I promised you explosions, so I guess I better deliver – and I have to blow my own trumpet a little bit here. That’s a very loud and sudden noise, but I promise not an explosion. I know more about noise from explosions and the propagation of noise from explosions than most people in Britain. That’s enough of that.
If we start fairly small and work up I think that would be ok, don’t you? Of course you do, you’ll just skip to the end to see the big stuff, but I’m warning you… It won’t make sense if you don’t read it all.
These smallest explosions then, well really it depends on whether we’re talking detonations or deflagrations which are slightly different but both can give you a good bang. A true explosion is a detonation, but I’m going to start small with a deflagration. It’s one you’ll probably all be familiar with, but would never consider an explosion – matches. When you light a match, you’ll get a flash of white light in the instant it lights – that’s the deflagration of the phosphor tip igniting.
A much larger deflagration that I’m sure you’ll all think of is black powder. You may be thinking of black powder rifles or, if you’re British, you may be thinking of Guy Fawkes and the gunpowder plot. Either way you’ll be thinking “But that’s an explosion, that’s why they use it. It goes bang!” But it’s easily demonstrable that once again, this is not a detonation.
When you put a pile of black powder on the floor and then touch it with a match, it will go “WHOOSH!” in a puff of smoke and disappear. Burns very quickly. Clearly not an explosion though if it whooshes in smoke like that, it’s like a big match but without the wood to burn and hold a flame. You could of course put it in a box and confine it – and doing this is what made you think black powder is an explosive. Here’s a pile of black powder burning.
Simple physics can tell you why black powder goes bang when you put it in a box. It starts as a solid – which can be very dense. But burning it turns it into a gas, which is much less dense – it takes up a lot more space for the same “amount”. So when it’s confined, the pressure will increase very very quickly until the container fails and all the gas escapes creating the “BANG” that you’re surely familiar with. This pressure can be very significant too – it’s what propels cannon balls, and they were used to knock castles down!
So there’s a tiny little bit about things going bang, without any real detonations. You can approximate a black powder explosion by popping a balloon for example, and you all know that sound. It’s just not as loud as a black powder bang. Real detonations will come up next time, and you’re in for a shock.