Energy is defined as the capacity to do work, while work is a force acting on an object that causes its displacement (i.e. force × distance). Both energy and work are measured in joules or, more fundamentally, in newton metres. Power (measured in watts, i.e. joules per second) is the rate of doing work or the rate at which energy changes from one form to another.
The Sun is by far the most important source of natural energy on Earth. The solar radiation that reaches the Earth contributes to winds, waves, atmospheric water circulation, atmospheric heating and surface water evaporation, and to organic activity.
The gravitational attractions of the Sun and the Moon combine to produce tides, and rocks in the Earth's interior also generate heat by the decay of radioactive isotopes in them. These are small but potentially exploitable sources of energy.
Most of the world's carbon is locked within carbonate rocks. A large amount of carbon also exists as preserved organic carbon, which includes fossil fuels.
Green plants use solar radiation to build carbohydrates and plant tissue from carbon dioxide and water in the atmosphere and dissolved in the oceans, in a process known as photosynthesis. Photosynthesis releases oxygen into the atmosphere and oceans. When they respire, organisms use oxygen to generate energy from food, releasing carbon dioxide and water vapour back into the atmosphere and oceans. These respiratory reactions are the reverse of photosynthesis.
Concentrations of marine phytoplankton occur in the upper sunlit layers of the oceans where upwelling currents bring nutrients. These form the basis of the marine food chain. There is only a build-up of carbon within marine sediments where there is an adequate supply of organic material and where physical conditions are right for its preservation.
The world is plentifully supplied with solar-derived energy, but most of it is not in a sufficiently concentrated form to be useful to modern industrial society.
Fuels are of immense value because they are concentrated sources of energy (they have a high energy density) that can easily be stored, transported and used at will. At the current level of technology, energy transport has become an essential aspect of energy use, dominated by electricity. The most convenient way of electricity generation is through the conversion of mechanical motion — most usually produced today from thermal energy — using generators driven by turbines.
Energy sources can be subdivided into renewables, like solar, wind and wave power, and non-renewables, like peat, coal, oil and gas. Renewables are effectively everlasting, but non-renewables are finite.