The Laws of Thermodynamics Book Summary - The Laws of Thermodynamics Book explained in key points

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The Laws of Thermodynamics summary

Name: The Laws of Thermodynamics
Rating: 4.3 (62 reviews)
Author: Peter Atkins

Peter Atkins

A Very Short Introduction

4.3 (62 ratings)

29 mins

Topics

Physics

Table of Contents

The Laws of Thermodynamics
Summary of 10 key ideas

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What’s in it for me? Learn about the laws of the universe.

Why does boiling water make the lid of a pot rattle, and why do the potatoes inside the pot cook? Why does burning gasolene make two-ton cars move? How do refrigerators cool your leftovers?

The answer to each of these seemingly simple questions takes us to the heart of thermodynamics, a form of theoretical physics that studies the transformation of energy. Everything that happens in the universe, from the expansion and contraction of gasses to the heating and cooling of metals, is governed by the discipline’s laws.

In these blinks, we’ll be following top physicist Peter Atkins as he explains the workings of the zeroth, first, second, and third laws of thermodynamics. His method? Assume nothing – not even seemingly obvious concepts like temperature. The result? A rock-solid theoretical structure that explains how our world really works.

You’ll also find out

why steam engines can’t use all the heat they produce;
what happens when you cool atoms to the lowest possible temperature; and
why crystals have higher entropy than gasses.

If two systems are in mechanical equilibrium, then a third system in equilibrium with one will also be in equilibrium with the other.

Thermodynamics concerns itself with systems. What we mean by this is: anything that has boundaries. A block of steel is a system. A combustion engine and the human body are also systems.

Beyond those boundaries, we find the system’s surroundings. This could be a bath of cool water in a laboratory or the atmosphere around a system. Together, a system and its surroundings make up the universe.

Systems can take different forms. This depends on the nature of their boundaries. Imagine a flask without a lid. That’s an “open system.” Pop a lid on the same flask and you’ve got a “closed system.” Then there are “isolated systems.” These aren’t affected by their surroundings at all. A vacuum flask is a good approximation of such a system.

Right, now that we’ve defined our terms, let’s unpack the first concept we’ll need to get to grips with to understand thermodynamics – mechanical equilibrium.

Picture two metal cylinders next to one another. Both are fully sealed except for a horizontal tube joining them together like a walkway between two buildings. This tube contains two pistons held together by a rigid rod.

The pistons move this rod back and forth according to the pressure in their respective cylinders. If the pressure is higher on the right than on the left, the right piston pushes the rod toward the left cylinder and vice versa. This is like a tug-of-war, but with pushing rather than pulling.

If the pistons don’t move, we can infer that the pressure in both of these cylindrical systems is equal. In this case, we say that they are in mechanical equilibrium.

At this stage we can give our two existing cylinder systems names – we’ll call them “A” and “B” – before adding a third, “C,” and seeing what happens.

Cylinder C is connected to A with another tube containing movable pistons. Let’s say these pistons don’t move and there’s no tug-of-war between A and C. We can now conclude that the pressure in both systems is the same and that A and C are in mechanical equilibrium.

But what happens if we detach C from A and connect it to B instead? In a word, nothing. There won’t be a tug-of-war here, either. If C and A and A and B are in mechanical equilibrium, then C and B will also be in mechanical equilibrium.

Why is this important? Let’s find out...

The zeroth law is about thermal equilibrium, and it allows us to introduce the concept of temperature.

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.

The Boltzmann distribution tells us how atoms are distributed across energy levels given a certain temperature.

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.

The first law of thermodynamics states that the internal energy of an isolated system remains constant if no work is done on it.

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.

Some heat is lost to the surroundings as it’s converted into work, and work must be done to transfer heat from a cold object to a hotter one.

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.

The second law of thermodynamics states that the entropy of the universe increases during spontaneous changes.

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.

The second law explains the way cold sinks work and the direction of heat transfers.

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?

Entropy is a measure of the probability of determining the energy state occupied by a molecule.

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.

Enthalpy, Helmholtz energy, and Gibbs energy are all useful accounting tools in thermodynamics. 

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.

The third law of thermodynamics states that crystalline substances at absolute zero have zero entropy.

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.

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What is The Laws of Thermodynamics about?

The Laws of Thermodynamics (2010) is a short and accessible introduction to thermodynamics, the field of physics concerned with the relationships between different forms of energy. Authored by one of the world’s preeminent authorities on the subject, Peter Atkins, it explains the four laws that govern the universe – the zeroth, first, second, and third laws. Along the way, The Laws of Thermodynamics unravels the workings of familiar-sounding concepts like temperature as well as more exotic ideas like entropy and energy states.

Who should read The Laws of Thermodynamics?

Quizzical types who’ve always wondered how the universe works
Humanities graduates looking for a gentle introduction to physics
Folks who love a good mental workout

About the Author

Peter Atkins is a renowned physicist and the author of over 60 books, including Physical Chemistry, a standard textbook used by students around the globe. Atkins is a Fellow of Lincoln College, University of Oxford, and a well-known face on the international lecture circuit. He has been a visiting professor in China, France, Israel, and New Zealand.

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