The Sun and Earth form an ongoing example of a heating process. Some of the Sun's thermal radiation strikes and heats the Earth. Compared to the Sun, Earth has a much lower temperature and so sends far less thermal radiation back to the Sun. The heat of this process can be quantified by the net amount, and direction (Sun to Earth), of energy it transferred in a given period of time.
In thermodynamics, heat is energy transferred from one system to another as a result of thermal interactions. The amount of heat transferred in any process can be defined as the total amount of transferred energy excluding any macroscopic work that was done and any transfer of part of the object itself. When two systems with different temperatures are put in contact, heat flows spontaneously from the hotter to the colder system. Transfer of energy as heat can occur through direct contact, through a barrier that is impermeable to matter (as in conduction), by radiation between separated bodies, by way of an intermediate fluid (as in convective circulation), or by a combination of these. By contrast to work, heat involves the stochastic (random) motion of particles (such as atoms or molecules) that is equally distributed among all degrees of freedom, while work is confined to one or more specific degrees of freedom such as those of the center of mass.
Like thermodynamic work, heat is a property of a process, not a property of a system. Energy exchanged as heat (a process function) changes the internal energy (a state function) of each system by equal and opposite amounts. This is to be distinguished from the common conception of heat as a property of high-temperature systems.
Although heat flows spontaneously from a hotter body to a cooler one, it is possible to construct a heat pump or refrigeration system that does work to increase the difference in temperature between two systems. In contrast, a heat engine reduces an existing temperature difference to do work on another system.
As a form of energy, the SI unit of heat is the joule (J). The conventional symbol used to represent the amount of heat exchanged in a thermodynamic process is Q. Heat is measured by its effect on the states of interacting bodies, for example, by the amount of ice melted or a change in temperature. The quantification of heat via the temperature change of a body is called calorimetry.
Use of the symbol Q for the total amount of energy transferred as heat is due to Rudolf Clausius in 1850:
"Let the amount of heat which must be imparted during the transition of the gas in a definite manner from any given state to another, in which its volume is v and its temperature t, be called Q"
Heat released by a system into its surroundings is by convention a negative quantity (Q < 0); when a system absorbs heat from its surroundings, it is positive (Q > 0). Heat transfer rate, or heat flow per unit time, is denoted by . This should not be confused with a time derivative of a function of state (which can also be written with the dot notation) since heat is not a function of state.Heat flux is defined as rate of heat transfer per unit cross-sectional area (units watts per square metre).