Your own pet cloud.........

In class today we were looking at the relationship between pressure and temperature. Did you have a go at making the pet cloud for the practical of the week? The science behind the cooling is known as adiabatic temperature effects. Ever pumped up a bicycle tyre? No? But you've eaten soup, right? Let me explain: When you work a bike pump to compress air into the tyre, you might have noticed that the valve gets hot. The valve heats up because you're squashing and squeezing air into a small space (reducing its volume), and that has two effects. It increases the pressure - which is what you want so your tyres don't sag - but it also increases the temperature of the air. The reverse happens when you let air out of the tyre: as the air rushes out it expands, the pressure falls, and you should feel both the jet of air and valve get cold.

How about eating soup? Well, when your soup is too hot, you blow on it, right? But if you just open your mouth and breathe out, the air feels warm (breathe on the back of your hand - it's true!). When you blow on soup, you purse your lips, which has the effect of squirting air out through a nozzle. The resulting jet expands as it leaves your mouth, and as you now know, an expanding gas is a cooling gas. So the jet feels cold. (You can do that on the back of your hand too – there, a science demo that requires no equipment whatsoever.)

The most graphic demonstration of adiabatic effects I've come across was on a TV programme in which some guys took a four-wheeled pedal-powered vehicle and welded a huge water tank to the back of it. The tank was half-full of a quarter of a tonne (250kg) of water, with the other half being full of air... only, the air was pressurised to ten atmospheres.

The idea was that the rider of this contraption would hammer down a racetrack as fast as he could, then pull a handle to open a valve at the back of the tank. The resulting jet of water would thrust him forward like a rocket, quickly doubling his speed.

Astonishingly, it worked, more-or-less. But the valve and nozzle were rubbish, and what with all the sloshing there wasn't a nice clean jet of water - just a spray of water and high-pressure air. Of course, the air expanded and cooled, but not just by a few degrees. Ten atmospheres pressure and an accidentally-rather-good expansion nozzle produced enough cooling effect to freeze water.

They made a rocket vehicle that was propelled not by a jet of water, but by a splatter of ice. It made a heck of a mess, of course, but watching an elderly chap thunder down a track while under ice cube power is quite funny.