The frequency of atomic collisions decrease as air expands, therefore the air gets
Temperature is just the average heat of a substance. That is, if you take the
kinetic energy (heat) of all the particles in a given volume, and divide by the
volume you get the energy density, which we call temperature.
If you have a certain amount of air, the particles have a fixed amount of heat
(unless you let the heat pass to some other substance or use some to do work) and
so if you let the air expand you decrease the temperature (mathematically you are
dividing the heat by a larger number). Thus the blast of a CO2 fire extinguisher
can be used to cool a can of your favorite beverage, or freeze an attacking Blob!
(for you monster movie buffs out there!)
Rob Landolfi, Science Teacher, Washington, DC
The Ideal Gas Law states pV = nRT, where P = The pressure of the gas in Pa, V =
Volume of gas in m3, n = Number of moles of gas, R = A constant of about 8.314 and
T = Temperature in K.
As a gas (like air) expands, the value of V increases and this has the effect of
increasing T (The temperature). As the energy needed to increase it's temperature
must be supplied from somewhere, the gas takes the energy from the surrounding
system giving the effect of cooling. This is a principle used in refrigeration.
Peter Talman, B.S., Post Grad Student, Portsmouth University, UK
1. While PRESSURE is related to the number of collisions, temperature is not.
Temperature is defined only by the average K.E. of all the gas molecules.
2. Temperature is also not directly based on an energy 'density'. It is an average
over the total number of molecules, which does NOT change when an enclosed gas is
3. This answer doesn't address the fact that as V increases, P decreases, which
would allow T to remain constant and still satisfy the relationship.
The third answer does come closest, however. Imagine an enclosed gas pushing
a piston surrounded by a vaccuum. Pressure will cause
the piston to move, expending energy. The energy has to come from somewhere, and
is the K.E. of the gas molecules. It's simply this loss of K.E. that lowers the gas
Michael Onstad's answer to this question perpetuates a
misconception. The question is, 'Why does air cool as it expands?' Temperature is
related to neither energy density nor frequency of molecular collisions.
Temperature *is* related to molecular KE. Air molecules lose KE only by doing work
on other molecules or by fighting Earth's gravity (i.e., rising). So, expanding air
does NOT always cool; it depends on the nature of the expansion.
'Physicists like to think that all you have to do is say, these are the conditions, now what happens next?'