NEET Physics Thermometry, Calorimetry & Thermal Expansion Thermometry

A branch of science which deals with the measurement of temperature of a substance is known as thermometry.

(1) The linear variation in some physical properties of a substance with change of temperature is the basic principle of thermometry and these properties are defined as thermometric property \[(x)\] of the substance.

(2) Thermometric properties \[(x)\] may be as follows

(i) Length of liquid in capillary

(ii) Pressure of gas at constant volume.

(iii) Volume of gas at constant pressure.

(iv) Resistance of a given platinum wire.

(3) In old thermometry, freezing point \[({{0}^{o}}C)\] and steam point \[({{100}^{o}}C)\] are taken to define the temperature scale. So if the thermometric property at temperature \[{{0}^{o}}C,\,{{100}^{o}}C\] and \[{{t}^{o}}C\] are \[{{x}_{0}},\,{{x}_{100}}\] and \[x\] respectively then

\[\frac{t-0}{100-0}=\frac{x-{{x}_{0}}}{{{x}_{100}}-{{x}_{0}}}\] Þ \[t{}^\circ C=\frac{x-{{x}_{0}}}{{{x}_{100}}-{{x}_{0}}}\times 100{}^\circ C\]

(4) In modern thermometry instead of two fixed points only one reference point is chosen (triple point of water 273.16 K) the other is itself 0 K where the value of thermometric property is assumed to be zero.

So if the value of thermometric property at 0 K, 273.16 K and TK are \[0,\,\,{{x}_{Tr}}\] and \[x\] respectively then

\[\frac{T}{273.16}=\frac{x}{{{x}_{Tr}}}\]  \[\Rightarrow \] \[T=273.16\,\left[ \frac{x}{{{x}_{Tr}}} \right]\,K\]