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Thermal energy is energy of microscopic constituents of matter, which may include both kinetic and potential energy.

In the context of physical sciences, several forms of energy have been identified. These include{{ safesubst:#invoke:Unsubst||$N=Request quotation |date=__DATE__ |$B= {{#invoke:Category handler|main}}[need quotation to verify] }}:

Forms of energy
Type of energy Description
Kinetic (≥0), that of the motion of a body
Potential A category comprising many forms in this list
Mechanical The sum of (usually macroscopic) kinetic and potential energies
Mechanical wave (≥0), a form of mechanical energy propagated by a material's oscillations
Chemical that contained in molecules
Electric that from electric fields
Magnetic that from magnetic fields
Radiant (≥0), that of electromagnetic radiation including light
Nuclear that of binding nucleons to form the atomic nucleus
Ionization that of binding an electron to its atom or molecule
Elastic that of deformation of a material (or its container) exhibiting a restorative force
Gravitational that from gravitational fields
Rest (≥0) that equivalent to an object's rest mass
Thermal A microscopic, disordered equivalent of mechanical energy
Heat an amount of thermal energy being transferred (in a given process) in the direction of decreasing temperature
Mechanical work an amount of energy being transferred in a given process due to displacement in the direction of an applied force


Some entries in the above list constitute or comprise others in the list. The list is not necessarily complete. Whenever physical scientists discover that a certain phenomenon appears to violate the law of energy conservation, new forms are typically added that account for the discrepancy.

Heat and work are special cases in that they are not properties of systems, but are instead properties of processes that transfer energy. In general we cannot measure how much heat or work are present in an object, but rather only how much energy is transferred among objects in certain ways during the occurrence of a given process. Heat and work are measured as positive or negative depending on which side of the transfer we view them from.

Classical mechanics distinguishes between kinetic energy, which is determined by an object's movement through space, and potential energy, which is a function of the position of an object within a field, which may itself be related to the arrangement of other objects or particles. These include gravitational energy (which is stored in the way masses are arranged in a gravitational field), several types of nuclear energy (which utilize potentials from the nuclear force and the weak force), electric energy (from the electric field), and magnetic energy (from the magnetic field).

Other familiar types of energy are a varying mix of both potential and kinetic energy. An example is mechanical energy which is the sum of (usually macroscopic) kinetic and potential energy in a system. Elastic energy in materials is also dependent upon electrical potential energy (among atoms and molecules), as is chemical energy, which is stored and released from a reservoir of electrical potential energy between electrons, and the molecules or atomic nuclei that attract them. {{ safesubst:#invoke:Unsubst||$N=Request quotation |date=__DATE__ |$B= {{#invoke:Category handler|main}}[need quotation to verify] }}.

Potential energies are often measured as positive or negative depending on whether they are greater or less than the energy of a specified base state or configuration such as two interacting bodies being infinitely far apart.

Wave energies (such as light or sound energy), kinetic energy, and rest energy are each greater than or equal to zero because they are measured in comparison to a base state of zero energy: "no wave", "no motion", and "no inertia", respectively.

It has been attempted to categorize all forms of energy as either kinetic or potential, but as Richard Feynman points out:

These notions of potential and kinetic energy depend on a notion of length scale. For example, one can speak of macroscopic potential and kinetic energy, which do not include thermal potential and kinetic energy. Also what is called chemical potential energy is a macroscopic notion, and closer examination shows that it is really the sum of the potential and kinetic energy on the atomic and subatomic scale. Similar remarks apply to nuclear "potential" energy and most other forms of energy. This dependence on length scale is non-problematic if the various length scales are decoupled, as is often the case ... but confusion can arise when different length scales are coupled, for instance when friction converts macroscopic work into microscopic thermal energy.

Also, at relativistic speeds, defining kinetic energy is problematic because the energy due to the body's motion does not simply contribute additively to the total energy as it does at classical speeds.

Energy may be transformed between different forms at various efficiencies. Items that transform between these forms are called transducers.


Forms of energy sections
Intro  Mechanical energy  Kinetic energy  Potential energy  Mechanical work  Surface energy  Sound energy  Gravitational potential energy  Thermal energy  Chemical energy  Electric energy  Nuclear energy   See also    References   

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