Adiabatic change

EI has been asked about this concept, and spent some time trying to explain it. However, although EI understood the concept, it actually became quite difficult to explain. So we’ve thought about it some more, and here is our attempt at trying to explain it.

Firstly, an adiabatic change is one in which NO HEAT is TRANSFERRED TO or FROM a fluid (gas/liquid) doing work, or having work done on it. Normally this occurs when there is a change in pressure in a gas.

In other words, as a gas is compressed, it’s temperature will increase. Have you ever pumped up your bicycle tyre, and the nozzle or barrel of the pump has got really hot, almost too hot to touch by the time you’ve finished pumping the last bit of air in? That’s because of adiabatic HEATING. Diesel engines work on the same process of compression generating enough heat to cause ignition. There is no external source of heat, but the temperature has still increased. This must all have come from the act of pumping, i.e. pressurising, the air. The energy of the pumping has been converted to heat energy (internal energy) of the compressed gas.

Conversely, if a gas is suddenly allowed to expand, it will cool. A CO2 fire extinguisher (They used to be solid black, didn’t they? Now we’ve just got those EU compliant red things with different labels on. How the hell are you supposed to recognise the difference in a hurry now?? – Ed.)…when it is used, or any gas cylinder opened and allowed to vent suddenly will rapidly cool. In fact, if you are using a CO2 extinguisher, don’t put your hand on the funnel, because it might freeze to it. Why does this happen? As the gas expands, it does work on the surrounding air, pushing it out of the way. Since energy cannot be created or destroyed, merely converted from one form to another, the energy has to come from somewhere, and it comes from the internal energy of the gas doing the expanding, which we conveniently refer to as temperature.

So, as a gas is compressed or expands rapidly, it’s temperature changes, but no HEAT energy has been transferred into or out of the system. If heat is added to or lost from the surroundings, this is NOT a-diabatic. So for example a gas expanding as a result of being heated is not adiabatic, and a gas contracting as a result of being cooled is not adiabatic either. These processes involve a transfer of heat energy.

Eventually, after the sudden compression or expansion, there will be a transfer of heat energy, but at the time of the expansion or compression, there is not.

And that, in a nutshell, is adiabatic changes.

(It also happens with magnets, apparently, and they’re not fluids…- Ed.) Okay, yes it does happen with magnets, but don’t try and complicate the issue. (If you want to know more, see Adiabatic Demagnetisation on Wikipedia, but make sure you have your maths head on. You have been warned!)