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Here’s another little bit of physics seen in everyday stuff.  When disconnecting the gas cylinder to our camp stove while on holiday, I got a bit of a shock at how cold it was.  It shouldn’t have shocked me – that’s how it should be. 

When gas is made to expand it cools down. And in the gas cylinder, compressed gas is allowed to exit and expand to atmospheric pressure, so naturally the nozzle will get very cold indeed. The reverse is true of the bicycle pump – here you are compressing gas from atmospheric pressure to tyre pressure, and it gets hot.

The cooling of gas as it expands is a crucial part of the cycle going on in your fridge or air conditioner. The refrigerant liquid is allowed to expand while it is in contact with the cold bit of the fridge, thus sucking heat out of it. The liquid is then pumped to the outside of the fridge where it is compressed again, dumping heat out. That’s why your fridge is warm at the back. The second law of thermodynamics says that more heat has to dumped on the outside than taken from the inside, that is, there needs to be an input of work (in a fridge’s case, electrical energy) to make this process possible. Feel the heat coming out of the outside unit of an air-conditioning system – that is why you get a big electricity bill if you like your house nice and cool in summer.

Though I have to say, we haven’t even thought about switching our units on so far this summer.