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### Energy

There are formulas for calculating a numerical quantity, we call *energy*. The joule J is its unity.

##### Explanation

In physics we have no knowledge of what energy is. It is a mathematical principle that has a large number of different forms:

• Chemical energy

• Elastic energy

• Electrical energy

• Gravitational energy

• Heat energy

• Internal energy

• Kinetic energy

• Mass energy

• Momentum energy

• Nuclear energy

• Photon energy

• Potential energy

• Radiant energy

• Thermal energy

• Work energie

When we observe a system in nature and calculate the quantity again, it always stays the same. The *conservation of energy* is one of the most basic laws of physics.

##### Additional information

The joule is named after the English physicist James Prescott Joule.