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Energy is defined as the capacity of a physical system to perform work. However, it's important to keep in mind that just because energy exists, that doesn't mean it's necessarily available to do work.
Forms of Energy
Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, and electrical energy.
- Heat - Heat or thermal energy is energy from the movement of atoms or molecules. It may be considered as energy relating to temperature.
- Kinetic Energy - Kinetic energy is the energy of motion. A swinging pendulum has kinetic energy.
- Potential Energy - This is energy due to an object's position. For example, a ball sitting on a table has potential energy with respect to the floor because gravity acts upon it.
- Mechanical Energy - Mechanical energy is the sum of the kinetic and potential energy of a body.
- Light - Photons are a form of energy.
- Electrical Energy - This is energy from the movement of charged particles, such as protons, electrons, or ions.
- Magnetic Energy - This form of energy results from a magnetic field.
- Chemical Energy - Chemical energy is released or absorbed by chemical reactions. It is produced by breaking or forming chemical bonds between atoms and molecules.
- Nuclear Energy - This is energy from interactions with the protons and neutrons of an atom. Typically this relates to the strong force. Examples are energy released by fission and fusion.
Other forms of energy may include geothermal energy and classification of energy as renewable or nonrenewable.
There may be overlap between forms of energy and an object invariably possesses more than one type at a time. For example, a swinging pendulum has both kinetic and potential energy, thermal energy, and (depending on its composition) may have electrical and magnetic energy.
Law of Conservation of Energy
According to the law of conservation of energy, the total energy of a system remains constant, though energy may transform into another form. Two billiard balls colliding, for example, may come to rest, with the resulting energy becoming sound and perhaps a bit of heat at the point of collision. When the balls are in motion, they have kinetic energy. Whether they are in motion or stationary, they also have potential energy because they are on a table above the ground.
Energy cannot be created, nor destroyed, but it can change forms and is also related to mass. The mass-energy equivalence theory states an object at rest in a frame of reference has a rest energy. If additional energy is supplied to the object, it actually increases that object's mass. For example, if you heat a steel bearing (adding thermal energy), you very slightly increase its mass.
Units of Energy
The SI unit of energy is the joule (J) or newton-meter (N * m). The joule is also the SI unit of work.