This article is about a mode of transfer of energy. For the system in a home, see Central heating. For other uses, see Heat (disambiguation).
Heat
A glowing-hot metal bar showing incandescence, the emission of light due to its temperature, is often recognized as a source of heat
Common symbols
SI unit
joule
Other units
British thermal unit, calorie
In SI base units
kg⋅m2⋅s−2
Dimension
Thermodynamics
The classical Carnot heat engine
Branches
Classical
Statistical
Chemical
Quantum thermodynamics
Equilibrium / Non-equilibrium
Laws
Zeroth
First
Second
Third
Systems
Closed system
Open system
Isolated system
State
Equation of state
Ideal gas
Real gas
State of matter
Phase (matter)
Equilibrium
Control volume
Instruments
Processes
Isobaric
Isochoric
Isothermal
Adiabatic
Isentropic
Isenthalpic
Quasistatic
Polytropic
Free expansion
Reversibility
Irreversibility
Endoreversibility
Cycles
Heat engines
Heat pumps
Thermal efficiency
System properties
Note: Conjugate variables in italics
Property diagrams
Intensive and extensive properties
Process functions
Work
Heat
Functions of state
Temperature / Entropy (introduction)
Pressure / Volume
Chemical potential / Particle number
Vapor quality
Reduced properties
Material properties
Property databases
Specific heat capacity
Compressibility
Thermal expansion
Equations
Carnot's theorem
Clausius theorem
Fundamental relation
Ideal gas law
Maxwell relations
Onsager reciprocal relations
Bridgman's equations
Table of thermodynamic equations
Potentials
Free energy
Free entropy
Internal energy
Enthalpy
Helmholtz free energy
Gibbs free energy
History
Culture
History
General
Entropy
Gas laws
"Perpetual motion" machines
Philosophy
Entropy and time
Entropy and life
Brownian ratchet
Maxwell's demon
Heat death paradox
Loschmidt's paradox
Synergetics
Theories
Caloric theory
Vis viva("living force")
Mechanical equivalent of heat
Motive power
Key publications
An Experimental Enquiry Concerning ... Heat
On the Equilibrium of Heterogeneous Substances
Reflections on the Motive Power of Fire
Timelines
Thermodynamics
Heat engines
Art
Education
Maxwell's thermodynamic surface
Entropy as energy dispersal
Scientists
Bernoulli
Boltzmann
Bridgman
Carathéodory
Carnot
Clapeyron
Clausius
de Donder
Duhem
Gibbs
von Helmholtz
Joule
Kelvin
Lewis
Massieu
Maxwell
von Mayer
Nernst
Onsager
Planck
Rankine
Smeaton
Stahl
Tait
Thompson
van der Waals
Waterston
Other
Nucleation
Self-assembly
Self-organization
Order and disorder
Category
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In thermodynamics, heat is the thermal energy transferred between systems due to a temperature difference.[1] In colloquial use, heat sometimes refers to thermal energy itself. Thermal energy is the kinetic energy of vibrating and colliding atoms in a substance.
An example of formal vs. informal usage may be obtained from the right-hand photo, in which the metal bar is "conducting heat" from its hot end to its cold end, but if the metal bar is considered a thermodynamic system, then the energy flowing within the metal bar is called internal energy, not heat. The hot metal bar is also transferring heat to its surroundings, a correct statement for both the strict and loose meanings of heat. Another example of informal usage is the term heat content, used despite the fact that physics defines heat as energy transfer. More accurately, it is thermal energy that is contained in the system or body, as it is stored in the microscopic degrees of freedom of the modes of vibration.[2]
Heat is energy in transfer to or from a thermodynamic system by a mechanism that involves the microscopic atomic modes of motion or the corresponding macroscopic properties.[3] This descriptive characterization excludes the transfers of energy by thermodynamic work or mass transfer. Defined quantitatively, the heat involved in a process is the difference in internal energy between the final and initial states of a system, and subtracting the work done in the process.[4] This is the formulation of the first law of thermodynamics.
Calorimetry is measurement of quantity of energy transferred as heat by its effect on the states of interacting bodies, for example, by the amount of ice melted or by change in temperature of a body.[5]
In the International System of Units (SI), the unit of measurement for heat, as a form of energy, is the joule (J).
^Van Wylen, Gordon; Sonntag, Richard (1978). Fundamentals of Classical Thermodynamics (Second edition, SI Version, Revised Printing ed.). Chapter 4.7, Definition of Heat: John Wiley & Sons. p. 76. ISBN 0-471-04188-2.{{cite book}}: CS1 maint: location (link)
^D.V. Schroeder (1999). An Introduction to Thermal Physics. Addison-Wesley. p. 15. ISBN 0-201-38027-7.
^Herbert B. Callen (1985). Thermodynamics and an Introduction to Thermostatics (2 ed.). John Wiley & Sons. http://cvika.grimoar.cz/callen/ Archived 17 October 2018 at the Wayback Machine or http://keszei.chem.elte.hu/1alapFizkem/H.B.Callen-Thermodynamics.pdf Archived 30 December 2016 at the Wayback Machine, p. 8: Energy may be transferred via ... work. "But it is equally possible to transfer energy via the hidden atomic modes of motion as well as via those that happen to be macroscopically observable. An energy transfer via the hidden atomic modes is called heat."
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