Heat

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 unitjoule
Other units
British thermal unit, calorie
In SI base unitskgm2s−2
Dimension

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).

  1. ^ 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)
  2. ^ D.V. Schroeder (1999). An Introduction to Thermal Physics. Addison-Wesley. p. 15. ISBN 0-201-38027-7.
  3. ^ 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."
  4. ^ Callen, p.19
  5. ^ Maxwell, J.C. (1871), Chapter III.

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