Negative Kelvin temperature.

Lord Kelvin found out that in the Volume vs Temperature graph for gases the volume tends to zero when temperature tends to -273.15°C. He stated that this is the minimum temperature a substance can attain.
However,  today scientists have successfully achieved negative kelvin temperatures (temperatures lower than zero Kelvin).

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Basically, what is the idea behind negative temperature and how do we identiy a negative temperature has been attained? ?

The property of gases called Entropy helps us to do so.
Entropy in the most simple meaning is randomness or disorder in a material. Higher the disorder higher the entropy.( gases have higher entropy than liquids as the molecules of gas are more disordered or less systematic )

Change in entropy in a system due to change of heat (taking place reversibly) at given temperature is given by-

             dS =  dQ/T
Where dS is change in entropy
dQ is heat added or removed in reversible manner
T is temperature of system.

Generally in our notion if heat is added to ice it tyrns liquid and entropy increases. Similarly if heat is added to any substance its entropy will increase. This can be seen from the formula dS is directly proportional to dQ as T is positive in these cases.

Now when a substance is at negative temperature addition of heat decreases its entropy and vice versa. Sounds strange!!
Scientists have experimentally confirmed the entropy changes at negative temperature.

So is the gas law valid only above zero Kelvin. Do materials show other strange behaviour below zero kelvin? Only time will tell.

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