THERMODYNAMIC TEMPERATURE AND SCALE
THERMODYNAMIC TEMPERATURE AND SCALE
Development of Thermodynamic Temperature
Temperature is due to the random translation, rotational motions and vibrations of the submicroscopic particles. These motions constitute the internal energy of a substance. Thermodynamic temperature is the measure of the average kinetic energy per degree of freedom of its constituent particles. Therefore,it is one of the important thermodynamic parameters. Kinetic energy is considered zero at absolute zero. Absolute zero is also called the Null point.”
Third law of thermodynamics
It defines the thermodynamic temperature. Lowest thermodynamic temperature is the absolute zero. Temperature cannot go below this temperature. Because, at this temperature, the constituents of matter have minimal motion. Zero absolute temperature has also been called the ground state of matter. At this, matter is its state of minimum energy level. Thermodynamic temperature has been called absolute temperature because of being proposed by Kelvin. Further,it also does not depend on the properties of the working medium.
Practical applications of Thermodynamic temperature
Normally temperature (o C, o F etc.) is not used in thermodynamics. Temperature is used only in equations involving temperature difference. Thermodynamic temperature is most convenient in all the equations used in thermodynamics . Boyle’ Law, Charles Law, Perfect gas equation etc. all use thermodynamic temperature i.e. absolute temperature. It is also used in finding the efficiency of Carnot heat engine, COP of a Reversed Carnot cycle for a refrigerator as well as a heat pump.
All other scales of temperature depend on the properties of the working substance. While thermodynamic temperature scale is independent of the working medium. It is based on the fact the efficiency of a reversible steam engine depends only on two temperatures. This efficiency does not depend on the properties of the working substance. Thermodynamic scale differs from empirical scales in that it is absolute. It is based on basic laws of thermodynamics. Thermodynamic scale is based on the efficiency of the reversible heat engines. Absolute thermodynamic temperature scale is developed from the second law of thermodynamics.
S I Units
The International System of Units has defined a scale for the thermodynamic temperature. It has been called the thermodynamic temperature scale. Its lower limit is absolute zero (0 K) or (-2730 C) and upper point is the triple point of water at 273.16 K or o.16o C.
Features Of The Thermodynamic Temperature Scale
(i) One Kelvin is 1/273.16 of difference between absolute zero and the triple point of water
(ii) It fixes one Kelvin equal to 10C.
(iii) Thermodynamic temperature scale covers temperatures only between absolute zero (0 K) and the triple point of water (273.16 K or 0.01 °C).
(iv) For temperature outside this range, many different thermometer designs are required to cover the entire range. Such thermometers are as given below:
(i) Helium vapor pressure thermometer
(ii) Helium gas thermometer
(iii) Standard platinum resistance thermometer
(iv) Monochromatic radiation thermometers
(v) Liquid (mercury) thermometers
Different temperature scales
(i)Empirical Temperature Scale — Celsius, Kelvin, Fahrenheit etc..
(ii) Ideal Temperature Scale — Thermodynamic Temperature Scale and International Temperature Scale
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