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Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

Thursday, 4 August 2011

It’s Getting Hot In Here

Unless you’ve been living on another planet you may have noticed it’s been getting a little bit hot and sticky recently, which naturally got me thinking about thermal expansion. This is the technical term for what happens to things as they get hot. Although, in the case of one thing I’ve been seeing a lot of in my drinks, ice, this relationship doesn’t exactly hold true.

We know that solids, liquids and gases are all made up of particles (or molecules), and that in solids these are relatively fixed, in liquids they are free to slide around, and in gases they bounce about freely. Well, the amount of movement isn’t just about what state they are in, it also depends on the temperature.

Let’s follow one specific example to demonstrate what happens. Imagine we have a block of steel. It’s a cold metal block so the molecules are neatly arranged and just sort of vibrating in place. If we start to heat it up, the molecules vibrate more, and with a larger amplitude. This results in a tiny volume increase in the size of our block, almost like the molecules are busting to get out.

Add some more heat (well, quite a bit more for a metal) and eventually we’ll get a liquid. The liquid has a slightly bigger volume than the original block. Now the molecules are zipping about and sliding over each other. If we add more heat they move faster, and again we get a volume increase.

After (admittedly quite a lot) more heat some of the molecules will escape the liquid they are moving so fast, followed by more and more, until we have a gas! The gas has a lot more volume than the liquid did before it and adding yet more heat only serves to make those molecules go even faster. Oh they must be tired!

Now whilst all of this is interesting, is it any use? Well, it’s actually very important to leave space for objects to expand and contract, otherwise you can get cracks and breaks. Which is not a good thing, especially if we’re talking about bridges or electricity pylons. Bridges are often built with one end on rollers, to allow for thermal expansion on hot days, whilst electricity pylons are purposely strung with loose wires so that on cold days when the cables contract they don’t snap.

All this heat makes me need something cold, so we’ll turn to ice. Water is a strange substance because as a liquid it actually has a smaller volume than as a solid. This is because when the solid is being made, the molecules form a very open structure which takes up a lot of space, whereas in the liquid they were all free to move around. It’s also why ice floats, because the open structure means it’s less dense too. Not many things act like this, so it’s strange that something so common can display such behaviour!

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