Zeroth Law of Thermodynamics and Absolute Temperature
Thermometers with liquid working fluids are usually used for measurement of temperature. When such a device is brought in contact with a body whose temperature is to be measured, the liquid column inside the thermometer expands due to heat conducted from the body. The expanded length can be said to represent the degree of hotness in a somewhat quantitative manner.
The Zeroth Law of Thermodynamics states that if two bodies are in thermal equilibrium with a third body, then the two given bodies will be in thermal equilibrium with each other. The zeroth law of thermodynamics is used for measurement of temperature. In the Celsius temperature scale, two fixed points – ice point and steam point – are used to devise the scale. Thus, the freezing point of water (at standard atmospheric pressure) is assigned a value of zero, while the boiling point of pure water (at standard atmospheric pressure) denoted as 100. However for introducing detail, the distance between the two end points of the liquid column marks is arbitrarily divided into 100 equal spaces called degrees. This exercise can be extended both below zero and above 100 to expand the range of the thermometer.
The entire exercise can be carried out with any other substance as the thermometric fluid.However, for any specific measured temperature the extent of expansion of the liquid column will vary with the thermometric fluid as each fluid would expand to different extent under the influence of temperature. To overcome this problem, the ideal gas (see next section) has been arbitrarily chosen as the thermometric fluid. Accordingly, the temperature scale of the SI system is then described by the Kelvin unit (T0K). Its relation to the Celsius (t0 C) scale is given by:
T( 0 K ) = 273.15 + t (0C )
Thus the lower limit of temperature, called absolute zero on the Kelvin scale, occurs at –273.150C.