Heat
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This article is about a mode of transfer of energy. For other uses, see Heat (disambiguation).
Thermodynamics
The classical Carnot heat engine
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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 in transfer to or from a thermodynamic system, by
mechanisms other than thermodynamic work or transfer of matter.[1][2][3][4][5][6][7] The various
mechanisms of energy transfer that define heat are stated in the next section of this
article.
Like thermodynamic work, heat transfer is a process involving more than one system,
not a property of any one system. In thermodynamics, energy transferred as heat
(a process function) contributes to change in the system's cardinal energy variable of
state, for example its internal energy, or for example its enthalpy. This is to be
distinguished from the ordinary language conception of heat as a property of an isolated
system.
The quantity of energy transferred as heat in a process is the amount of transferred
energy excluding any thermodynamic work that was done and any energy contained in
matter transferred. For the precise definition of heat, it is necessary that it occur by a
path that does not include transfer of matter.[8]
Though not immediately by the definition, but in special kinds of process, quantity of
energy transferred as heat can be measured by its effect on the states of interacting
bodies. For example, respectively in special circumstances, heat transfer can be
measured by the amount of ice melted, or by change in temperature of a body in the
surroundings of the system.[9] Such methods are called calorimetry.
The conventional symbol used to represent the amount of heat transferred in a
thermodynamic process is Q. As an amount of energy (being transferred), the SI unit of
heat is the joule (J).
Contents
1Mechanisms of transfer that define heat
2Notation and units
3Classical thermodynamics
o 3.1Heat and entropy
o 3.2Heat and enthalpy
4History
o 4.1Carathéodory (1909)
5Heat transfer
o 5.1Heat transfer between two bodies
o 5.2Heat engine
o 5.3Heat pump or refrigerator
o 5.4Macroscopic view
o 5.5Microscopic view
o 5.6Calorimetry
o 5.7Engineering
6Latent and sensible heat
7Heat capacity
8"Hotness"
9See also
10References
o 10.1Quotations
o 10.2Bibliography of cited references
o 10.3Further bibliography
11External links
Mechanisms of transfer that define heat
The mechanisms of energy transfer that define heat include conduction, through direct
contact of immobile bodies, or through a wall or barrier that is impermeable to matter;
or radiation between separated bodies; or friction due to isochoric mechanical or
electrical or magnetic or gravitational work done by the surroundings on the system of
interest, such as Joule heating due to an electric current driven through the system of
interest by an external system, or through a magnetic stirrer. When there is a suitable
path between two systems with different temperatures, heat transfer occurs necessarily,
immediately, and spontaneously from the hotter to the colder system. Thermal
conduction occurs by the stochastic (random) motion of microscopic particles (such as
atoms or molecules). In contrast, thermodynamic work is defined by mechanisms that
act macroscopically and directly on the system's whole-body state variables; for
example, change of the system's volume through a piston's motion with externally
measurable force; or change of the system's internal electric polarization through an
externally measurable change in electric field. The definition of heat transfer does not
require that the process be in any sense smooth. For example, a bolt of lightning may
transfer heat to a body.
Convective circulation allows one body to heat another, through an intermediate
circulating fluid that carries energy from a boundary of one to a boundary of the other;
the actual heat transfer is by conduction and radiation between the fluid and the
respective bodies.[10][11][12] Convective circulation, though spontaneous, does not
necessarily and immediately occur simply because of some slight temperature
difference; for it to occur in a given arrangement of systems, there is a threshold that
must be crossed.
Although heat flows spontaneously from a hotter body to a cooler one, it is possible to
construct a heat pump which expends work to transfer energy from a colder body to a
hotter body. In contrast, a heat engine reduces an existing temperature difference to
supply work to another system. Another thermodynamic type of heat transfer device is
an active heat spreader, which expends work to speed up transfer of energy to colder
surroundings from a hotter body, for example a computer component. [13]
