Temperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. Temperature is a measure of the average kinetic energy of the particles in a sample of matter. When stuff moves. ... Kinetic energy (also called vis viva, or living force) is energy possessed by a body by virtue of its motion. ...

General description

The formal properties of temperature are studied in thermodynamics. This article needs to be cleaned up to conform to a higher standard of quality. ...

Formally, temperature is that property which governs the transfer of thermal energy, or heat, between one system and another. When two systems are at the same temperature, they are in thermal equilibrium and no heat transfer will occur. When a temperature difference does exist, heat will tend to move from the higher temperature system to the lower temperature system, until thermal equilibrium is established. This heat transfer may occur via conduction, convection or radiation (see heat for additional discussion of the various mechanisms of heat transfer). A red-hot iron rod cooling after being worked by a blacksmith. ... In thermodynamics, a thermodynamic system is in thermodynamic equilibrium if its energy distribution equals a Maxwell-Boltzmann-distribution. ... Heat flow along perfectly insulated wire Heat conduction is the transmission of heat across matter. ... Convection is the transfer of heat by the motion of or within a fluid. ... Radiation generally means the transmission of waves, objects or information from a source into a surrounding medium or destination. ... A red-hot iron rod cooling after being worked by a blacksmith. ...

Temperature is related to the amount of thermal energy or heat in a system. As more heat is added the temperature rises, similarly a decrease in temperature corresponds to a loss of heat from the system. On the microscopic scale this heat corresponds to the random motion of atoms and molecules in the system. Thus, an increase in temperature corresponds in an increase in the rate of movement of the atoms in the system.

Temperature is an intrinsic property of a system, meaning that it does not depend on the system size or the amount of material in the system. Other intrinsic properties include pressure and density. By contrast, mass and volume are extrinsic properties, and depend on the amount of material in the system. This article is not about the concepts of intrinsic and extrinsic properties in philosophy. ... Pressure is the application of force to a surface, and the concentration of that force in a given area. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... Volume (also called capacity) is a quantification of how much space an object occupies. ... This article is not about the concepts of intrinsic and extrinsic properties in philosophy. ...

Applications

Temperature plays an important role in almost all fields of science, including physics, chemistry, and biology.

Many physical properties of materials including the phase (solid, liquid, gaseous or plasma), density, solubility, vapor pressure, and electrical conductivity depend on the temperature. Temperature also plays an important role in determining the rate and extent to which chemical reactions occur. This is one reason why the human body has several elaborate mechanisms for maintaining the temperature at 37 °C, since temperatures only a few degrees higher can result in harmful reactions with serious consequences. Temperature also controls the type and quantity of thermal radiation emitted from a surface. One application of this effect is the incandescent light bulb, in which a tungsten filament is electrically heated to a temperature at which significant quantities of visible light are emitted. In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... A solid is a state of matter, characterized by a definite volume and a definite shape (i. ... A liquid will assume the shape of its container. ... Gas (actually, as), the GNU assembler, is the default GCC back-end. ... The word plasma has a Greek root which means to be formed or molded (the word plastic shares this root). ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ... The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor. ... Electrical conductivity is a measure of how well a material accommodates the transport of electric charge. ... A chemical reaction is a process involving one, two or more substances (called reactants), characterized by a chemical change and yielding one or more product(s) which are different from the reactants. ... Thermal radiation is electromagnetic radiation from an object that is simply caused by its temperature. ... The incandescent light bulb (archaically known as the electric lamp) uses a glowing wire filament heated to white-hot by electrical resistance, to generate light (a process known as thermal radiation or incandescence). ... General Name, Symbol, Number tungsten, W, 74 Chemical series transition metals Group, Period, Block 6, 6, d Appearance grayish white, lustrous Atomic mass 183. ... Lightning strikes during a night-time thunderstorm. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. ...

For information on temperature changes relevant to climate change or Earth's geologic past see: Temperature record Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years The term climate change is used to refer to changes in the Earths global climate or regional climates. ... Earth, also known as the Earth, Terra, and (mostly in the 19th century) Tellus, is the third planet outward from the Sun. ... Geology (from Greek γη- (ge-, the earth) and λογος (logos, word, reason)) is the science and study of the Earth, its composition, structure, physical properties, history, and the processes that shape it. ... The temperature record shows the fluctuations of the temperature of the atmosphere and the oceans through various spans of time. ...

Temperature measurement

Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid. Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint is the gas thermometer mentioned below.

Other important devices for measuring temperature include:

• Thermocouples
• Thermistors
• Resistance Temperature Detector (RTD)
• Pyrometers
• Langmuir probes (for electron temperature of a plasma)
• Other thermometers

One must be careful when measuring temperature to ensure that the measuring instrument (thermometer, thermocouple, etc) is really the same temperature as the material that is being measured. Under some conditions heat from the measuring instrument can cause a temperature gradient, so the measured temperature is different from the actual temperature of the system. In such a case the measured temperature will vary not only with the temperature of the system, but also with the heat transfer properties of the system. An extreme case of this effect gives rise to the wind chill factor, where the weather feels colder under windy conditions than calm conditions even though the temperature is the same. What is happening is that the wind increases the rate of heat transfer from the body, resulting in a larger reduction in body temperature for the same ambient temperature. In electronics, thermocouples are a widely used type of temperature sensor. ... A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. ... A Resistance Thermometer Detector measures the relationship between electrical resistance and temperature. ... A pyrometer is non-contact temperature measuring device, generally the term is applied to instruments measuring temperatures above 600 degrees Celsius. ... A Langmuir probe is a device named after Nobel Prize winning physicist Irving Langmuir which is used to determine the electron temperature, electron density, and plasma potential. ... The word plasma has a Greek root which means to be formed or molded (the word plastic shares this root). ... A thermometer is a device used to measure temperatures or temperature changes. ... Wind chill is the apparent temperature felt on the exposed human (or animal) body due to the combination of air temperature and wind speed. ...

Under some conditions it becomes possible to measure temperature by a direct use of the Planck's law of black body radiation. For example, the cosmic microwave background temperature has been measured from the spectrum of photons observed by satellite observations such as the WMAP. In the study of the quark-gluon plasma through heavy-ion collisions, single particle spectra sometimes serve as a thermometer. Black body spectrum as a function of wavelength In physics, the spectral intensity of electromagnetic radiation from a black body at temperature T is given by the Plancks law of black body radiation: where: I(ν) is the amount of energy per unit time per unit surface area per unit... WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003) The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ... Alternate meaning: WMAP (AM) Artist depiction of the WMAP satellite at the L2 point The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. ... Quark gluon plasma is a phase of Quantum Chromodynamics (QCD) which exists at extremely high temperature and density. ... Quark gluon plasma is a phase of Quantum Chromodynamics (QCD) which exists at extremely high temperature and density. ...

See also: color temperature, Timeline of temperature and pressure measurement technology, Planck temperature White light is commonly described by its color temperature. ... Timeline of temperature and pressure measurement technology 1592 - Galileo Galilei builds a crude thermometer using the contraction of air to draw water up a tube 1612 - Santorre Santorio puts thermometer to medical use 1643 - Evangelista Torricelli invents the mercury barometer 1714 - Daniel Gabriel Fahrenheit invents the mercury in glass thermometer... The Planck temperature, named after German physicist Max Planck, is the natural unit of temperature, denoted by TP. TP = (c5 / G)1/2 / k ≈ 1. ...

History of temperature measurement

In 1701, Ole Christensen Røemer (1644-1710) made one of the first practical thermometers. As a temperature indicator it used red wine. The temperature scale used for his thermometer had 0 representing the temperature of a salt and ice mixture (at about 259 K). Ole Rømer. ...

In 1708 Gabriel Fahrenheit (1686-1736) modified Rømer's scale and switched to mercury for more precise measurement. The Fahrenheit scale is still used in parts of the world. G.D. Fahrenheit Daniel Gabriel Fahrenheit (born May 24, 1686, Danzig (GdaÅ„sk); died September 16, 1736, The Hague, Netherlands), was a German physicist and an engineer, who worked for most of his life in the Netherlands. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery white Atomic mass 200. ... Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ...

In 1731, René-Antoine Ferchault de Réaumur (1683-1757) made a simpler temperature scale. On this scale 0 represented the freezing point of water (273.15 K) and 80 represented the boiling point (373.15 K). René Antoine Ferchault de Réaumur. ...

In 1742, Anders Celsius (1701-1744) invented the centigrade or Celsius temperature scale in which 100° represented the boiling point of water (373.15 K) and 0° represented the freezing point (273.15 K). Anders Celsius The observatory of Anders Celsius, from a contemporary engraving. ... The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed a similar system in 1742. ...

Units of temperature

The basic unit of temperature (symbol: T) in the International System of Units (SI) is the kelvin (K). One kelvin is formally defined as 1/273.16 of the temperature of the triple point of water (the point at which water, ice and water vapor exist in equilibrium). The temperature 0 K is called absolute zero and corresponds to the point at which the molecules and atoms have the least possible thermal energy. An important unit of temperature in theoretical physics is the Planck temperature (1.4 × 10³² K). The International System of Units (abbreviated SI from the French phrase, Système International dUnités) is the most widely used system of units. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. ... Water (from the Old English word wæter and the German word Wasser) is a colorless, tasteless, and odorless substance in its pure form that is essential to all known forms of life and is known also as the most universal solvent. ... Icicles A natural, 4 tonne, block of ice on a beach in Iceland Ice is the solid form of water. ... Vapor (US English) or vapour (British English) is the gaseous state of matter. ... For the 2002 science fiction movie see Equilibrium (2002 movie) Equilibrium or balance is any of a number of related phenomena in the natural and social sciences. ... Absolute zero is the highest temperature that cannot be obtained in any macroscopic system. ... A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... Properties For alternative meanings see atom (disambiguation). ... Thermal energy is kinetic energy due to disordered motions and vibrations of microscopic particles such as molecules and atoms. ... The Planck temperature, named after German physicist Max Planck, is the natural unit of temperature, denoted by TP. TP = (c5 / G)1/2 / k ≈ 1. ...

In the field of plasma physics, because of the high temperatures encountered and the electromagnetic nature of the phenomena involved, it is customary to express temperature in electron volts (eV) or kilo electron volts (keV), where 1 eV = 11,605 K. In the study of QCD matter one routinely meets temperatures of the order of a few hundred MeV, equivalent to about 10¹² K. A Plasma lamp In physics and chemistry, a plasma is an ionized gas, and is usually considered to be a distinct phase of matter. ... Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... Quark Matter refers to any of a number of phases of matter built out of quarks and gluons. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ...

For everyday applications, it is often convenient to use the Celsius scale, in which 0 °C corresponds to the temperature at which water freezes and 100 °C corresponds to the boiling point of water at sea level. In this scale a temperature difference of 1 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the kelvin scale, but offset by the temperature at which water freezes (273.15 K). Thus the following equation can be used to convert from Celsius to kelvin. The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed a similar system in 1742. ... The melting point of a solid is the temperature at which it changes state from solid to liquid. ... The boiling point of a substance is the temperature at which it can change state from a liquid to a gas throughout the bulk of the liquid. ...

In the United States, the Fahrenheit scale is widely used. On this scale the freezing point of water corresponds to 32 °F and the boiling point to 212 °F. The following formula can be used to convert from Fahrenheit to Celsius: Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ...

See temperature conversion formulas for conversions between most temperature scales. Summary Kelvin Celsius Fahrenheit Rankine Delisle Newton Réaumur Rømer See also Conversion of units External links Online Temperature Conversion Categories: Physical quantity | Thermodynamics ...

¹ Only the kelvin, Celsius, Fahrenheit, and Rankine scales are in use today.
² Some numbers in this table have been rounded off.
³ Normal human body temperature is 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F. The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed a similar system in 1742. ... Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ... Rankine is a now rarely used temperature scale named after the Scottish engineer and physicist William John Macquorn Rankine, who proposed it in 1859. ... The Delisle scale is a temperature scale invented in 1732 by the French astronomer Joseph-Nicolas Delisle (1688–1768). ... Around 1700, Isaac Newton (1642–1727) applied his mind to the problem of heat. ... The degree Réaumur is a unit of temperature named after René Antoine Ferchault de Réaumur, who first proposed it in 1731. ... Rømer is a disused temperature scale named after the Danish astronomer Ole Christensen Rømer, who proposed it in 1701. ... Absolute zero is the highest temperature that cannot be obtained in any macroscopic system. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ... General Name, Symbol, Number titanium, Ti, 22 Chemical series transition metals Group, Period, Block 4, 4, d Appearance silvery metallic Atomic mass 47. ... The photosphere of an astronomical object is the region at which the optical depth becomes one. ...

Articles about temperature ranges:

• 10⁻¹² K = 1 picokelvin (pK)
• 10⁻⁹ K = 1 nanokelvin (nK)
• 10⁻⁶ K = 1 microkelvin (µK)
• 10⁻³ K = 1 millikelvin (mK)
• 10⁰ K = 1 kelvin
• 10¹ K = 10 kelvins
• 10² K = 100 kelvins
• 10³ K = 1,000 kelvin = 1 kilokelvin (kK)
• 10⁴ K = 10,000 kelvins = 10 kK
• 10⁵ K = 100,000 kelvins = 100 kK
• 10⁶ K = 1 megakelvin (MK)
• 10⁹ K = 1 gigakelvin (GK)
• 10¹² K = 1 terakelvin (TK)

To help compare different orders of magnitude this page lists temperatures between 1 picokelvin (10-12 K) and 1 nanokelvin (10-9 K). ... To help compare different orders of magnitude this page lists temperatures between 1 nanokelvin and 1 microkelvin. ... To help compare different orders of magnitude this page lists temperatures between 1 microkelvin and 1 millikelvin. ... To help compare different orders of magnitude this page lists temperatures between 1 millikelvin and 1 kelvin. ... To help compare different orders of magnitude this page lists temperatures between 1 kelvin and 10 kelvins. ... To help compare different orders of magnitude this page lists temperatures between 10 kelvins and 100 kelvins. ... To help compare different orders of magnitude this page lists temperatures between 100 kelvins and 1000 kelvins. ... To help compare different orders of magnitude this page lists temperatures between 1,000 kelvins and 10,000 kelvins. ... To help compare different orders of magnitude this page lists temperatures between 10,000 kelvins and 100,000 kelvins. ... To help compare different orders of magnitude this page lists temperatures between 100,000 kelvins and 1,000,000 kelvins. ... To help compare different orders of magnitude this page lists temperatures in the megakelvin range between 1 million kelvins and 1000 million kelvins. ... To help compare different orders of magnitude this page lists temperatures in the gigakelvin range between 109 kelvins and 1012 kelvins. ... To help compare different orders of magnitude this page lists temperatures in the terakelvin range between 1012 kelvins and 1015 kelvins. ...

Theoretical foundation of temperature

Zeroth-law definition of temperature

While most people have a basic understanding of the concept of temperature, its formal definition is rather complicated. Before jumping to a formal definition, let's consider the concept of thermal equilibrium. If two closed systems with fixed volumes are brought together, so that they are in thermal contact, changes may take place in the properties of both systems. These changes are due to the transfer of heat between the systems. When a state is reached in which no further changes occur, the systems are in thermal equilibrium. In thermodynamics, a thermodynamic system is in thermodynamic equilibrium if its energy distribution equals a Maxwell-Boltzmann-distribution. ...

Now a basis for the definition of temperature can be obtained from the so-called zeroth law of thermodynamics which states that if two systems, A and B, are in thermal equilibrium and a third system C is in thermal equilibrium with system A then systems B and C will also be in thermal equilibrium (being in thermal equilibrium is a transitive relation; moreover, it is an equivalence relation). This is an empirical fact, based on observation rather than theory. Since A, B, and C are all in thermal equilibrium, it is reasonable to say each of these systems shares a common value of some property. We call this property temperature. The zeroth law of thermodynamics is the law which states that thermal equilibrium is transitive. ... In mathematics, a binary relation R over a set X is transitive if it holds for all a, b, and c in X, that if a is related to b and b is related to c, then a is related to c. ... In mathematics, an equivalence relation on a set X is a binary relation on X that is reflexive, symmetric and transitive, i. ...

Generally, it is not convenient to place any two arbitrary systems in thermal contact to see if they are in thermal equilibrium and thus have the same temperature. Also, it would only provide an ordinal scale. Scaling is the measurement of a variable in such a way that it can be expressed on a continuum. ...

Therefore, it is useful to establish a temperature scale based on the properties of some reference system. Then, a measuring device can be calibrated based on the properties of the reference system and used to measure the temperature of other systems. One such reference system is a fixed quantity of gas. The ideal gas law indicates that the product of the pressure and volume (P · V) of a gas is directly proportional to the temperature: The ideal gas law, or universal gas equation, is an equation of state of an ideal gas. ... This article is about proportionality, the mathematical relation. ...

(1)

where T is temperature, n is the number of moles of gas and R is the gas constant. Thus, one can define a scale for temperature based on the corresponding pressure and volume of the gas: the temperature in kelvins is the pressure in pascals of one mole of gas in a container of one cubic metre, divided by 8.31... In practice, such a gas thermometer is not very convenient, but other measuring instruments can be calibrated to this scale. The mole (symbol: mol) is one of the seven SI base units and is commonly used in chemistry. ... U R ALL NERDS HAHAHAHAHAHAHAHAHAHA ...

Equation 1 indicates that for a fixed volume of gas, the pressure increases with increasing temperature. Pressure is just a measure of the force applied by the gas on the walls of the container and is related to the energy of the system. Thus, we can see that an increase in temperature corresponds to an increase in the thermal energy of the system. When two systems of differing temperature are placed in thermal contact, the temperature of the hotter system decreases, indicating that heat is leaving that system, while the cooler system is gaining heat and increasing in temperature. Thus heat always moves from a region of high temperature to a region of lower temperature and it is the temperature difference that drives the heat transfer between the two systems.

Temperature in gases

As mentioned previously for a monatomic ideal gas the temperature is related to the translational motion or average speed of the atoms. The kinetic theory of gases uses statistical mechanics to relate this motion to the average kinetic energy of atoms and molecules in the system. For this case 7736 K = 7463 degrees Celsius corresponds to an average kinetic energy of one electronvolt; to take room temperature (300 K) as an example, the average energy of air molecules is 300/7736 eV, or 0.0388 electronvolt. This average energy is independent of particle mass, which seems counterintuitive to many people. Although the temperature is related to the average kinetic energy of the particles in a gas, each particle has its own energy which may or may not correspond to the average. However, after an examination of some basic physics equations it makes perfect sense. The second law of thermodynamics states that any two given systems when interacting with each other will later reach the same average energy. Temperature is a measure of the average kinetic energy of a system. The formula for the kinetic energy of an atom is:
The kinetic theory of gases is a theory that explains the macroscopic properties of gases by consideration of their composition at a molecular level. ... Statistical mechanics is the application of statistics, which includes mathematical tools for dealing with large populations, to the field of mechanics, which is concerned with the motion of particles or objects when subjected to a force. ... To help compare different orders of magnitude this page lists temperatures between 1,000 kelvins and 10,000 kelvins. ... To help compare different orders of magnitude we list here energies between 10−19 joules and 10−18 joules (0. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... To help compare different orders of magnitude this page lists temperatures between 100 kelvins and 1000 kelvins. ... To help compare different orders of magnitude we list here energies between 10−21 joule and 10−20 joule (0. ... Kinetic energy (also called vis viva, or living force) is energy possessed by a body by virtue of its motion. ...

(Note that a calculaton of the kinetic energy of a more complicated object, such as a molecule, is slightly more involved. Additional degrees of freedom are available, so molecular rotation or vibration must be included.)

Thus, particles of greater mass (say a neon atom relative to a hydrogen molecule) will move slower than lighter counterparts, but will have the same average energy. This average energy is independent of the mass because of the nature of a gas, all particles are in random motion with collisions with other gas molecules, solid objects that may be in the area and the container itself (if there is one). A visual illustration of this from Oklahoma State University makes the point more clear. Not all the particles in the container have different velocities, regardless of whether there are particles of more than one mass in the container, but the average kinetic energy is the same because of the ideal gas law. In a gas the distribution of energy (and thus speeds) of the particles corresponds to the Boltzmann distribution. The phrase degrees of freedom is used in three different branches of science: in physics and physical chemistry, in mechanical and aerospace engineering, and in statistics. ... General Name, Symbol, Number neon, Ne, 10 Chemical series noble gases Group, Period, Block 18, 2, p Appearance colorless Atomic mass 20. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Gas (actually, as), the GNU assembler, is the default GCC back-end. ... The ideal gas law, or universal gas equation, is an equation of state of an ideal gas. ... The Maxwell-Boltzmann distribution is a probability distribution with applications in physics and chemistry. ...

An electronvolt is a very small unit of energy, approximately 1.602×10^-19 joule. An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... The joule (symbol J, also called newton meter, watt second, or coulomb volt) is the SI unit of energy and work. ...

Temperature of the vacuum

A system in a vacuum will radiate its thermal energy, i.e. convert heat into electromagnetic waves. It will do so until an equilibrium with the vacuum is found. This equilibrium will not be at 0 K if the vacuum is filled with electromagnetic waves. Conversely, the system can absorb energy from the vacuum if it contains intense electromagnetic waves. Radiation generally means the transmission of waves, objects or information from a source into a surrounding medium or destination. ... Absorption has a number of meanings: In physics, absorption is a process in which particles of some sort encounter another material and are taken up by or even disappear in it. ...

At equilibrium, the radiation's spectrum will be the same as the radiation of a black body at the equilibrium temperature, so that one can say that the vacuum has that temperature. Far from equilibrium, the spectrum usually have very different shapes, and one temperature cannot be assigned to the vacuum anymore. Sometimes, a part of the spectrum follows that shape: for example one can say that the cosmic microwave background radiation, a part of the cosmic radiation, has a temperature of about 3 K. The power spectrum is a plot of the portion of a signals power (energy per unit time) falling within given frequency bins. ... As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ... // Basic information The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ...

Second-Law definition of temperature

In the previous section temperature was defined in terms of the Zeroth Law of thermodynamics. It is also possible to define temperature in terms of the second law of thermodynamics, which deals with entropy. Entropy is a measure of the disorder in a system. The second law states that any process will result in either no change or a net increase in the entropy of the universe. This can be understood in terms of probability. Consider a series of coin tosses. A perfectly ordered system would be one in which every coin toss would come up either heads or tails. For any number of coin tosses, there is only one combination of outcomes corresponding to this situation. On the other hand, there are multiple combinations that can result in disordered or mixed systems, where some fraction are heads and the rest tails. As the number of coin tosses increases, the number of combinations corresponding to imperfectly ordered systems increases. For a very large number of coin tosses, the number of combinations corresponding to ~50% heads and ~50% tails dominates and obtaining an outcome significantly different from 50/50 becomes extremely unlikely. Thus the system naturally progresses to a state of maximum disorder or entropy. In physics, the second law of thermodynamics, in its many forms, is a statement about the quality and direction of energy flow, and it is closely related to the concept of entropy. ... The thermodynamic entropy S, often simply called the entropy in the context of thermodynamics, is a measure of the amount of energy in a physical system that cannot be used to do work. ...

Now, we have stated previously that temperature controls the flow of heat between two systems and we have just shown that the universe, and we would expect any natural system, tends to progress so as to maximize entropy. Thus, we would expect there to be some relationship between temperature and entropy. In order to find this relationship let's first consider the relationship between heat, work and temperature. A Heat engine is a device for converting heat into mechanical work and analysis of the Carnot heat engine provides the necessary relationships we seek. The work from a heat engine corresponds to the difference between the heat put into the system at the high temperature, q_H and the heat ejected at the low temperature, q_C. The efficiency is the work divided by the heat put into the system or: A heat engine performs the conversion of heat energy to work by exploiting the temperature gradient between a hot source and a cold sink. Heat is transferred to the sink from the source, and in this process some of the heat is converted into work. ... A heat engine is an engine that uses heat to produce mechanical work by carrying a working substance through a cyclic process. ...

(2)

where w_cy is the work done per cycle. We see that the efficiency depends only on q_C/q_H. Because q_C and q_H correspond to heat transfer at the temperatures T_C and T_H, respectively, q_C/q_H should be some function of these temperatures:

(3)

Carnot's theorem states that all reversible engines operating between the same heat reservoirs are equally efficient. Thus, a heat engine operating between T₁ and T₃ must have the same efficiency as one consisting of two cycles, one between T₁ and T₂, and the second between T₂ and T₃. This can only be the case if: A heat engine is an engine that uses heat to produce mechanical work by carrying a working substance through a cyclic process. ...

which implies:

q₁3 = f(T₁,T₃) = f(T₁,T₂)f(T₂,T₃)

Since the first function is independent of T₂, this temperature must cancel on the right side, meaning f(T₁,T₃) is of the form g(T₁)/g(T₃) (i.e. f(T₁,T₃) = f(T₁,T₂)f(T₂,T₃) = g(T₁)/g(T₂)· g(T₂)/g(T₃) = g(T₁)/g(T₃)), where g is a function of a single temperature. We can now choose a temperature scale with the property that:

(4)

Substituting Equation 4 back into Equation 2 gives a relationship for the efficiency in terms of temperature:

(5)

Notice that for T_C = 0 K the efficiency is 100% and that efficiency becomes greater than 100% below 0 K. Since an efficiency greater than 100% violates the first law of thermodynamics, this implies that 0 K is the minimum possible temperature. In fact the lowest temperature ever obtained in a macroscopic system was 20 nK, which was achieved in 1995 at NIST. Subtracting the right hand side of Equation 5 from the middle portion and rearranging gives:

where the negative sign indicates heat ejected from the system. This relationship suggests the existence of a state function, S, defined by:

(6)

where the subscript indicates a reversible process. The change of this state function around any cycle is zero, as is necessary for any state function. This function corresponds to the entropy of the system, which we described previously. We can rearranging Equation 6 to get a new definition for temperature in terms of entropy and heat:

(7)

For a system, where entropy S may be a function S(E) of its energy E, the termperature T is given by:

(8)

The reciprocal of the temperature is the rate of increase of entropy with energy.

Negative temperatures

See main article: Negative temperature.

For some systems and specific definitions of temperature, it is possible to obtain a negative temperature. A system with a negative temperature is not colder than absolute zero, but rather it is, in a sense, hotter than infinite temperature (sic). Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Absolute zero is the highest temperature that cannot be obtained in any macroscopic system. ... Infinity is a word carrying a number of different meanings in mathematics, philosophy, theology and everyday life. ...

See also

• Maxwell's demon
• Heat conduction
• ITS-90 International Temperature Scale

Maxwells demon is a character in an 1867 thought experiment by the Scottish physicist James Clerk Maxwell, meant to raise questions about the second law of thermodynamics. ... Heat flow along perfectly insulated wire Heat conduction is the transmission of heat across matter. ... ITS-90 is the current version of the International Temperature Scale. ...

References

• Kroemer, Herbert; Kittle, Charles (1980). Thermal Physics (2nd ed.). W. H. Freeman Company. ISBN 0716710889.

External links

Look up Temperature in Wiktionary, the free dictionary

• Temperature Conversions: Celsius (Centigrade), Fahrenheit, Kelvin, Réaumur and Rankine
• An elementary introduction to temperature aimed at a middle school audience
• Why do we have so many temperature scales?
• A Brief History of Temperature Measurement