Types of energy

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, work, and transfer of matter 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. These energy transfers 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 movementthrough 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.

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.

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.