to make things move. there may be large forces producing motion you can see, or small forces producing motion you cannot see, like the movements of atoms or their particles.
These are the main forms of energy(with examples of things that have them):kinetic energy(anything moving):potential energy(anything stretched, compressed, or held above the ground); heat energy(things which can cool down); chemical energy(food, fuels, batteries); radiant energy(light, sound); electrical energy(electric currents); nuclear energy(nucleus of an atom)
Changing but Keeping
Energy can change from one form to another. Think about the energy changes which occur before and after you eat an apple. The apple tree has absorbed energy radiated from the Sun. As the apple grew, some of this was stored as chemical energy. When you eat the apple, its chemical energy is stored in your body. You release this energy by a chemical reaction called respiration. As a result, you can move your muscles. So some of the chemical energy is changed into kinetic energy.
You can think of this process as an energy chain, with energy being changed into different forms as it passes along the chain. However, at each stage of the chain, some energy is wasted as heat. Even the kinetic energy you have when you move about is eventually changed into heat, as friction from the ground and the air finally slows you down.
Scientists have studied many energy chains. As a result, they have discovered a law about energy: it can change into different forms, but it can never be made, and it can never be lost or destroyed. This is called the law of conservation of energy. Conservation means 'keeping'. The law has this name because it tells you that the same total amount of energy is kept all the way along a chain.
Everything that happens in the Universe is part of an energy chain. We often talk about 'using energy', but we never really use it up. We just pass it on in some other form. Eventually it ends up as heat, but this becomes so spread out that it can be impossible to detect or use.
Supplying the Energy
Industrial societies need a huge amount of energy to run their factories, homes, and transport systems. Much of this energy comes from burning oil, coal, and natural gas. The majority of power stations use these fuels, and most car, buses and trucks run on petrol or diesel fuel, both of which are made from oil.
Oil, coal, and natural gas are extracted from the ground. They are called fossil fuels because they were formed from the remains of plants and tiny sea creatures which lived on Earth many millions of years ago. The decaying remains became buried and then crushed. More and more mud built up above them and eventually turned to rock, which trapped the fossil fuels underneath.
The Earth has only limited supplies of fossil fuels. Once gone, there will be no way of replacing them. At present rate of use, there is probably enough oil and natural gas left to last 50 years or so, and enough coal to last about 300 years.
Apart from supply problems, fossil fuels have a heavy environmental cost. When they burn, they pollute the atmosphere. Some pollutants can be removed by, for example, catalytic converters on cars and desulphurization units in coal-burning power stations. However, even the cleanest exhaust gases still contain plenty of carbon dioxide gas. In the atmosphere, carbon dioxide traps the Sun's heat, rather like the glass in a greenhouse. This adds to global warming, which is also called the greenhouse effect. In nuclear power stations, the heat to raise steam for the turbines comes not from burning fuel, but from fission in nuclear reactor. Power stations like this do not produce large amounts of polluting gases. However, they are expensive to build, and expensive to shut down at the end of their working life. They need extremly high safety standards, and also produce radioactive waste which is difficult to dispose of safely.
There are some alternatives to fossil fuels and nuclear power. Hydroelectric schemes generate electricity using the potential energy of water behind a dam. Aerogenerators(generators driven by windmills)use the kinetic energy of the wind. Solar panels absorb the radiant energy from the Sun and use it to heat up water.
One advantage of these alternative energy sources is that they are renewable-when the energy has been delivered, there is always more to take its place. However, one disadvantage is that none can challenge oil as a convenient and concentrated source of energy for vehicles. Electricity from power stations can be used to charge up batteries in electric cars, but manufacturers are reluctant to make electric cars because they cannot offer the performance of fuel-burning vehicles.
Waste or Save?
In industrialized countries, people tend to be very wasteful with energy. However, there are some ways that savings can be made.
Transporting people by car takes twice as much energy per person as transporting them by bus. If a car is only carrying one person, the figure rises to eight times. So encouraging people to use public transport is one way of saving energy.
Fuel-burning power stations waste more than a half of their energy as heat. Production of this heat is unavoidable, but it can be used to supply the surrounding district with hot water for heating systems.
Household rubbish is often dumped in landfill sites, such as old quarries. But it can be used as a source of energy. Rotting waste gives off methane gas, which can be collected and used as a fuel. Some power stations can burn waste directly as fuel.
Without good insulation, houses waste energy. The walls and roof account for nearly half the energy lost. Draught accounts for about a third. Heat losses and draughts can be reduced, but with people living in the house, there must be regular air changes.
Energy from The Sun
In one way or another, nearly all of our energy has come from the Sun. Plants absorb the energy in sunlight, so their energy comes from the Sun. Animals including humans feed on plants or on animals which have fed on plants, so their energy also comes from the Sun. Fossil fuels contain energy from the Sun which was trapped by plants many millions of years ago. Even the energy from wind and water originally came from the Sun. Winds arise because the Sun heats some parts of the Earth more than others, and this makes air move. In hydroelectric schemes, where lakes are filled by rainwater, the rain got into the air in the first place because the Sun's heat made water evaporate from rivers, seas, and oceans.
Where does the Sun get its energy?
The Sun has been shining for 4500 million years. Humans have been relying on its heat and light for over a million years. But only during the last century has the secret of its energy source been discovered.
By the middle of the 19th century, scientists had measured the Sun's heat output. From geological evidence, they had also realized that the Earth was probably many hundreds of millions old. The Sun must have been heating the Earth for all this time. How could it have been pouring out energy for so long? No known process could explain it; if the Sun burnt its fuel, like a fire, ,its chemical energy would have long since run out. Science had to come up with some new energy source!
By the early 1900s, researchers were investigating the recently discovered phenomenon of radioactivity. In time, it became clear that the nucleus of an atom could release energy, and that this might be the source of the Sun's energy. Today, we know that the Sun gets its energy from nuclear fusion. Deep in core, hydrogen nuclei combine to form helium nuclei, releasing huge amounts of energy as they do so. Nuclear energy has kept the Sun shining for 4500 million years - and should keep it shining for another 6000 million.
Imitating the Sun
Scientists and engineers are now trying to design fusion reactors which can be used as an energy source on Earth. However the technical problems are immense. For fusion to start, atoms must collide at very high speeds, which means heating gas to a temperature of 100 million centigrade or more. The super hot gas, called a plasma, cannot be held in any ordinary container, so it will have to be trapped by a strong magnetic field. In the Sun, fusion happens at only 15 million centigrade, but the Sun uses a different fusion process which would not give enough energy if scales down to the conditions possible on Earth.
Fusion reactors will have many advantages over today's nuclear power station. Some of the hydrogen they need can be extracted from sea water, Their main waste product, helium, is not radioactive, so there is no long-term storage problem with nuclear waste. Also the reactors have built-in safety: if a fault develops, fusion stops automatically.
One day, fusion reactors may meet much of the world's energy needs. Until then, existing energy resources must be managed carefully so that supplies do not run out and pollution is kept to a minimum.
1. What is energy?
2. In what unit energy can be measured by scientists?
3. Would you explain the law of conservation of energy?
4. Resultantly all the energy comes from, where?
5. How the Sun can generate that powerful energy for a long time?
6. What is the Green House effect?