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In physics, energy is one of the basic quantitative properties describing a physical system or object's state. Energy can be transformed (converted) among a number of forms that may each manifest and be measurable in differing ways. The law of conservation of energy states that the (total) energy of a system can increase or decrease only by transferring it in or out of the system. The total energy of a system can be calculated by simple addition when it is composed of multiple non-interacting parts or has multiple distinct forms of energy. Common energy forms include the kinetic energy of a moving object, the radiant energy carried by light and other electromagnetic radiation, and various types of potential energy such as gravitational and elastic. Common types of energy transfer and transformation include processes such as heating a material, performing mechanical work on an object, generating or making use of electric energy, and many chemical reactions.

Forms of energy

Energy exists in many forms. In the context of physical sciences, several forms of energy have been defined. These include:

  • 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, e.g. that of ocean surface waves or that of sound
  • Chemical
  • Electric
  • Magnetic
  • 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
  • Gravitational
  • Intrinsic, the rest energy (≥0) equivalent to an object's rest mass
  • Thermal, a microscopic equivalent of mechanical energy

Thermal energy is energy of microscopic constituents of matter, which may include both kinetic and potential energy. Heat is just that amount of thermal energy being transferred (in a given process) in the direction of decreasing temperature. Mechanical work is just that amount of (mechanical) 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. 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.