Third Law of Thermodynamics

What is the Third Law of Thermodynamics?

The third law of thermodynamics states that the entropy of a perfect crystal at a temperature of zero Kelvin (absolute zero) is equal to zero.

Entropy, denoted by ‘S’, is a measure of the disorder/randomness in a closed system. It is directly related to the number of microstates (a fixed microscopic state that can be occupied by a system) accessible by the system, i.e. the greater the number of microstates the closed system can occupy, the greater its entropy. The microstate in which the energy of the system is at its minimum is called the ground state of the system.

At a temperature of zero Kelvin, the following phenomena can be observed in a closed system:

  • The system does not contain any heat.
  • All the atoms and molecules in the system are at their lowest energy points.

Therefore, a system at absolute zero has only one accessible microstate – it’s ground state. As per the third law of thermodynamics, the entropy of such a system is exactly zero.




Why is it Impossible to Achieve a Temperature of 0 Kelvin?

For an isentropic process(idealized thermodynamic process that is both adiabatic and reversible)that reduces the temperature of some substance by modifying some parameter X to bring about a change from ‘X2’ to ‘X1’, an infinite number of steps must be performed in order to cool the substance to zero Kelvin.

This is because the third law of thermodynamics states that the entropy change at absolute zero temperatures is zero. The entropy v/s temperature graph for any isentropic process attempting to cool a substance to absolute zero is illustrated below.

Absolute Zero and the Third Law of Thermodynamics


From the graph, it can be observed that – the lower the temperature associated with the substance, the greater the number of steps required to cool the substance further. As the temperature approaches zero kelvin, the number of steps required to cool the substance further approaches infinity.


Consequences of the Third Law of Thermodynamics

While scientists have never been able to achieve absolute zero in laboratory settings, they get closer and closer all the time. This makes sense because the third law suggests a limit to the entropy value for different systems, which they approach as the temperature drops.

Most importantly, the third law describes an important truth of nature: Any substance at a temperature greater than absolute zero (thus, any known substance) must have a positive amount of entropy. Furthermore, because it defines absolute zero as a reference point, we are able to quantify the relative amount of energy of any substance at any temperature.
This is a key difference from other thermodynamic measurements, such as energy or enthalpy, for which there is no absolute reference point. Those values make sense only relative to other values.

Putting together the second and third laws of thermodynamics leads to the conclusion that eventually, as all energy in the universe changes into heat, it will reach a constant temperature. Called thermal equilibrium, this state of the universe is unchanging, but at a temperature ​higher​ than absolute zero.

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