Kelvin thermodynamic scale

The ITS-90 scale is designed to give temperatures T90 that do not differ from the Kelvin Thermodynamic Scale by more than the uncertainties associated with the measurement of the fixed points on the date of adoption of ITS-90 (January 1, 1990), to extend the low-temperature range previously covered by EPT-76, and to replace the high-temperature thermocouple measurements of IPTS-68 with platinum resistance thermometry. The result is a scale that has better agreement with thermodynamic temperatures, and much better continuity, reproducibility, and accuracy than all previous international scales. 

The relation between the Centigrade and Kelvin thermodynamic scales is determined by... 

This equation is identical to Equation (3.11). Therefore, the Kelvin thermodynamic scale and the ideal gas scale become identical when the temperature of the triple point of water is assigned the value of 273.16 K. 

Note that Table 2.29 refers to the temperature K or the temperature °C. Both of these measurement scales are temperature measurement units. There is another scale, also known as K, which is the unit of measurement for the Kelvin thermodynamic scale. Because heat is a thermodynamic property, temperature... 

In 1854, Lord Kelvin recommended that when a single fixed point that was sufficiently stable was developed, it would be preferable to define the scale using only that one point (plus the absolute zero) (Thomson, 1854). Accepting this recommendation, the 10th General Conference in 1954 redefined the Kelvin thermodynamic scale (Comite International, 1955). The redefinition of the scale was accomplished by assigning a value of 273.16 K to the temperature of the triple point of water. The unit, K, was defined as 1/273.16 of the thermodynamic temperature of the triple point. The zero of the Celsius thermodynamic scale was defined to be 0.01°C below the triple point, that is, a temperature on the scale was to be expressed in terms of its difference from that of the thermal state 0.01 K lower than the triple point of water. The temperature expressed in this way is the Celsius thermodynamic temperature, t, and is defined by... 

Temperature Scales A quantitative description of temperature requires the definition of a temperature scale. The two most commonly encountered in thermodynamics are the absolute or ideal gas (°A) scale and the thermodynamic or Kelvin (K) scale."... 

The Thermodynamic or Kelvin Temperature Scale Description of the Kelvin temperature scale must wait for the laws of thermodynamics. We will see that the Kelvin temperature is linearly related to the absolute or ideal gas temperature, even though the basic premises leading to the scales are very different, so that... 

It was Lord Kelvin who recognized that Carnot s hypothetical engine was of fundamental importance, and used it to define a thermodynamic scale of temperature that has become known as the Kelvin temperature. He set the thermodynamic temperature T of the reservoirs proportional to the amount of heat exchanged at each that is. 

Kelvin (degree Kelvin) K Defined in the thermodynamic scale by assigning 273.16 K to the triple point of water (freezing point, 273.15 K = 0°C)... 

The Kelvin scale is thus defined in terms of an ideal reversible heat engine. At first such a scale does not appear to be practical, because all natural processes are irreversible. In a few cases, particularly at very low temperatures, a reversible process can be approximated and a temperature actually measured. However, in most cases this method of measuring temperatures is extremely inconvenient. Fortunately, as is proved in Section 3.7, the Kelvin scale is identical to the ideal gas temperature scale. In actual practice we use the International Practical Temperature Scale, which is defined to be as identical as possible to the ideal gas scale. Thus, the thermodynamic scale, the ideal gas scale, and the International Practical Temperature Scale are all consistent scales. Henceforth, we use the symbol T for each of these three scales and reserve the symbol 9 for any other thermodynamic scale. 

Thermal conductivity is expressed in several different internationally recognized ways. One method of expressing thermal conductivity (A) is in terms of the heat flux under steady conditions per square meter for one meter of thickness of one degree Kelvin difference in temperature. Kelvin is a thermodynamic scale and is centigrade starting at absolute zero. 

This is sometimes called the Kelvin temperature and measured in degrees Kelvin. More rigorously, the Kelvin temperature scale is defined by the second law of thermodynamics, but, numerically, it is the same as T. We will use the symbol K to designate absolute temperature as defined by Eq. (3). 

The Kelvin degree scale was originally introduced by William Thomson. It is a thermodynamic temperature scale based on the second law of thermodynamics and is identical to the absolute temperature scale based on the above volume expansion arguments. Substituting Equation (1.4) into Equation (1.3) gives ... 

The general concept of temperature scales is discussed briefly in Exp. 1. The thermodynamic temperature scale, based on the second law of thermodynamics, embraces the Kelvin (absolute) scale and the Celsins scale, the latter being defined by the equation... 

The size of the kelvin, the SI temperature unit with symbol K, is defined by the statement that the triple point of pure water is exactly 273.16 K. The practical usefulness of the thermodynamic scale suffers from the lack of convenient instruments with which to measure absolute temperatures routinely to high precision. Absolute temperatures can be measured over a wide range with the helium-gas thermometer (appropriate corrections being made for gas imperfections), but the apparatus is much too complex and the procedure much too cumbersome to be practical for routine use. 

It will be seen in Chapter VII ( 18k) that it is possible to develop an absolute temperature scale, also independent of the nature of the thermometric substance, based on the second law of thermodynamics. This is sometimes called the Kelvin scale, in honor of its originator. Lord Kelvin (William Thomson). Actually, the thermodynamic scale can be shown to be identical with the absolute ideal gas scale, as defined above hence, temperatures on the latter, as well as the former, scale are represented by the symbol K. The ice point is consequently 273.16 K. It may be noted, incidentally, that the thermodynamic derivation of the absolute temperature scale provides a more definite interpretation of the absolute zero, i.e., the lowest limit of temperature, than is possible by means of the ideal gas thermometer. ... 

In view of the elaborate experimental techniques usually required to make accurate thermodynamic temperature measurements, the need for a practical scale above about 0.5 K that is close to the Kelvin thermodynamic temperature scale remains great. There are several modifications that can be anticipated for a future IPTS. They include assigned values of fixed points that are in closer agreement with thermodynamic temperatures (as determined by recent experiments), extension of the range covered to lower temperature, improved standard instruments for interpolation procedures. It is expected that there will be a scale revision which will encompass the above, and that the new scale will be adopted by about 1987. 

Since we are assuming, for the present, that only the ideal gas Kelvin temperature scale has a firm thermodynamic basis, we will use it. rather than the Fahrenheit and Celsius scales, in all thermodynamic calculations. (Another justification for the use of an absolute-temperature scale is that the interrelation between pressure, volume, and temperature for fluids is simplest when absolute temperature is used.) Consequently, if the data for a thermodynamic calculation are not given in terms of absolute temperature, it will generally be necessary to convert these data to absolute temperatures using Eqs. 1.4-4. ... 

Clearly, any number of thermodynamic scales exist, but one must be chosen, and the Kelvin scale is that one. Equally clearly, any number of empirical scales exist that are not thermodynamic scales, since for them q /q2 T1/T2. For example, the Celsius scale is not a thermodynamic scale since 100/0 is not even close to 1.3661. 

Equation (2.6) defines a new temperature scale, called a gas scale of temperature or, more exactly, an ideal gas scale of temperature. The importance of this scale lies in the fact that the limiting value of Kq, and consequently I/kq, has the same value for all gases. On the other hand, o does depend on the scale of temperature used originally for t. If t is in degrees Celsius (symbol °C), then 1/ao = 273.15 °C. The resulting T-scale is numerically identical to the thermodynamic temperature scale, which we will discuss in detail in Chapter 8. The SI unit of thermodynamic temperature is the kelvin (symbol K). Temperatures on the thermodynamic scale are frequently called absolute temperatures or kelvin temperatures. According to Eq. (2.6) (see also Appendix III, Sect. A-III-6),... 

The current definition of the temperature scale is based on one fixed point, the triple point of water. The absolute temperature of that point is defined arbitrarily as 273.16 K exactly. (The triple point of water is that temperature at which pure liquid water is in equilibrium with ice and water vapor.) This definition fixes the size of the kelvin, the degree on the thermodynamic scale. The size of the Celsius degree is defined to be equal to one kelvin exactly and the origin of the Celsius scale of temperature is defined as 273.15 K exactly. 

Remember that all equations in thermodynamics use the absolute or kelvin temperature scale, so that if you are given temperatures in °C, you must convert them to the kelvin scale before using them. The standard temperature of 25 °C for example is 298.15 K. 

Thomson, wniiam, 1st Baron Kelvin (1824-1907) Irish-born Scottish physicist and mathematician who proposed the absolute, or Kelvin, temperature scale (1848) and, at around the same time as Rudolf Clausius, established the second law of thermodynamics. He also invented a tide predictor and a harmonic analyzer. 

Kelvin scale The fundamental temperature scale, also called the absolute or thermodynamic scale, in which the temperature measure is based on the average kinetic energy per molecule of a perfect gas. The zero of the Kelvin scale is -273.15°C (-459.67°F). A unit kelvin change is equivalent to a degree Celsius (°C) change. To convert from degree Fahrenheit (°F) to kelvin (K) use the formula Tk = (t°p + 459.67)71.8. Symbol K. 

The temperature scale used in kinetic theory is the absolute or thermodynamic (Kelvin) scale. The thermodynamic scale of temperature uses units of kelvins (K), which have the same size as the more familiar degrees Celsius (°C) but whose zero is absolute zero (-273.15 C)... 

The Celsius and Fahrenheit scales are based on the physical properties of water but the absolute or thermodynamic scale of temperature is not based on the physical properties of any substance. The kelvin is the SI unit of temperature and is based on absolute zero (approximately -273 °C)... 

The standard hydrogen electrode is therefore a practical, if not a convenient, reference, whereas some references in chemistry are theoretical references, for example the absolute or thermodynamic scale. Zero kelvin is experimentally unobtainable. Absolute zero is not just a reference - it is a fundamental rather than arbitrary point. This fact is not devalued by practical difficulties in making matter at absolute zero as a standard reference, and it could be said that the true standard hydrogen electrode reference is also unobtainable as, in principle, equilibrium takes forever to establish. 

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