The Laws of Thermodynamics summary

Now that we’ve gotten to grips with mechanical equilibrium, it’s time to unpack the first law of thermodynamics – law zero or, as physicists call it, the zeroth law.

We’ll move forward in the same way we did in the previous blink – that is, without assuming anything. Before we can talk about temperature or heat, we must first show our workings. To do this, we’ll begin by introducing a new concept: thermal equilibrium.

So far we’ve looked at pistons and cylinders from the outside, but what’s going on inside these systems? To answer that question, we need to zoom in to the atomic level. We won’t be talking about individual atoms in this blink, however – we’re going to be exploring groups of atoms.

Zooming in like this takes us beyond classical thermodynamics, a field of physics established in the nineteenth century – a time before the existence of atoms had been widely accepted. Instead, we’ll be dealing with statistical thermodynamics. This approach emerged later and is concerned with the probabilities of certain behaviors when you look at lots of atoms simultaneously.

To understand the first law of thermodynamics, we need to explain a new concept – work. The word itself is pretty familiar, but we’re not talking about what you do when you get to your office. In this context, work is a mechanical concept.

Here’s the basic definition: work is motion against an opposing force. Think of a pulley lifting a heavy object, for example – the pulley’s work is a constant battle against the opposing force of gravity. When you struggle to remain upright in a strong wind, your body is also “working” in this sense.

The second law of thermodynamics is notoriously tricky, so let’s bring things down to earth with a concrete example – the steam engine – before we get into the more abstract stuff.

On one level, steam engines are extremely complex feats of engineering, combining hundreds of separate parts and carefully designed pistons and valves. The principles behind the mechanism, however, are relatively straightforward. Steam engines essentially have three components. First, a hot energy source – steam. Second, a device that transforms that heat into work – that’s the job of pistons and turbines. Finally, there’s the cold sink, a vent that extracts energy that hasn’t been used as heat.

So far we’ve discovered two important things. First, when heat is converted into work some of the heat is transferred to the surroundings. Second, heat cannot be transferred from high-temperature systems to low-temperature systems without work occurring somewhere along the line.

The challenge we now face is combining these two insights into a single statement. To do this we need to understand entropy.

Before we move on to the third law of thermodynamics, we need to answer a couple of questions. First off: does the second law as we’ve formulated it make sense of our first statement that some heat will be transferred to the system’s surroundings when heat is converted into work?

Put differently, what would happen if a steam engine consisted of a hot energy source and a turbine but lacked a cold sink?

What do we mean when we say that entropy is a measure of disorder? To answer that question, we need to zoom in and look at the matter at a molecular level.

Think back to our gymnasium storeroom. The balls were either on one of the many shelves or on the floor – there weren’t any balls floating between any two “levels.” This was how we illustrated the distribution of particles across their available energy states.

Think back to your first paycheck. It looked like a lot of cash, right? It was probably at that point that you realized that a good chunk of that money was going to be deducted to cover taxes. When heat is converted into work, something similar happens. Every time a system produces heat, as when burning fuel creates steam, that system has to pay a “heat tax.”

Imagine you’re burning a hydrocarbon like coal in a cylinder fitted with a movable piston. The combustion of the fuel produces carbon dioxide and water vapor. Together, these take up more space than the original fuel and oxygen. To accommodate this extra volume, the piston is driven outward. This requires work, and some of the heat produced by the reaction is used to cover this energy expenditure.

Now that we understand the zeroth, first, and second laws of thermodynamics and seen what entropy is all about, we can move on to the final law – the third law. This law is a little different from the others as it won’t require us to introduce any new concepts. Instead, it will allow us to tie together the concepts we’ve discussed so far.

The first thing we need to say about the third law is that it draws on an important insight into cyclical processes. As the name suggests, these processes go full circle, meaning that the system in question returns to its original state. You can see how this works if you think about your refrigerator. When you place something inside a fridge, it removes heat from this object and transfers it to its surroundings. The refrigerator itself, however, always stays the same temperature.