The second law of thermodynamics

So now you know the first law – what’s the second law? This one is a whole lot easier to define since you don’t need to fret over what exactly energy IS, and, historically speaking, it was understood long before the first law. (It is relegated to being the second law of thermodynamics since the first law was considered more important).

There are many ways of describing the second law. One highly relevant (to me, anyway) example from yesterday’s evening meal at home is ‘If you drop a bottle of wine onto the kitchen floor you end up with lots of pieces of glass splattered to all corners of the room – but if you pick up the pieces, place them into a bag, and shake them a bit, they don’t reassemble themselves into a wine bottle (to say nothing of the contents)’.  The issue here is that processes that occur on their own are irreversible – they don’t undo themselves naturally.

A more traditional example is this one. Take one jar of hot water and one jar of cold water. Tip them into a large container, mix it around, and pour back into the two jars. You now have two jars of warm water. Mix them again, and you still have two jars of warm water. There is no way that, when you pour the water back into the jars, you will get back hot water in one jar and cold water in another. In a nutshell heat will from a hot body to a colder body, but not the other way around. Really simple. If you don’t get it, download Flanders and Swann’s very comical song on thermodynamics.

So, if heat flows from hot to cold, how come a fridge works. Here, you take heat out of the already cold inside, and dump it to the outside. The second law says it can’t happen? That’s true in that it can’t happen naturally – you switch the fridge off, and the inside will eventually become the same temperature as the outside. To make it happen we need to pump the heat out, and that requires extra energy, in the form of a supply of electricity to your fridge.  The colder you want to make the inside of the fridge, the more energy you need to use.  (Where does the energy go? To the outside of the fridge in the form of heat, in addition to the heat that you’ve pumped out.) Which brings us back to Tuesday’s comments – useful things (like making whole wine bottles, or cooling fridges) don’t happen on their own – they require energy – there’s no getting around the fact. And that means using an energy source, which, unfortunately is often of a fossilised nature.

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