Absolute scale of temperature

Kelvin then replotted his data, this time extrapolating each graph till the volume of the gas was zero, which he found to occur at a temperature of -273.15 °C see Figure 1.5. He then devised a new temperature scale in which this, the coldest of temperatures, was the zero. He called it absolute zero, and each subsequent degree was equal to 1 °C. This new scale of temperature is now called the thermodynamic (or absolute) scale of temperature, and is also sometimes called the Kelvin scale. 

TRIPLE POINT. The temperature and pressure at which the solid, liquid, and vapor of a substance are in equilibrium with one another. Also applied to similar equilibrium between any three phases, Le., two solids and a liquid, etc. The triple point of water is +0.072 C at 4.6 mmHg it is of special importance because it is the fixed point for the absolute scale of temperature. 

An absolute scale of temperature can be designed by reference to the Second Law of Thermodynamics, viz. the thermodynamic temperature scale, and is independent of any material property. This is based on the Carnot cycle and defines a temperature ratio as ... 

Charles law defines the change of volume with changing temperature the volume of a definite amount of a gas under constant pressure is directly proportional to the absolute temperature. The absolute temperature is 273° plus the centigrade temperature but really the determination of the absolute scale of temperature depends entirely on the behavior of gases. 

The work of Carnot, published in 1824, and later the work of Clausius (1850) and Kelvin (1851), advanced the formulation of the properties of entropy and temperature and the second law. Clausius introduced the word entropy in 1865. The first law expresses the qualitative equivalence of heat and work as well as the conservation of energy. The second law is a qualitative statement on the accessibility of energy and the direction of progress of real processes. For example, the efficiency of a reversible engine is a function of temperature only, and efficiency cannot exceed unity. These statements are the results of the first and second laws, and can be used to define an absolute scale of temperature that is independent of ary material properties used to measure it. A quantitative description of the second law emerges by determining entropy and entropy production in irreversible processes. 

This mental experiment would not invalidate our deductions any more than the assumption of perfectly reversible processes, which also is only justifiable in the limiting case. Lord Kelvin avoids the use of the ideal gas altogether by defining the absolute scale of temperature in terms of the second law. 

The traditional way to approach the subject is to state the Second Law as it has been deduced on the basis of years of experience, and then show through use of the Carnot cycle the logical consequences (such as the existence of the entropy and an absolute scale of temperature). Two logically equivalent ways of stating the Second Law are... 

William Thomson (Lord Kelvin) (1824-1907) taught natural philosophy at Glasgow. In 1854 he proposed the absolute scale of temperature... 

Thomson, William (Lord Kelvin) (1824-1907), Scottish mathematician and physicist, was born in Belfast, Ireland. He proposed his absolute scale of temperature in 1848. During his life he published more than 600 papers and was elected to the Royal Society in 1851. He is buried in Westminster Abbey next to Isaac Newton. 

James Prescott Joule (1818-1889) An English physicist who discovered the relationship of heat to mechanical work (theory of conservation of energy, first law of thermodynamics). He collaborated from 1852 to 1856 with William Thomson (see box below). They developed the absolute scale of temperature and discovered the Joule-Thomson effect. Joule also frrund the relationship between the flow of current through a resistance and the dissipated heat, now called Joule s law. 

The fact that the efficiency of a reversible heat engine is independent of the physical and chemical nature of the engine has an important consequence which was noted by Lord Kelvin, William Thomson (1824-1907). Following Carnot s work, Lord Kelvin introduced the absolute scale of temperature. The efficiency of a reversible heat engine is a function only of the temperatures of the hot and cold reservoirs, independent of the material properties of the engine. Furthermore, the efficiency cannot exceed 1, in accordance with the First Law. These two facts can be used to define an absolute scale of temperature which is independent of any material properties. 

We refer to absolute zero as T= 0, not T= 0 K. There are other absolute scales of temperature, all of which set their lowest value at zero. Insofar as it is possible, all expressions in science should be independent of the units being employed, and in this case the lowest attainable temperature is T= 0 regardless of which absolute scale we are using. On the other hand, we write 6 = 0°C not 8 = 0 because the Celsius scale has an arbitrarily defined zero point. 

The Simple Gas Laws—The most common simple gas laws are Boyle s law relating gas pressure and volume (equation 6.5, Fig. 6-6) Charles s law relating gas volume and temperature (equation 6.8, Fig. 6-7) and Avogadro s law, relating volume and amount of gas. Some important ideas that originate from the simple gas laws are the Kelvin (absolute) scale of temperature (equation 6.6), the standard conditions of temperature and pressure (STP), and the molar volume of a gas at STP— 22.7 L/mol (expression 6.10). 

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