Why Does Charles' Law Give a Different Answer Without Converting to Kelvin?

[MRCHEM]

why is the answer different if you didnt convert T into kelvin considering that
they have the same temp in actual process

 

[HallsofIvy]

The same temperature? Yes, but Kelvin and celsius use different numbers to represent that temperature! Using different numbers in a formula typically gives different answers.

Oh, I see what you mean! Charles Law says that $V_1/T_1= V_2/T_2$ so $V_1/V_2= T_1/T_2$ and you are asking why the differences don't "cancel". They would IF Kelvin temperature were simply a multiple of celsius Temperature. If K= aC, then $K_1/K_2= (aC_1)/(aC_2)= C_1/C_2$. But they are not: Kelvin temperature is celsius temperature minus 273.15. That is K= C-a. $K_1/K_2= (C_1- a)/(C_2-a)$ and we can't cancel.

 

[dextercioby]

And keep it then in Celsius, Fahrenheit, Reaumur, Beufort, Mercali, Richter or other scales ? Maybe it's probably Charles law was valid only when both the volume and the temperature are expressed in SI units, with T the absolute thermodynamical temperature measured in Kelvin.

Daniel.