Thermometry

**Category : **JEE Main & Advanced

The centigrade \[{{(}^{o}}C),\] Farenheite \[{{(}^{o}}F),\] Kelvin (K), Reaumer (R), Rankine (Ra) are commonly used temperature scales.

(1) To construct a scale of temperature, two fixed points are taken. First fixed point is the freezing point (ice point) of water, it is called lower fixed point (LFP). The second fixed point is the boiling point (steam point) of water, it is called upper fixed point (UFP).

(2) Celsius scale : In this scale LFP (ice point) is taken \[{{0}^{o}}\] and UFP (steam point) is taken \[{{100}^{o}}\]. The temperature measured on this scale all in degree Celsius \[{{(}^{o}}C)\].

(3) Farenheite scale : This scale of temperature has LFP as \[{{32}^{o}}F\] and UFP as \[{{212}^{o}}F\]. The change in temperature of \[{{1}^{o}}F\] corresponds to a change of less than \[{{1}^{o}}\]on Celsius scale.

(4) Kelvin scale : The Kelvin temperature scale is also known as thermodynamic scale. The triple point of water is also selected to be the zero of scale of temperature. The temperature measured on this scale are in Kelvin (K).

The triple point of water is that point on a P-T diagram where the three phases of water, the solid, the liquid and the gas, can coexist in equilibrium. Different measuring scales

Scale |
Symbol for each degree |
LFP |
UFP |
Number of divisions on the scale |

Celsius | \[^{o}C\] | \[{{0}^{o}}C\] | \[{{100}^{o}}C\] | 100 |

Fahrenheit | \[^{o}F\] | \[{{32}^{o}}F\] | \[{{212}^{o}}F\] | 180 |

Reaumer | \[^{o}R\] | \[{{0}^{o}}R\] | \[{{80}^{o}}R\] | 80 |

Rankine | \[^{o}Ra\] | 460 Ra | 672 Ra | 212 |

Kelvin | K | 273.15 K | 373.15 K | 100 |

(5) Temperature on one scale can be converted into other scale by using the following identity.

\[\frac{\text{Reading on any scale }-\text{LFP}}{\text{UFP}-\,\text{LFP}}\] = Constant for all scales

(6) All these temperatures are related to each other by the following relationship

\[\frac{C-0}{100}=\frac{F-32}{212-32}=\frac{K-273.15}{373.15-273.15}=\frac{R-0}{80-0}=\frac{Ra-460}{672-460}\]

or \[\frac{C}{5}=\frac{F-32}{9}=\frac{K-273}{5}=\frac{R}{4}=\frac{Ra-460}{10.6}\]

(7) The Celsius and Kelvin scales have different zero points but the same size degrees. Therefore any temperature difference is the same on the Celsius and Kelvin scales \[{{({{T}_{1}}-{{T}_{2}})}^{o}}C=({{T}_{2}}-{{T}_{1}})K\].

*play_arrow*Temperature*play_arrow*Thermometry*play_arrow*Thermometers*play_arrow*Thermal Expansion*play_arrow*Scales of Temperature*play_arrow*Application of Thermal Expansion in Solids*play_arrow*Thermal Expansion in Liquids*play_arrow*Variation of Density with Temperature*play_arrow*Expansion of Gases*play_arrow*Heat*play_arrow*Specific Heat*play_arrow*Specific Heat of Solids*play_arrow*Specific Heat of Liquid (Water)*play_arrow*Specific Heat of Gases*play_arrow*Phase Change and Latent Heat*play_arrow*Thermal Capacity and Water Equivalent*play_arrow*Some Important Terms*play_arrow*Joule's Law (Heat and Mechanical Work)*play_arrow*Principle of Calorimetry*play_arrow*Heating Curve

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