The Laws of Thermodynamics

If there’s anything that physicists love, it’s obeying laws.

Thermodynamics is the branch of physics that deals with the relationship between heat and work. Unlike Newton’s laws of motion which were all proposed by the one genius, the laws of thermodynamics have developed over centuries by the work of multiple scientists. However, it can be said that French engineer Nicolas Léonard Sadi Carnot is the ‘father of thermodynamics’, with other scientists who made major contributions to the field being Maxwell, Bernoulli and Clausius.

The laws of thermodynamics may seem quite simple but many consequences arise from them. There are four laws in total – the first two laws aren’t inherently scientific if I’m honest – it takes a bit of common sense really. But scientists like to set out blunt laws and word things with scientific terminology… so off we go.

The Zeroth Law: If two systems are both in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

In other words, if A is in equilibrium with C, and B is in equilibrium with C, then A is in equilibrium with B. (Two systems are in thermal equilibrium if there is no overall heat transfer in between them.)

But wait – did you say zeroth? Wut?

Okay, so there were originally three laws of thermodynamics. Although the concept behind the zeroth law existed very early on, it was not yet stated outright as a law. When scientists decided to establish this new law, calling it the Fourth Law did not make logical sense since it was far more significant than the other three laws. However, renaming the laws would also cause confusion and contradiction with scientific literature, so the solution was to introduce the ‘zeroth’ law.

The zeroth law gives rise to the concept of temperature and the ability to measure temperature using apparatus such as thermometers. There have been many attempts to standardise temperature, ending with the Kelvin being the most scientifically appropriate choice of scale.

The First Law: Energy cannot be created or destroyed, only transformed from one form to another, or:

First Law of Thermodynamics

The first law is essentially the law of conservation of energy, but adapted for thermodynamic processes. 

In words, the change in the internal energy of a system (ΔU) is equal to the heat supplied to the system (Q) minus the work done by the system (W).

Perpetual motion machines violate the first law of thermodynamics as they would generate infinite energy from a finite supply of energy.

The Second Law: The entropy of an isolated system will always increase over time.

Entropy is a measure of how disordered a system is. You can see this in daily life – it is much easier to make an environment untidy than it is to make it tidy.

The second law results in the irreversibility of all thermodynamic processes. There cannot exist perfectly reversible or perfectly efficient heat engines, and as a result perpetual motion machines also violate the second law of thermodynamics.

Personally I find this to be extremely thought-provoking. If we consider the universe to be an isolated system, then it will gradually become more and more disordered over time, and this has been a heavily considered factor in theories of the fate of the universe.

The Third Law: The entropy of a perfect crystal at absolute zero is equal to zero.

Like many things in thermodynamics, achieving zero entropy is only theoretical and not practically possible. One reason why is because it would be a violation of the second law. Once you cool a system to absolute zero, an increase in entropy would cause an increase in temperature, and you would have to continue cooling forever. You can approach absolute zero, but you can never maintain it.



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