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.
In calorimetry, sensible heat is defined with respect to a particular state variable of the system; it causes change of temperature, leaving that particular state variable unchanged. Heat transfer that occurs with the system at constant temperature and that changes that particular state variable is called latent heat with respect to that variable. For infinitesimal changes, the total incremental heat transfer is then the sum of the latent and sensible heat increments. This is a basic paradigm for thermodynamics, and was important in the historical development of the subject.
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.
By Dean Hanley