In physics, energy (from the Greek ἐνέργεια - energeia, "activity, operation", from ἐνεργός - energos, "active, working") is a quantity that can be assigned to any particle, object, or system of objects as a consequence of its physical state. Different forms of energy include kinetic, potential, thermal, gravitational, sound, elastic and electromagnetic energy. The forms of energy are often named after a related force. German physicist Hermann von Helmholtz established that all forms of energy are equivalent - energy in one form can disappear but the same amount of energy will appear in another form. Energy is subject to a conservation law. Energy is a scalar physical quantity. In the International System of Units (SI), energy is measured in joules, but in some fields other units such as kilowatt-hours and kilocalories are also used.

Any form of energy can be transformed into another form. When energy is in a form other than thermal energy, it may be transformed with good or even perfect efficiency, to any other type of energy. With thermal energy, however, there are often limits to the efficiency of the conversion to other forms of energy, due to the second law of thermodynamics. As an example, oil is reacted with oxygen, potential energy is released, since new chemical bonds are formed in the products which are more powerful than those in the oil and oxygen. The released energy resulting from this process may be converted directly to electricity (as in a fuel cell) with good efficiency. Alternately it may be converted into thermal energy, if the oil is simply burned in order to heat the combustion gases to a certain temperature. In the latter case, however, some of the thermal energy can no longer be used to perform work at that temperature, and is said to be "degraded." As such, it exists in a form unavailable for further transformation. The remainder of the thermal energy may be used to produce any other type of energy, such as electricity.

In all such energy transformation processes, the total energy remains the same. Energy may not be created nor destroyed. This principle, the conservation of energy, was first postulated in the early 19th century, and applies to any isolated system. According to Noether's theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time.

Although the total energy of a system does not change with time, its value may depend on the frame of reference. For example, a seated passenger in a moving airplane has zero kinetic energy relative to the airplane, but non-zero kinetic energy (and higher total energy) relative to the Earth.

Energy is a fundamental concept in physics that is often defined as the capacity to do mechanical work — a definition disputed by many physicists. It also has a specific connotation of the forms of energy used for powering technological society.

Sourced

• It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount.
  • Richard Feynman, in The Feynman Lectures on Physics (1964) Volume I, 4-1
• Hold somebody's hand and feel its warmth. Gram per gram, it converts 10 000 times more energy per second that the sun. You find this hard to believe? Here are the numbers: an average human weighs 70 kilograms and consumes about 12 600 kilojoules / day; that makes about 2 millijoules / gram.second, or 2 milliwatts / gram. For the sun it's miserable 0.2 microjoules / gram.second. Some bacteria, such as the soil bacterium "Azotobacter" convert as much as 10 joules / gram.second, outperformung the sun by a factor 50 million. I am warm because inside each of my body cells there are dozens, hundreds or even tousands of mitochondria that burn the food I eat.
  • Gottfried Schatz, in "Jeff's view on science and scientists", Amsterdam, Elsevier Butterworth-Heinemann, 2006, ISBN 978-0-444-52133-0, ISBN 0-444-52133-X (pbk.), p. 43, "The tragic matter"

Unsourced

• For those who want some proof that physicists are human, the proof is in the idiocy of all the different units which they use for measuring energy.
  • Richard Feynman

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Wed Jul 14 12:48:09 2010