For other uses, see Heat (disambiguation).
Heat generated from the nuclear fusion in the Sun, and transported to Earth as electromagnetic radiation, is one of the driving forces of life on Earth.
In physics, chemistry, engineering, and thermodynamics, heat is energy produced or transferred from one body, region, set of components, or thermodynamic system to another in any way other than as work.[1][2]
In ordinary language, as distinct from technical language, heat has a broader meaning.[3] This can lead to confusion if the diversity of usage of words is forgotten.[4][5][6][7]
Thermodynamically, energy can be produced or transferred as heat by thermal conduction[8], by thermal radiation,[9] by friction and viscosity,[10] and by chemical dissipation.[11][12][13]
The engineering discipline of heat transfer recognizes heat transfer by conduction, by convection, by mass transfer, and by radiation.
Heat transfer by conduction and by radiation from a hotter to a colder body is spontaneous. The second law of thermodynamics requires that the transfer of energy from one body to another with an equal or higher temperature can only occur with the aid of a heat pump by mechanical work, or by some other similar process in which entropy is increased in the universe in a manner that compensates for the decrease of entropy in the cooled body, due to the removal of the heat from it.[14] For example, energy may be removed against a temperature gradient by spontaneous evaporation of a liquid.
In physics, especially in calorimetry, and in meteorology, the concepts of latent heat and of sensible heat are used.
A related and potentially confusing term is thermal energy, loosely defined as the energy of a body that increases with its temperature. Potentially confusingly, thermal energy is sometimes referred to as heat, although the thermodynamic definition of heat requires it to be in transfer between two systems or in production in a dissipative process such as friction, viscosity, or chemical reaction.
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Contents
- 1 Overview
- 2 Notation and units
- 3 Semantics
- 4 Internal energy and enthalpy
- 4.1 Path-independent examples for an ideal gas
- 4.2 Incompressible substances
- 5 Latent and sensible heat
- 6 Specific heat
- 7 Entropy
- 8 Heat transfer in engineering
- 9 Application
- 10 See also
- 11 Notes
- 12 References
- 13 External links
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Overview
Heat flows spontaneously only from systems of higher temperature to systems of lower temperature. When two systems come into thermal contact, they always exchange thermal energy due to the microscopic interactions of their particles. When the systems are at different temperatures, the net flow of thermal energy is not zero and is directed from the hotter region to the cooler region, until their temperatures are equal and the flow of heat ceases. Then they have reached a state of thermal equilibrium, exchanging thermal energy at an equal rate in both directions.
The first law of thermodynamics requires that the energy of an isolated system is conserved. To change the energy of a system, energy must be transferred to or from the system. For a closed system, heat and work are the mechanisms by which energy can be transferred. For an open system, total energy can be changed also by transfer of matter.[15]
Work performed on a system is, by definition [2], an energy transfer to the system that is due to a change to external or mechanical parameters of the system, such as the volume, magnetization, center of mass in a gravitational field.
For a closed system (with no external transfer of matter), heat is defined as energy transferred to the system in any way other than as work. Heat transfer is an irreversible process, which leads to the systems coming closer to mutual thermodynamic equilibrium. In the case of systems close to thermodynamic equilibrium where temperature can be defined, some heat transfer can be related to temperature difference between systems. Heat transfer can also arise by friction and by viscosity. For an open system, far from thermodynamic equilibrium, where temperature cannot be defined, there the distinction between heat and work may not be feasible.
Human notions such as hot and cold are relative terms and are generally used to compare one system’s temperature to another or its surroundings.
Heat may flow across the boundary of the system and thus change its internal energy.
Definitions
Scottish physicist James Clerk Maxwell, in his 1871 classic Theory of Heat, was one of the first to enunciate a modern definition of heat. Maxwell outlined four stipulations for the definition of heat:
- It is something which may be transferred from one body to another, according to the second law of thermodynamics.
- It is a measurable quantity, and thus treated mathematically.
- It cannot be treated as a substance, because it may be transformed into something that is not a substance, e.g., mechanical work.
- Heat is one of the forms of energy.
Several modern definitions of heat are as follows:
- The energy transferred from a high-temperature system to a lower-temperature system is called heat.[16]
- Any spontaneous flow of energy from one system to another caused by a difference in temperature between the systems is called heat.[17]
In a thermodynamic sense, heat is never regarded as being stored within a system. Like work, it exists only as energy in transit from one system to another or between a system and its surroundings. When energy in the form of heat is added to a system, it is stored as kinetic and potential energy of the atoms and molecules in the system.[18]
Notation and units
As a form of energy heat has the unit joule (J) in the International System of Units (SI). However, in many applied fields in engineering the British Thermal Unit (BTU) and the calorie are often used. The standard unit for the rate of heat transferred is the watt (W), defined as joules per second.
The total amount of energy transferred as heat is conventionally written as Q for algebraic purposes. 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
- .
Heat flux is defined as rate of heat transfer per unit cross-sectional area, resulting in the unit watts per square metre.
Semantics
There is some diversity of usage of the word heat, even in technical scientific writings.[6] In current scientific usage, the language surrounding the term can be conflicting and even misleading. One study showed that several popular textbooks used language that implied several meanings of the term, that heat is the process of transferring energy, that it is the transferred energy (i.e., as if it were a substance), and that is an entity contained within a system, among other similar descriptions. The study determined it was not uncommon for a combination of these representations to appear within the same text.[7] They found the predominant use among physicists to be as if it were a substance.
Internal energy and enthalpy
In the case where the number of particles in the system is constant, the first law of thermodynamics states that the differential change in internal energy dU of a system is given by the differential heat flow δQ into the system minus the differential work δW exerted by the system:[note 1]
- .
The differential transfer of heat, δQin, makes differential contributions, not only to internal energy, but also to the work done by the system:
- .
The work done by the system includes boundary work, which causes the boundaries of the system to expand, in addition to other work (e.g. shaft work performed by a compressor fan):
- .
is equal to the differential enthalpy change (dH) of the system. Substitution gives:
- .
Both enthalpy, H, and internal energy, U, are state functions. In cyclical processes, such as the operation of a heat engine, state functions return to their initial values. Thus, the differentials for enthalpy and energy are exact differentials, which are dH and dU, respectively. The symbol for exact differentials is the lowercase letter d.
In contrast, neither Q nor W represents the state of the system (i.e. they need not return to their original values when returning to same step in the following cycle). Thus, the infinitesimal expressions for heat and work are inexact differentials, δQ and δW, respectively. The lowercase Greek letter delta, δ, is the symbol for inexact differentials. The integral of any inexact differential over the time it takes to leave and return to the same thermodynamic state does not necessarily equal zero. However, for slow enough processes involving no change in volume (i.e. dV = 0), applied magnetic field, or other external parameters (i.e. δWout = 0 and δWin = 0), δQ forms the exact differential, , wherein the following relation applies:
- dU = TdS = δQrev.
Likewise, for an isentropic process (i.e. δQ = 0 and dS = 0), δW forms the exact differential, , wherein the following relation applies:
- dU = − pdV = − δWrev.
Path-independent examples for an ideal gas
For a simple compressible system such as an ideal gas inside a piston, the internal energy change ΔU at constant volume and the enthalpy change ΔH at constant pressure are modeled by separate heat capacity values, which are Cv and Cp, respectively.
Constrained to have constant volume, the heat, Q, required to change its temperature from an initial temperature, T0, to a final temperature, Tf, is given by this formula:
Removing the volume constraint and allowing the system to expand or contract at constant pressure, the heat, Q, required to change its temperature from an initial temperature, T0, to a final temperature, Tf, is given by this formula:
Note that when integrating an exact differential (e.g. dU), the lowercase letter d is substituted for Δ (e.g. ΔU), and when integrating an inexact differential (e.g. δWboundary), the lowercase Greek letter δ is removed with no replacement (e.g. Wboundary).
Incompressible substances
For incompressible substances, such as solids and liquids, the distinction between the two types of heat capacity (i.e. Cp which is based on constant pressure and Cv which is based on constant volume) disappears, as no work is performed.
Latent and sensible heat
In a 1847 lecture entitled On Matter, Living Force, and Heat, James Prescott Joule characterized the terms latent heat and sensible heat as components of heat each effecting distinct physical phenomena, namely the potential and kinetic energy of particles, respectively.[19] He described latent energy as the energy of interaction in a given configuration of particles, i.e. a form of potential energy, and the sensible heat as an energy affecting the thermal energy, which he called the living force.
Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a change of state that occurs without a change in temperature. Such a process may be a phase transition, such as the melting of ice or the boiling of water.[20][21] The term was introduced around 1750 by Joseph Black as derived from the Latin latere (to lie hidden), characterizing its effect as not being directly measurable with a thermometer.
Sensible heat, in contrast to latent heat, is the heat exchanged by a thermodynamic system that has as its sole effect a change of temperature.[22] Sensible heat therefore only increases the thermal energy of a system.
Consequences of Black's distinction between sensible and latent heat are examined in the Wikipedia article on calorimetry.
Specific heat
Specific heat, also called specific heat capacity, is defined as the amount of energy that has to be transferred to or from one unit of mass (kilogram) or amount of substance (mole) to change the system temperature by one degree. Specific heat is a physical property, which means that it depends on the substance under consideration and its state as specified by its properties.
The specific heats of monatomic gases (e.g., helium) are nearly constant with temperature. Diatomic gases such as hydrogen display some temperature dependence, and triatomic gases (e.g., carbon dioxide) still more.
Entropy
Main article: Entropy
In 1856, German physicist Rudolf Clausius defined the second fundamental theorem (the second law of thermodynamics) in the mechanical theory of heat (thermodynamics): "if two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent, then the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:"[23][24]
In 1865, he came to define this ratio as entropy symbolized by S, such that, for a closed, stationary system:
and thus, by reduction, quantities of heat δQ (an inexact differential) are defined as quantities of TdS (an exact differential):
In other words, the entropy function S facilitates the quantification and measurement of heat flow through a thermodynamic boundary.
To be precise, this equality is only valid, if the heat δQ is applied reversibly. If, in contrast, irreversible processes are involved, e.g. some sort of friction, then instead of the above equation one has
This is the second law of thermodynamics.
Heat transfer in engineering
A red-hot iron rod from which heat transfer to the surrounding environment will be primarily through radiation.
The discipline of heat transfer, typically considered an aspect of mechanical engineering and chemical engineering, deals with specific applied methods by which thermal energy in a system is generated, or converted, or transferred to another system. Although the definition of heat implicitly means the transfer of energy, the term heat transfer encompasses this traditional usage in many engineering disciplines and laymen language.
Heat transfer includes the mechanisms of heat conduction, thermal radiation, and mass transfer. In engineering, the term convective heat transfer is used to describe the combined effects of conduction and fluid flow and is often regarded as an additional mechanism of heat transfer. Although distinct physical laws may describe the behavior of each of these methods, real systems often exhibit a complicated combination which are often described by a variety of complex mathematical methods.
Application
In accordance with the first law, heat may be converted to or from work by so-called heat engines, e.g. the steam engine. Heat engines achieve maximum efficiency when the difference between initial and final temperature is largest, resulting in minimal loss. Heat pumps on the other hand operate at a small temperature difference, transferring heat at low temperatures from a reservoir, e.g. from the soil, and deliver it by means of electrical work for heating purposes. Now the temperature difference should be small, to keep the lost electrical work small.
See also
- Effect of sun angle on climate
- Heat death of the Universe
- Heat diffusion
- Heat equation
- Heat exchanger
- Heat flux sensor
- Heat transfer coefficient
- History of heat
- Sigma heat
- Shock heating
- Thermal management of electronic devices and systems
- Thermometer
- Relativistic heat conduction
- Waste heat
Notes
- ^ An alternate convention is to consider the work performed on the system by its surroundings. This leads to a change in sign of the work. This is the convention adopted by many modern textbooks of physical chemistry, such as those by Peter Atkins and Ira Levine, but many textbooks on physics define work as work done by the system.
References
- ^ Bryan, G.H. (1907). Thermodynamics. An Introductory Treatise dealing mainly with First Principles and their Direct Applications, B.G. Tuebner, Leipzig, page 47.
- ^ a b F. Reif (2000). Fundamentals of Statistical and Thermal Physics. Singapore: McGraw-Hll, Inc.. p. 67. ISBN 0-07-Y85615-X.
- ^ Oxford English Dictionary, second edition, Oxford University Press, Oxford, UK.
- ^ Planck, M. (1897/1903), p. 1, "This direct sensation, however, furnishes no quantitative scientific measure ..."
- ^ Truesdell, C. (1980). The Tragicomical History of Thermodynamics 1822-1854, Springer, New York, ISBN 0–387–90403–4, page 15: "What they meant is not always clear."
- ^ a b "A review of selected literature on students' misconceptions of heat". Boğaziçi University Journal of Education 20 (1): 25–41. 2003. http://buje.boun.edu.tr/upload/revizeedilmis/45bc61ceeb94344a0664C646d01.pdf.
- ^ a b Brookes, D.; Horton, G.; Van Heuvelen, A.; Etkina, E. (2005). "Concerning Scientific Discourse about Heat". 2004 Physics Education Research Conference (AIP Conference Proceedings) 790: 149–152. doi:10.1063/1.2084723. http://research.physics.illinois.edu/per/David/perc2004_revised.pdf.
- ^ Guggenheim, E.A. (1949/1967), p. 8.
- ^ Planck. M. (1914). The Theory of Heat Radiation, a translation by Masius, M. of the second German edition, P. Blakiston's Son & Co., Philadelphia.
- ^ Lebon, G., Jou, D., Casas-Vásquez, J. (2008). Understanding Non-equilibrium Thermodynamics. Foundations, Applications, Frontiers, Springer, ISBN 978–3–540–74252–4, pages 120 and 62.
- ^ Planck 1927, p. 39.
- ^ Partington, J.R. (1949), pp. 151–155.
- ^ Guggenheim, E.A. (1949/1967), pp. 241–242.
- ^ Planck, M. (1922/1927), p. 89.
- ^ Kondepudi, D. (2008), p. 59.
- ^ Discourse on Heat and Work - Department of Physics and Astronomy, Georgia State University: Hyperphysics (online)
- ^ Schroeder, Daniel V. (2000). An introduction to thermal physics. San Francisco, California: Addison-Wesley. p. 18. ISBN 0-321-27779-1. "Heat is defined as any spontaneous flow of energy from one system to another, caused by a difference in temperature between the systems."
- ^ Smith, J.M., Van Ness, H.C., Abbot, M.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw-Hill. ISBN 0073104450.
- ^ J. P. Joule (1884), The Scientific Paper of James Prescott Joule, The Physical Society of London, p. 274, "I am inclined to believe that both of these hypotheses will be found to hold good,—that in some instances, particularly in the case of sensible heat, or such as is indicated by the thermometer, heat will be found to consist in the living force of the particles of the bodies in which it is induced; whilst in others, particularly in the case of latent heat, the phenomena are produced by the separation of particle from particle, so as to cause them to attract one another through a greater space." , Lecture on Matter, Living Force, and Heat. May 5 and 12, 1847
- ^ Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.
- ^ Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-7607-4616-8.
- ^ Ritter, Michael E. (2006). "The Physical Environment: an Introduction to Physical Geography". http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/energy/energy_balance.html.
- ^ Published in Poggendoff’s Annalen, Dec. 1854, vol. xciii. p. 481; translated in the Journal de Mathematiques, vol. xx. Paris, 1855, and in the Philosophical Magazine, August 1856, s. 4. vol. xii, p. 81
- ^ Clausius, R. (1865). The Mechanical Theory of Heat] –with its Applications to the Steam Engine and to Physical Properties of Bodies. London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
Bibliography
- Guggenheim, E.A. (1967) [1949], Thermodynamics. An Advanced Treatment for Chemists and Physicists (fifth ed.), Amsterdam: North-Holland Publishing Company.
- Kondepudi, D. (2008), Introduction to Modern Thermodynamics, Chichester UK: Wiley, ISBN 978–0–470–01598–8.
- Partington, J.R. (1949), An Advanced Treatise on Physical Chemistry., volume 1, Fundamental Principles. The Properties of Gases, London: Longmans, Green and Co.
- Planck, M. (1903) [1897] (in English), Treatise on Thermodynamics (first ed.), London: Longmans, Green and Co., http://www.onread.com/reader/145819
- Planck, M. (1927) [1922] (in English), Treatise on Thermodynamics (third ed.), London: Longmans, Green and Co.
External links
- Heat on In Our Time at the BBC. (listen now)
- Plasma heat at 2 gigakelvins - Article about extremely high temperature generated by scientists (Foxnews.com)
- Correlations for Convective Heat Transfer - ChE Online Resources
- An Introduction to the Quantitative Definition and Analysis of Heat written for High School Students
