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energy

Capacity for doing work. This work may be as simple as reading a book, using a computer, or driving a car. Without energy no activity is possible. Energy can exist in many different forms. For example, potential energy (PE) is energy deriving from position; thus a stretched spring has elastic PE, and an object raised to a height above the Earth's surface, or the water in an elevated reservoir, has gravitational PE. Moving bodies possess kinetic energy (KE). Energy can be converted from one form to another, but the total quantity in a system stays the same (in accordance with the conservation of energy principle). Energy cannot be created or destroyed. For example, as an apple falls it loses gravitational PE but gains KE. Although energy is never lost, after a number of conversions it tends to finish up as the kinetic energy of random motion of molecules (of the air, for example) at relatively low temperatures. This is ‘degraded’ energy that is difficult to convert back to other forms.

Energy sources

There are two main sources of energy: the Sun, the ultimate source; and decay of radioactive elements in the Earth. Plants use the Sun's energy and convert it into food and oxygen. The remains of plants and animals that lived millions of years ago have been converted into fossil fuels such as coal, oil, and natural gas.

Energy types and transfer

For energy to be useful it has to be converted into a form that can do work. Energy occurs in many forms - for example, potential (stored energy), kinetic (movement), chemical, heat, light, electrical, sound, and nuclear energy.

A flat battery in a torch will not light the torch. If the battery is fully charged, it contains enough chemical energy to illuminate the torch bulb. When one body A does work on another body B, A transfers energy to B. The energy transferred is equal to the work done by A on B. Energy is therefore measured in joules. The rate of doing work or consuming energy is called power and is measured in watts (joules per second). Energy can be converted from any form into another. A ball resting on a slope possesses potential energy that is gradually changed into kinetic energy of rotation and translation as the ball rolls down. As a pendulum swings, energy is constantly being changed from a potential form at the highest points of the swing to kinetic energy at the lowest point. At positions in between these two extremes, the system possesses both kinetic and potential energy in varying proportions. A weightlifter changes chemical energy from the muscles into potential energy of the weight when the weight is lifted. If the weightlifter releases the weight, the potential energy is converted to kinetic energy as it falls, and this in turn is converted to heat energy and sound energy as it hits the floor. A lump of coal and a tank of petrol, together with the oxygen needed for their combustion, have chemical energy. Other sorts of energy include electrical and nuclear energy, light, and sound. However, all of these types are ultimately classifiable as either kinetic or potential energy.

Resources

So-called energy resources are stores of convertible energy. Non-renewable resources include the fossil fuels (coal, oil, and gas) and nuclear-fission ‘fuels’ - for example, uranium-235. The term ‘fuel’ is used for any material from which energy can be obtained. We use up fuel reserves such as coal and oil, and convert the energy they contain into other, useful forms. The chemical energy released by burning fuels can be used to do work. Renewable resources, such as wind, tidal, and geothermal power, have so far been less exploited. Hydroelectric projects are well established, and wind turbines and tidal systems are being developed.

Energy conservation and efficiency

All forms of energy are interconvertible by appropriate processes. Energy is transferred from one form to another, but the sum total of the energy after the conversion is always the same as the initial energy. This is the principle of conservation of energy. This principle can be illustrated by the use of energy flow diagrams, called Sankey diagrams, which show the energy transformations that take place. When a petrol engine is used to power a car, about 75% of the energy from the fuel is wasted. The total energy input equals the total energy output, but a lot of energy is wasted as heat so that the engine is only about 25% efficient. The combustion of the petrol-air mixture produces heat energy as well as kinetic energy. All forms of energy tend to be transformed into heat and cannot then readily be converted into other, useful forms of energy.

Heat transfer

A difference in temperature between two objects in thermal contact leads to the transfer of energy as heat. Heat is energy transferred due to a temperature difference. Heat is transferred by the movement of particles (that possess kinetic energy) by conduction, convection, and radiation. Heat conduction involves the movement of heat through a solid material by the transfer of vibrational energy from atom to atom. For example, thermal energy is lost from a house by conduction through the walls and windows. Convection involves the transfer of energy by the movement of fluid particles. All objects radiate heat in the form of radiation of electromagnetic waves. Hotter objects emit more energy than cooler objects. Methods of reducing energy transfer as heat through the use of insulation are important because the world's fuel reserves are limited and heating homes costs a lot of money in fuel bills. Heat transfer from the home can be reduced by a variety of methods, such as loft insulation, cavity wall insulation, and double glazing. The efficiencies of insulating materials in the building industry are compared by measuring their heat-conducting properties, represented by a U-value. A low U-value indicates a good insulating material.

E = mc²

Mass can be converted into energy under certain conditions, according to Einstein's theory of relativity. This conversion of mass into energy is the basis of atomic power. Einstein's special theory of relativity (1905) correlates any gain, E, in energy with a gain, m, in mass, by the equation E = mc², in which c is the speed of light. The conversion of mass into energy in accordance with this equation applies universally, although it is only in nuclear reactions that the percentage change in mass is large enough to detect.

Burning fossil fuels causes acid rain and is gradually increasing the carbon dioxide content in the atmosphere, with unknown consequences for future generations. Nuclear power stations do not release carbon dioxide but produce highly dangerous waste that must be disposed of, and pose a small but non-zero risk of a catastrophic release of radioactive material. The ultimate non-renewable but almost inexhaustible energy source would be nuclear fusion (the process by which energy is generated in the Sun), but controlled fusion is a long way off. (The hydrogen bomb is a fusion bomb.) Harnessing resources generally implies converting their energy into electrical form, because electrical energy is easy to convert to other forms and to transmit from place to place, though not to store. Almost a third of all energy consumption in Europe is by transport.

activation energy conservation of energy

endothermic reaction energy conservation

energy of reaction energy transfer

potential energy renewable energy

respiration