Cold Gas
Classical Mechanics
Level
1
Why does the gas that is coming out of an aerosol spray canister feel cold to the skin?
Aerosols canisters are filled with coolants
Our skin cannot judge the temperature of the gas correctly
The gas expands adiabatically
There is an endothermic reaction between the gas and the skin
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1 solution
The gas inside the can is highly compressed. When the valve of the container is opened, the gas escapes very quickly.
It expands to a larger volume, and does work against the atmosphere. Since it expands very quickly, there is very little heat transfer between the gas and the surroundings, so the expansion can be treated as adiabatic.
We can make use of the first law of thermodynamics:
Δ U = Q − W
Since there is no heat exchange, Q = 0 . Since the gas does work, W is positive, and hence Δ U is negative. This means that the internal energy of the gas decreases, and its temperature falls too. □