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temperature, measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used. A temperature scale is determined by choosing two reference temperatures and dividing the temperature difference between these two points into a certain number of degrees. The two reference temperatures used for most common scales are the melting point of ice and the boiling point of water. On the Celsius temperature scale, or centigrade scale, the melting point is taken as 0℃ and the boiling point as 100℃, and the difference between them is divided into 100 degrees. On the Fahrenheit temperature scale, the melting point is taken as 32℉ and the boiling point as 212℉, with the difference between them equal to 180 degrees. The Réaumur scale, used in some parts of Europe, also sets the melting point at zero, but it has an 80-degree temperature difference between 0°Re and the boiling point at 80°Re. The temperature of a substance does not measure its heat content but rather the average kinetic energy of its molecules resulting from their motions. A one-pound block of iron and a two-pound block of iron at the same temperature do not have the same heat content. Because they are at the same temperature the average kinetic energy of the molecules is the same; however, the two-pound block has more molecules than the one-pound block and thus has greater heat energy. A temperature scale can be defined theoretically for which zero degree corresponds to zero average kinetic energy (see gas laws). Such a point is called absolute zero, and such a scale is known as an absolute temperature scale. The Kelvin temperature scale is an absolute scale having degrees the same size as those of the Celsius temperature scale; the Rankine temperature scale is an absolute scale having degrees the same size as those of the Fahrenheit temperature scale. The relationship between absolute temperature and average molecular kinetic energy is one result of the kinetic-molecular theory of gases. See heat; thermodynamics.
A concept related to the flow of heat from one object or region of space to another. The term refers not only to the senses of hot and cold but to numerical scales and thermometers as well. Fundamental to the concept are the absolute scale and absolute zero and the relation of absolute temperatures to atomic and molecular motions.
Thermometers do not measure a special physical quantity. They measure length (as of a mercury column) or pressure or volume (with the gas thermometer at the National Institute of Standards and Technology) or electrical voltage (with a thermocouple). The basic fact is that if a mercury column has the same length when touching two different, separated objects when the objects are placed in contact, no heat will flow from one to the other. See Thermometer
The numbers on the thermometer scales are merely historical choices; they are not scientifically fundamental. The most widely used scales are the Fahrenheit (°F) and the Celsius (°C). The centigrade scale with 0° assigned to ice water (ice point) and 100° assigned to water boiling under one atmosphere pressure (steam point) was formerly used, but it has been succeeded by the Celsius scale, defined in a different way than the centigrade scale. However, on the Celsius scale the temperatures of the ice and steam points differ by only a few hundredths of a degree from 0° and 100°, respectively. The illustration shows how the Celsius and Fahrenheit scales compare and how they fit onto the absolute scales.
In 1848 William Thomson (Lord Kelvin), following ideas of Sadi Carnot, stated the concept of an absolute scale of temperature in terms of measuring amounts of heat flowing between objects. Most important, Kelvin conceived of a body which would not give up any heat and which was at an absolute zero of temperature. Experiments have shown that absolute zero corresponds to -273.15°C or -459.7°F. Two absolute scales, shown in the illustration, are the Kelvin (K) and the Rankine (°R).
In practice, absolute temperatures are measured by using low-density helium gas and dilute paramagnetic crystals, the most nearly ideal of real materials. The measurement of a single temperature with a gas or magnetic thermometer is a major scientific event done at a national standards laboratory. Only a few temperatures have been measured, including the freezing point of gold (1337.91 K or 1948.57°F), and the boiling points of sulfur (717.85 K or 832.46°F), oxygen (90.18 K or -297.35°F), and helium (4.22 K or -452.07°F). Various types of thermometers (platinum, carbon, and doped germanium resistors; thermocouples) are calibrated at these temperatures and used to measure intermediate temperatures. See Temperature measurement, Thermodynamic principles
temperatureSymbol: T . A property of a body or region that determines the direction of heat flow under thermal contact – always from a higher-temperature to a lower-temperature body or region. Numerical values of temperature are assigned by means of an internationally accepted scale of temperature, and for scientific purposes are now expressed in kelvin (K) or degrees Celsius (°C): a temperature difference of 1 K is equal to a temperature difference of 1 °C, and a Celsius temperature t and a thermodynamic temperature T are related by the formula t = T – 273.15.
Astronomical temperatures are usually determined from spectroscopic measurements. Since astronomical bodies do not generally have a uniform temperature distribution and do not obey exactly the laws determining temperature, there are several different types of temperature; each characterizes a particular property of the body and has a slightly different value. They include effective, color, brightness, ionization, and excitation temperature. The latter two are determined from the Saha ionization equation and the Boltzmann equation. See also thermodynamic temperature scale.
in astrophysics, a parameter that characterizes the physical state of a medium. In astrophysics the temperature of celestial objects is determined by investigating their radiation on the basis of certain theoretical assumptions; in particular, it is assumed that the medium is in thermodynamic equilibrium and that the laws of blackbody radiation apply. Since, however, the conditions prevailing in celestial objects, such as stars and nebulas, differ markedly from thermodynamic equilibrium, temperature determinations by different methods may yield considerably different results. The following types of temperature are in use: effective temperature, brightness temperature, color temperature, excitation temperature, kinetic temperature, electron temperature, ion temperature, and ionization temperature.
The effective temperature of a star or some other object, such as the solar corona, is the temperature of a blackbody having the same dimensions and producing the same total radiant flux as the star or object.
The brightness temperature is the temperature of a blackbody whose radiation intensity at a specific wavelength equals the intensity observed in a given direction.
The color, or spectrophotometric, temperature is the temperature of a blackbody whose distribution of relative radiation intensity in the region of the spectrum in question is closest to the observed distribution. This temperature may vary greatly for different spectral regions.
The excitation temperature is a parameter that characterizes the distribution of atoms according to excitation states (the “population” of electron energy levels). It is assumed that this distribution can be represented by Boltzmann’s equation
where x0 is the excitation potential, k is the Boltzmann constant, n0 is the number of atoms in the normal, unexcited state, n is the number of atoms in the excited state, and T is the temperature. The excitation temperature in a given medium may vary for different atoms and energy levels.
The kinetic temperature is a parameter that characterizes the average kinetic energy of thermal motion of the particles according to the equation
where m is the mass and v is the particle velocity.
The electron and ion temperatures are the kinetic temperatures of electrons and ions, respectively.
The ionization temperature is a parameter that characterizes the degree of ionization of matter and is determined from the relative intensity of spectral lines on the assumption that certain theoretical hypotheses (Saha’s ionization equation) are valid.
For the state of thermodynamic equilibrium, all definitions of temperature yield the same quantity.
REFERENCETeoreticheskaia astrofizika. Moscow, 1952.
a physical quantity that characterizes the state of thermodynamic equilibrium of a macroscopic system.
The temperature is the same for all parts of an isolated system in thermodynamic equilibrium. If an isolated system is not in equilibrium, then in time heat transfer from the hotter to the colder parts of the system leads to temperature equalization throughout the entire system; this phenomenon is described by the zeroth law of thermodynamics.
Temperature determines the energy-level distribution of the particles in a system (seeBOLTZMANN STATISTICS); the particle velocity distribution (seeMAXWELLIAN DISTRIBUTION); the degree of ionization of matter (seeSAHA EQUATION); the properties of the equilibrium electromagnetic radiation of bodies, namely, the spectral density of the radiation (seePLANCK’S RADIATION LAW); and the total energy radiated per unit volume (seeSTEFAN-BOLTZMANN LAW).
The temperature appearing as a parameter in the Boltzmann distribution is sometimes called the excitation temperature, that appearing in the Maxwellian distribution is called the kinetic temperature, that appearing in the Saha equation is called the ionization temperature, and that found in the Stefan-Boltzmann law is called the radiation temperature. Since all these parameters are equivalent for a system in thermodynamic equilibrium, they are simply called the temperature of the system.
In the kinetic theory of gases and other branches of statistical mechanics, temperature is defined quantitatively in such a manner that the average kinetic energy of the translational motion of a particle having three degrees of freedom is (3/2)kT, where k is the Boltzmann constant and T is the temperature of the body. In the general case, temperature is defined as the derivative of the energy of a body as a whole with respect to its entropy. This temperature is always positive, since the kinetic energy is positive, and is called the absolute temperature or the temperature on the thermodynamic temperature scale. The kelvin (K) has been adopted as the unit of absolute temperature in the International System of Units; it is also known as the degree Kelvin (°K). Temperature is frequently measured on the Celsius scale (t). The relation between t and T is given by the equation t = T – 273.15°K; the degree Celsius is equal to the kelvin. Methods for measuring temperature are discussed in THERMOMETRY and THERMOMETER.
Only the equilibrium state of bodies is described by a strictly defined temperature. There are, however, systems whose state may be approximately characterized by several temperatures that are not equal to each other. A plasma consisting of light and heavy charged particles (electrons and ions, respectively) provides an example of such a system. In plasma particle collisions, energy is rapidly transferred from electrons to electrons and from ions to ions, but energy is slowly transferred from electrons to ions and vice versa. There are plasma states in which the systems of electrons and ions, taken separately, are near equilibrium, and an electron temperature and an ion temperature that are not the same can be introduced.
In bodies whose particles have a magnetic moment, energy is usually transferred slowly from translational to magnetic degrees of freedom, which are associated with possible changes in the direction of the magnetic moment. For this reason, states exist in which the system of magnetic moments is characterized by a temperature that is not equal to the kinetic temperature corresponding to the translational motion of the particles. The magnetic temperature determines the magnetic portion of the internal energy and may be either positive or negative (seeNEGATIVE TEMPERATURE). As the temperatures equalize, energy is transferred from particles (degrees of freedom) with a higher temperature to particles (degrees of freedom) with a lower temperature if the temperatures are simultaneously positive or negative, but energy is transferred in the opposite direction if one temperature is positive and the other is negative. In this sense, a negative temperature is “higher” than any positive temperature.
The concept of temperature is also used to characterize non-equilibrium systems (see). For example, the brightness of celestial bodies is characterized by a brightness temperature and the spectral composition of radiation, by a color temperature.
A. F. ANDREEV