Why does the release of compressed air make a can cold?
Answer:
The famous formula PV = nRT explains it all.
P=pressure, V=volume, T=temperature, nR are constants.
When the pressure suddenly drops in a container of constant volume (such as a can) then the temperature drops as well...you can see from the formula, that if V remains constant, and nR is a constant, that the formula simplifies to P = kT (where k is some constant) and when P decreases, then T decreases as well.
The NON-geeky answer is as follows...when pressure drops, such as releasing compressed air, then there are fewer molecules bumping into each other (which produces heat) so when pressure drops, there is less particle interaction, and less heat produced.
The geeky explanation explains why the temperature drop is so dramatic and so sudden. Nature has to balance the equation!
It is the same principal that makes your air conditioner work. The molecules in the gas in its uncompressed form have more room to move around and require more energy. This energy is sucked out of its surroundings, thus cooling them.
When gas expands it cools itself. Since you spray some out, theres more room in the can for the gas to spread out, thus it cools because it expands.
The rapid decompression of the previously compressed air. By the ideal gas approximation PV=nRT . Since the pressure is decreasing and the volume of remaining constant the nRT product must also decrease. We know however that n is decreasing so only when n decreases slower than P does the temp in the can decrease.
Same reason that evaporating water cools you off. It takes energy to expand that gas, and losing that energy cools things off. At the hospital, we have huge tanks of liquid oxygen that have ice form on the outside of them. The oxygen is not cooled by any refrigerant, it is cooled by its own evaporation.
When you compress a gas it gets hotter, as you can feel a bicycle pump getting hot. When its released it gets colder.
It gets hotter because you are pushing on it and that increases the energy in the gas. When its expanded, the energy in the gas decreases, and the temperature decreases. It is the same as having a box of bouncing balls. As the box is expanded, the balls will lose energy and slow down.
Inside the can, the gas is expanding adiabatically. That is, no heat is exchanged with the surroundings. In this case, the ideal gas law can be used to show that VT^alpha=constant, so increasing the volume of the remaining gas decreases the temperature.
http://en.wikipedia.org/wiki/adiabatic_p...
When gas leaves a can, there are fewer gas molecules remaining, which means the kintetic energy (and therefore temperature) within the can drops due to fewer collisions between molecules.
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