What are the Laws of Thermodynamics?

This blog post is based on something I wrote for a 1st year undergraduate who was interested to know more than just the First Law of Thermodynamics.  It's helpful to write things like this for your own sake so that you can check for yourself if your understanding makes sense and has a sort of internal consistency to it.  Hence, as well as it hopefully having helped my undergraduate student, it helps me too.

As an introduction it is worth clarifying that the laws of thermodynamics are about as fundamental as it gets if we are talking about how the universe works.  They apply (as far as we know, and there's no reason to doubt this) everywhere within our universe, and yet interestingly were devised in an age when their application was primarily directed towards the operation of steam engines.  It was the desire to understand how to extract the maximum possible work from steam engines, whether for driving the machinery in a textile mill or as the motive power of a railway locomotive, that led to these laws, and indeed a steam engine is still an ideal context through which to learn about them.  Although the steam engines of the Victorian industrial age are now mostly obsolete apart from as museum pieces, the modern equivalent is the steam turbine which is still at the heart of many electricity generation processes in power stations.

The Zeroth Law of Thermodynamics

Devised as a sort of afterthought to the 1st, 2nd and 3rd Laws in order to formalise the concept of temperature. The law describes that bodies which are in thermal equilibrium are at the same temperature. Essentially this means we can use temperature as a measurable property which allows us to tell when things are in thermal equilibrium (the condition in which there is no net transfer of energy by heating between the things).  Without the zeroth law we have three laws in which temperature is a central foundation, and yet it is not defined.   

The reason it is called 'zeroth' and not 'fourth' is somewhat odd, but is because of the fact that it was realised that this law was necessary to define before any of the others could really make sense, and hence logically needed to precede them in the order.  By this time however the identities of the first, second and third laws were so well established that it was felt that to change them in order to accommodate a new 'first law' was simply a step too far.  Hence 'zeroth'.   

The First Law of Thermodynamics

This is essentially the same as the principle of conservation of energy. In the study of thermodynamics this is usually of relevance to the concepts of work and heat, which are two modes of energy transfer. It also involves the concept of system, surroundings and universe, in that energy transfers occur between a system and its surroundings. Collectively these are called the universe. According to the First Law, the quantity of energy in the universe remains constant; it is simply transferred between the system and the surroundings. In chemistry a system means all the atoms involved in a chemical reaction (reactants and products). Of course the surroundings are also made of atoms, but they are not undergoing changes in terms of bonding like the ones in the system are.

The Second Law of Thermodynamics

This is 'the big one'.  The understanding of this law is frequently cited as a sort of litmus test of someone’s science knowledge (probably because of an influential book by CP Snow, “Two Cultures”), which is perhaps a bit unfair but it gives some indication of its importance and also that it might sometimes be misunderstood. 

The version of this law (as in the way of explaining it) that I prefer is that a change that is not spontaneous (such as the cooling which occurs in a fridge) is only possible if a spontaneous change is occurring somewhere else. This means we can make energy flow in the ‘wrong’ direction (ie. cold to hot), but only if somewhere else some energy is flowing in the ‘right’ direction (hot to cold) in order to make it happen. In terms of the fridge, this effectively means that it has to be switched on, working because of the energy transferred from a spontaneous process which is occurring in a power station (like the combustion of a fuel). In the context of chemistry and the Second Law, ‘spontaneous’ simply means something that will happen as opposed to not happening (iron rusting, rather than rusty iron turning into pristine iron). Refrigeration is a good context through which the Second Law can be understood. Things do not naturally cool down to a temperature lower than their surroundings.  

Could we use a different example, other than the fridge?  Yes, we could use another kitchen example such as the heating of a pan of water on a gas hob.  A pan of water will not spontaneously get hotter than its surroundings.  It requires the process of natural gas reacting exothermically with oxygen - a spontaneous process.  Choosing appropriate examples is an important consideration when thinking about something abstract like this.  The key difference between the fridge and the gas hob is that in the case of the fridge the spontaneous process could be happening hundreds of miles away where the electrical energy is being generated.  What if it is being generated by a wind turbine?  This is still derived from a spontaneous process - wind is the result of the expansion of a gas (air) when it gets heated (or contracts when it cools).  Ultimately all of this leads back to the process of nuclear fusion in the sun; which is also a spontaneous process.   

As with the other laws, there are different ways of stating it, but the basic principles rest on the observation that there are some processes in the universe which happen spontaneously, and others which don’t. (For example, hot things always cool down in colder surroundings, gases always spread out in containers etc). This also applies to chemical reactions – it applies to everything (literally). Chemical reactions are observed to favour a particular ‘direction’ – for example carbon dioxide and water never spontaneously turn back into into methane and oxygen. It is not that this is not possible; it is hypothetically possible but it just doesn’t happen. The big question is – why?

The reason for this ‘direction’ of change is because of a property which is invoked by the Second Law – entropy. Entropy is often described as ‘disorder’ but it is a little more than this. Specifically it is about the distribution of energy, rather than just the arrangement of particles. (However, analogies with physical objects which are in a state of disarray are useful in thinking about entropy). All changes (which really means all transfers of energy) result in an increase in the universe’s entropy. What this basically means is that energy is getting spread out amongst all the atoms in the universe. We can’t observe or measure entropy directly (although we can calculate it), but we can observe that all energy transfers ultimately end up as ‘heat’ in our surroundings. This spreading out of energy corresponds to the ‘disorder’.

There is a statistical dimension to thermodynamics, and it is especially relevant to the Second Law. A good analogy is to think about something like shuffling a pack of cards. If you start with all the suits in order, and all cards in order in each suit, how likely is it that you could shuffle the cards and end up with the cards back in this order? It is statistically possible, but the odds are vanishingly small and in practice some other arrangement of the cards always ensues, because there are so many other possibilities. This applies similarly to the distribution of energy amongst atoms; basically the spontaneous changes represent the more statistically favourable distribution of the energy in the universe, and this can correspond to changes in the way atoms are bonded. Hence, the reason methane and oxygen do not spontaneously re-form from carbon dioxide and water is because to do so would be the equivalent of getting the deck of cards back into its original arrangement. It would require energy to go from a state in which it is spread out amongst more possible places to a state in which it’s spread out amongst fewer. You could make it happen but you’d have to do some work, using processes which are spontaneous and which ultimately increase the universe’s entropy more than the amount you’d reduce it by in reversing this reaction. This is why the Second Law is such a big deal – it represents a one-way street of change, and hence the increase in the universe’s entropy is sometimes called “time’s arrow”.

You can look at many everyday processes as being a consequence of the Second Law. For example, things tend to get dirty rather than getting clean.  This represents a particular movement of particles, but it is because this movement of particles favours a greater distribution of energy.

In chemistry the Second Law is important in determining conditions for spontaneity of reactions – basically the temperature at which reactions can be made to ‘go’.  

There is a theory, which makes scientific sense even if it is impossible to confirm, that there is a maximum value for the amount of entropy in the universe.  This value will be reached when all the energy in the universe is uniformly spread out.  If it is all uniformly spread out, no energy can flow from one place to another and consequently no further change will be possible in the universe.  At the present time, the energy is not uniformly spread out, and this is what enables everything to happen that we are accustomed to.  The future energy uniformity is usually referred to as the 'heat death' of the universe.  It does not, as its name might imply, mean that the universe will get hot - it just means an end of events caused by the process of heating.  For this to happen, it means all atoms would have to be in such a condition that no more chemical bonds can be formed.  Hence stellar and planetary matter would probably not exist like it does now.  As I said, it's a theoretical idea, and how long it will take is anyone's guess.  I wouldn't cancel your Guardian subscription just yet...  

An interesting tangent to the Second Law is that creationists (who are not really scientists) call on it to try to bolster their arguments against evolution. They claim that evolution of more complex biological organisms and structures from simpler ones represents the creation of ‘order’ from ‘disorder’.  Hence, they say, if evolution were true, it would go against the Second Law of Thermodynamics. They use pseudo-scientific terms like 'irreducible complexity', in order to invoke the explanation of a divine presence (an intelligent designer).  This simply demonstrates that they don’t really understand the Second Law (and possibly evolution too). The processes of natural selection, favouring particular inherited characteristics, are not prohibited by the Second Law. Genetic variation in organisms is caused by random processes such as mutations in DNA; the environment then dictates which characteristics are likely to prevail according to whether they convey some advantage to the organism.  This is how organisms evolve, and the random events such as mutations are spontaneous processes, leading to an increase in entropy in the universe.  It can be explained more than adequately by processes which have been well studied and confirmed.

The Third Law of Thermodynamics

The Third Law is less important than the First or Second Laws, but has some significance. Peter Atkins puts it succinctly as, “no finite sequence of cyclic processes can succeed in cooling a body to absolute zero.” (There is a striking parallel with another physical limit, the speed of light). Put simply, you can’t reach absolute zero. The reason for this is that as the temperature decreases it takes an increasing amount of work to extract energy from the body being cooled.  The amount of work required to achieve the task is infinite - hence the impossibility.