Heat

In physics, heat is energy in transfer other than as work or by transfer of matter. When there is a suitable physical pathway, heat flows from a hotter body to a colder one. The transfer results in a net increase in entropy. The pathway can be direct, as in conduction and radiation, or indirect, as in convective circulation.

Heat refers to a process of transfer between two systems, the system of interest, and its surroundings considered as a system, not to a state or property of a single system. If heat transfer is slow and continuous, so that the temperature of the system of interest remains well defined, it can be described by a process function.

Kinetic theory explains heat as a macroscopic manifestation of the motions and interactions of microscopic constituents such as molecules and photons.

The quantity of energy transferred as heat is a scalar expressed in an energy unit such as the joule (J) (SI), with a sign that is 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.