The Laws of Thermodynamics Explained Simply

There’s a good chance you’ve heard about the laws of thermodynamics at some point. I know for me, it always seemed quite daunting. This doesn’t have to be the case.

In this article, I’ll attempt to explain simply the laws of thermodynamics, as well as some related concepts.

Zeroth Law of Thermodynamics

This is known as the zeroth law because it was assumed while the other laws were developed. Only later was it realised that it was significant. It is stated as:

If two systems are separately in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

Two systems are in thermal equilibrium if their state variables (pressure, temperature, volume etc.) do not change when they are put into contact via diathermal walls.

Diathermal walls are those which allow the transfer of heat to and from the system.

From this, we can infer that the two systems must have something in common, so we can equivalently state:

Two systems are in thermal equilibrium if and only if they have the same temperature.

Temperature is a difficult concept to define, but it’s related to the potential and kinetic energy of all the molecules in the system.

In fact, it’s only dependent on the internal energy, U, which is essentially the sum of all energies of all the components of a system.

First Law of Thermodynamics

This one is just a version of the conservation of energy, stated as:

The change in internal energy of a system is equal to the difference between the heat input to the system, and the work done by the system.

If you’re unsure about the concept of work, just think of it as converting energy from one form to the other.

In fact, energy is just defined as the ability to do work.

From experiments, it has been proven that the following statement is equivalent:

The change in internal energy of a system from one state to another is independent of the process.

i.e., U only depends on the current state of the system.

For example, the internal energy of a cup of tea will depend only on the amount of water, tea and milk (no sugar thanks), and its temperature. It doesn’t matter how that cup of tea was made.

Got some biccies with this one.

Second Law of Thermodynamics

This one is probably the one you’ve heard before, and this formulation of it is quite simple. We will see another way to phrase it later on.

Firstly, we have the Kelvin-Planck statement:

It is impossible to build a heat engine which converts heat into work with 100% efficiency.

Or equivalently the Clausius statement:

Heat will not spontaneously flow from a cold body to a hot body.

What these statements are basically saying is, you can’t convert heat to work (or work to heat) without some energy loss to the surroundings.

Historically, this law was not proven and was only empirical; it has never been seen to fail.

However, using statistical mechanics, you can actually prove this law microscopically. I won’t get into the details of that here.

Third Law of Thermodynamics

This one can actually be shown to be a sort of continuation of the second law. You could prove it if you assumed the second law to be true, but I won’t go into that.

It is impossible to reduce the temperature of a body to absolute zero (0 K) in a finite number of steps.

Basically, you can’t reach absolute zero!

As you might know, the unit Kelvin is named after Lord Kelvin. However, did you know his name was William Thomson, and the name Kelvin comes from a Glaswegian river?


Here, we’ll take a brief detour to introduce an important idea which will allow us to reformulate the second law more succinctly.

What on earth actually is entropy? It’s another concept which is hard to define, but I’ll try nonetheless.

Imagine transitioning from a solid to a liquid; the order of the system decreases.

Well, the state variable associated with order is entropy; it measures the disorder in the system.

As an example, let’s consider another one of my articles on here, where I baked a cake with random ingredients.

In the beginning, all of the ingredients were separated, and there was a lot of order, and hence low entropy.

After I had combined them all together though, the ingredients had been mixed randomly in the bowl. This means that the entropy is greater.

Cake Ingredients
Low entropy state with no mixing.
Mixed Ingredients
High entropy state, randomly mixed.

Second Law (Again)

Now that we know what entropy is, we can restate the second law in a form which you may have heard before.

In an isolated system, the entropy cannot decrease.

From which we can say that the entropy of the universe tends to a maximum.

Using the last example, it is easy to see how you would mix up the ingredients, and increase the entropy. However, you could never un-mix them back to the individual ingredients; thus you could never reduce the entropy!

Closing Remarks

So there you have it, the three (or four) laws of thermodynamics. These form the basis of all classical thermodynamic calculations.

If you want something more powerful however, you’ll have to use statistical mechanics, or even quantum mechanics in some places, but I won’t go into that here.

Thanks for reading, I hope you enjoyed. If you’d like to know more about this, have a look at this Wikipedia article.

You can leave a comment, or get in touch with me here, if you have any queries about anything I’ve said,.

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