Why does air get colder as it expands?

Asked by: Mike


The frequency of atomic collisions decrease as air expands, therefore the air gets cooler.
Answered by: Michael Onstad

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!)
Answered by: 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.
Answered by: 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 expanded.

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 against a piston surrounded by a vaccuum. Pressure will cause the piston to move, expending energy. The energy has to come from somewhere, and that is the K.E. of the gas molecules. It's simply this loss of K.E. that lowers the gas temperature.
Answered by: Paul Walorski

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.
Answered by: Tucker Hiatt