Thermodynamics is the science of how heat, and more broadly, energy, flows. The Laws of Thermodynamics are universal principles that explain how these flows work, much as Newton’s Laws of Motion describes how physical objects move. For a more detailed explanation, take a look at how NASA defines thermodynamics.
Originally, there were three laws. More recently a fourth law has been put forward, although somewhat confusingly, this is termed the Zeroth Law because it needs to precede the other three.
The Zeroth Law tells scientists that if object A and object B are at the same temperature, and that object B and object C are at the same temperature, then objects A and C are also at the same temperature. That’s what makes it possible to measure temperature on a thermometer and it’s important because it introduces the concept of thermal equilibrium. Imagine a hot poker dropped into a bowl of cold water. Heat will flow from the poker into the water until they reach the same temperature, at which point thermal equilibrium has been achieved.
Next up is the First Law of Thermodynamics. This tells scientists that the total amount of energy in a system is constant. It can neither be created nor destroyed but merely transformed from one form to another. Sometimes this is referred to as the “conservation of energy”. Think back to the poker example. The poker lost heat but the water gained heat. There was no change in the total amount of heat in the system.
The Second Law of Thermodynamics is where things get really complicated. In essence this tells scientists that energy wants to flow from where it is concentrated to where it can spread out as much as possible. In the poker example, the heat flows from where it is – in the poker – to where it is not – the cold water. The poker never gets hotter and the water colder because heat doesn’t move that way.
The scientific term for the tendency of energy to flow like this is entropy. Entropy is a measure of how heat likes to spread out and is expressed as energy divided by temperature. In the SI system of units this is represented as Joules per degree Kelvin.
So the Second Law says that the entropy of a system increases or stays the same but can never reduce. There’s a great Q&A on the Second Law on the website of Occidental College.
The Third Law of Thermodynamics tells scientists that as temperature approaches absolute zero, entropy tends to a minimum. As explained succinctly on the physicalgeography.net website, this leads to the concept of absolute zero – the temperature at which a molecule comes to a complete stop. In everyday life this Law has less value than the others, except for telling scientists that everything is going to cool down.
To wrap up it is necessary to discuss why scientists and engineers think these Laws are such a big deal.
For scientists it’s because they provide a way of understanding just about every phenomenon in the universe, from the Big Bang to how ice melts. Engineers on the other hand, are more interested in how they can apply these Laws. They look at energy in terms of the “work” that can be done. Consider a steam engine. Coal gives off heat when it is burnt. This heat is used to boil water. The steam that results needs more space than the water, so it expands, creating the pressure to move pistons in their cylinders.
Without an understanding of the Laws of Thermodynamics there would be no engines, no turbines, no generators and so no mass transportation and no electricity. The Laws of Thermodynamics underpin the modern world.