Energy transformation

# Overview
In physics, the term energy describes the amount of work which may be done by forces within a system. Energy changes within systems can be accomplished by adding or subtracting energy from them, as energy is a quantity which is conserved. Changes in the energy of systems may also coincide with changes in the system's mass.
Energy in a system may be transformed so that it resides in a different state. Energy in many states may be used to do a variety of physical work. Energy may be used in natural processes, machines, or to provide some service to society (such as heat, light, or motion). For example, an internal combustion engine converts the potential chemical energy in gasoline and oxygen into heat, which is then transformed into the propulsive energy (kinetic energy) that moves a vehicle. A solar cell converts solar radiation into electrical energy that can then be used to light a bulb or power a computer.

# Potential to kinetic energy
For every transformation of energy from one form into another there is usually a reverse process. When energy can be released in an exo-energetic form, more kinetic energy is released than was present previously. The reverse process (endo-energetic) transforms kinetic energy into potential energy, for example, nuclear fusion converts hydrogen nuclei, heat and pressure, usually with a catalyst, into heavier isotopes.

## Nuclear decay
In nuclear decay, potential energy is released from within the nuclei of heavy isotopes (such as uranium and thorium). The rate with which the energy is released can be dramatic. When these heavy isotopes are exposed to significant levels of neutrons, the transformation can be induced to release energy over very short times, as in a nuclear power reactor or nuclear fission bombs.
The reverse process (nuclear fusion) can transform lighter isotopes such as hydrogen and helium up through iron into heavier isotopes. It usually requires heat and pressure or like other processes, a catalyst.

# Forms or classes of energy and conversions
- Thermoelectric (Heat  Electricity)
- Geothermal power (Heat Electricity)
- Heat engines, such as the internal combustion engine used in cars, or the steam engine (Heat  Mechanical energy)
- Ocean thermal power (Heat  Electricity)
- Hydroelectric dams (Gravitational potential energy  Electricity)
- Electric generator (Kinetic energy or Mechanical work   Electricity)
- Fuel cells (Chemical energy  Electricity)
- Lamp (Chemical energy  Heat and Light)
- Microphone (Sound  Electricity)
- Nuclear reactor (Nuclear energy  Heat)
- Nuclear power plant (Nuclear energy  Heat  Electricity)
- Fossil fuel power plant (Chemical energy  Heat  Electricity)
- Solar cells (Light  Electricity), including downconversion and upconversion
- Solar thermal (Light  Heat)
- Wave power (Mechanical energy  Electricity)
- Windmills (Wind energy  Electricity or Mechanical energy)
- Piezoelectrics (Strain   Electricity)
- Acoustoelectrics (Sound   Electricity)
- Friction (Kinetic energy  Heat)

# Examples of energy conversion pathways in machines
A coal-fired power plant involves these power transfers:
- chemical energy in the coal converted to thermal energy,
- thermal energy converted to kinetic energy in steam,
- kinetic energy converted to mechanical energy in the turbine,
- mechanical energy of the turbine converted to electrical energy, which is the ultimate output.
In such a system, the last step is almost perfectly efficient, the first and second steps are fairly efficient, but the third step is relatively inefficient.
In a conventional automobile, these power transfers are involved:
- potential energy in the fuel is transformed to kinetic energy of expanding gas via combustion,
- kinetic energy of expanding gas is converted to linear piston movement,
- linear piston movement is converted to rotary crankshaft movement,
- rotary crankshaft movement is conveyed by the transmission assembly,
- rotary movement is passed out of transmission assembly,
- rotary movement is passed through the differential,
- rotary movement is passed out of the differential to drive wheels,
- rotary movement of the drive wheels converts to linear motion of the vehicle.

# Acknowledgements
The content on this page was first contributed by: Henry A. Hoff.
Initial content for this page in some instances came from Wikipedia.