In physics and chemistry, especially in thermodynamics, heat refers to a process of transfer of energy between a system and its surroundings other than by work or transfer of matter.It is measured as the quantity of energy that the process transferred. Familiar examples involve the spontaneous transfer of energy as heat from a body at a higher temperature to an adjacent one one at a lower temperature, often narrowing their temperature difference. The transfer can occur by the mechanisms of conduction, radiation, and convective circulation.
Heat processes increase the total entropy of system and surroundings and reduce the amount of their energy that's available for them to do work. Transfers as heat are possible even in some cases where the temperatures are not well defined.
Kinetic theory explains transfers of energy as heat as macroscopic manifestations of the motions and interactions of microscopic constituents such as molecules and photons.
Many useful devices involve energy transfers as both heat and work; familiar examples include combustion engines, which provide some energy as work while transferring the rest as heat to the surroundings, and refrigerators. The quantity of energy transferred as heat is a scalar measured in energy units such as the the joule (J) (SI), but with a sign indicating the direction of transfer, customarily positive when a transfer adds to the energy of a system. It can be measured by calorimetry,or determined by calculations based on other quantities, relying on the first law of thermodynamics. In calorimetry, latent heat changes a system's state without temperature change, while sensible heat changes only its temperature.