The third law of thermodynamics, like the second, centres around entropy. However, for the third law, it is focused on the relationship between entropy and temperature. The third law can be regarded as, for closed systems in thermodynamic equilibrium:

The entropy of a system tends to a constant value as its temperature approaches absolute zero.

In simpler terms, the law is saying that the entropy of a perfect crystal of clean substance is zero when the temperature reaches zero. To help explain this we look at the relationship between entropy and microstates, where microstates are the number of possible combinations of atoms:

S = kb ln(nms)

S = Entropy of the system

kb = Boltzmann constant

nms = Number of micro-states

At absolute zero, the number of microstates is one as at this temperature due to all the atoms being identical (only for a pure substance) and as a result, all the orders are duplicates hence ln(1)=0. If a substance is not pure, the entropy will not reach zero because the system is locked in place with some energy or the lowest energy state is non-distinctive. The value it approaches is the residual entropy.

The third law is stating that the cold items are more orderly than heated items. When the object reaches absolute zero, the particles which make the structure stop vibrating helping it form the most stable, low energy structure. When described like this, it is a lot easier to understand and visualise.

Example of The Third Law of Thermodynamics

A great visual example is fire and ice. Ice has a very low temperature and a stable structure, which is reflected by the small number of micro-states. The low temperature allows the entropy of the ice to remain stable. On the other hand, looking at ice (water) in a different state, boiling water has a much higher temperature and is less ordered. As a result it has more micro-states and the entropy is a lot less stable.

Related Posts: The First Law of Thermodynamics, The Second Law of Thermodynamics

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