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Being a measurable property, an energy manifests as an ability to perform work or action (either physical or mental). It is also synonymous with activity, power or strength. There are different forms of energy and some of them are overlapping:

- Heat or thermal energy
- Kinetic energy
- Potential Energy (stored)
- Mechanical Energy
- Light
- Electrical energy
- Magnetic Energy
- Chemical Energy
- Nuclear Energy

A said object possesses more than one type of energy at all times. Energy can also be classified as renewable and non-renewable. According to the law of conservation of energy, it only can be transferred and converted, but not be created or destroyed, and thus, the total amount of energy in a system is a constant variable. Energy is related to mass and velocity. The Massâ€“Energy Equivalence Principle indicates that whatever has a mass has an equivalent amount of energy and the other way around. Units for different kinds of energy: Food calorie, Gram calorie, Electron volt, joule, British Thermal Unit (BTU), Kilowatt-hour, Therm, foot-pound, Erg, Hartree, a tonne of oil equivalent (toe), Megaton TNT equivalent etc.

Kinetic energy (one determined by the motion of an object) and potential (represents the potential of an object to have and perform a movement, it depends on position) energy are categories that include all other forms of energy. In addition, the combination of kinetic and potential energy is called mechanical energy.

Obsolete units of energy: Poncelet, sthene, technical atmosphere.

(some results are rounded)

BTUs | calories | Ergs | Gallons of Gas | Joules | Kilocalories | Kilojoules | Kilowatt Hours | |
---|---|---|---|---|---|---|---|---|

1 BTU = | 1 | 252.1644 | 1.0551 x 10^{10} |
8.7719 x 10^{-6} |
1055 | 0.2522 | 1.0551 | 0.00029307 |

1 calorie = | 0.003966 | 1 | 41840000 | 3.4787 x 10^{-8} |
4.184 | 0.001 | 0.004184 | 1.1622 x 10^{-6} |

1 Erg = | 9.4782 x 10^{-11} |
2.3901 x 10^{-8} |
1 | 8.3142 x 10^{-16} |
1 x 10^{-7} |
2.3901 x 10^{-11} |
1 x 10^{-10} |
2.7778 x 10^{-14} |

1 Gallon of Gas = | 114000 | 28746746 | 1.2028 x 10^{15} |
1 | 120276384 | 28747 | 120276 | 33.4101 |

1 Joule = | 0.00094782 | 0.239 | 10000000 | 8.3142 x 10^{-9} |
1 | 0.00023901 | 0.001 | 2.7778 x 10^{-7} |

1 Kilocalorie = | 3.9657 | 1000 | 4.184 x 10^{10} |
3.4787 x 10^{-5} |
4184 | 1 | 4.184 | 0.001162 |

1 Kilojoule = | 0.9478 | 239.0057 | 1 x 10^{10} |
8.3142 x 10^{-6} |
1000 | 0.239 | 1 | 0.00027778 |

1 Kilowatt Hour = | 3412 | 860421 | 3.6 x 10^{13} |
0.0299 | 3600000 | 860.4207 | 3600 | 1 |

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