How did scientists knew they needed kelvin scale?read description

Question

My book says suppose this and that and there you go you got your kelvin scale.But the exact question which comes to my mind is if Charles law wasnt working for celsius scale how exactly did they know that they needed to make the kelvin scale as it is.And i still dont understand why the kelvin scale works and celsius doesnt?

Answer 1

The Temperature is proportional to the average kinetic energy of the gas particles. (Kinetic Theory of Gases). For proportionality to work, the zero in the temperature scale has to correspond to the zero of the kinetic energy and random motion of the molecules. Kelvin is an absolute scale based on the size of Celsius units. (Rankine is the one based on Fahrenheit degrees) so that differences in T are the same for K and °C. The zero for Celsius is based on the freezing point of water - not a notable point for all materials, just easy to measure. Charles Law was used to solve for Absolute Zero in known degrees to set the 0 for the absolute scale.

You can only do proportionality with absolute scales. If you measure the pressure inside a football, the gauge reads 12 psig (pounds per square inch where atmospheric is 14.7psi). You might try to do a proportionality analysis on how this pressure changes with temperature. This would be incorrect because the gauge pressure tells you the difference in pressure with atmospheric pressure (like an open manometer) - the actual pressure inside the football is 26.7 psia.

Hope that helps.