Thermodynamics

What are thermodynamics? I’ve heard the term a lot before, but what does it really deal with? Well, it turns out thermodynamics is a branch of physics that essentially studies energy… how it moves and changes form, (through heat and work) and how it affects matter.

We often talk about, or hear people talking about, the laws of thermodynamics… it seems like one of those concepts in physics that people love to reference despite not really understanding. Call me a nerd, but I refuse to mention physics concepts without at least having a basic understanding of them. So, what are the laws of thermodynamics and what are their implications?

The First Law of Thermodynamics is essentially the law of conservation of energy, something we’re introduced to as early as eight grade physics: the fact that energy cannot be created or destroyed, it can only be converted form one form to another. It turns out there’s a formula which expresses this (much more scientifically)…

ΔU = Q – W

Or in layman terms: the change in internal energy (ΔU) is equal to the heat added (Q) minus the work done by the system (W). But how does that relate to the law of conservation of energy? Well, essentially, what the formula tells us is that the total internal energy of a system must equal the energy given/added to it… but, if the system does work on it’s surroundings, some of that energy must be lost, therefore reducing the total internal energy of the system.

The Second Law of Thermodynamics states that the total entropy of an isolated system will always increase over time… okay, let’s break that down. What’s entropy?

Entropy is essentially a measure of disorder or randomness in a system. And the second law tells us that this is always increasing, which seems like it’s saying the universe is generally moving towards disorder. So what does that imply?

• First, that heat flows naturally from hotter to colder objects.
• Second, that energy’s transfers are never 100% efficient; some energy is always lost as unusable heat.
• Third, that processes will generally move towards greater ‘randomness’ (A glass dropped will break because there are many combinations of ways it can shatter, but it won’t recollect itself into it’s original shape because that requires a single unique combination.)

The Third Law of Thermodynamics states that as a systems temperature approaches absolute zero, its entropy approaches zero too. What this basically says is that at 0 Kelvin, everything stops moving and so there is no disorder. However, this is impossible in practice; removing every bit of energy from a system is incredibly difficult.

I may have cheated here by leaving this to last but there is a Zeroth Law of Thermodynamics, which fundamentally states that if two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other. In simpler words, if two systems have the same temperature as a third one, then all three have the same temperature. This is why thermometers work.

And that’s all for thermodynamics… for now. At some point I want to revisit this to look at the arrow of time; why does time run in one direction?