An insulated piston and cylinder contains an ideal gas. We pull the piston and expand the gas volume inside the cylinder.

I understand the temperature drops due to this expansion. But, how can we explain this on the microscopic level? How does pulling the piston affect the speed of particles (hence, the average kinetic energy of gas particles, thus lower gas temperature)?


2 Answers 2


Pulling the piston does work on the particles. If we consider the pressure of gas as a result of particles hitting the container and being reflected back, then being scattered from a moving piston means that they scatter back with a higher or lower velocity (depending on whether the gas is being compressed or expanding.)

After some energy has been transferred to the gas, it comes to equilibrium via inter-molecular/atomic collisions (although in case of a quasistatic process, the process is so slow that gas always remains in equilibrium.)

The following might be a good image for illustrating the collision between a molecule (B) and a much heavier piston (A):

enter image description here (image source)

Related: Proof of pressure of ideal gas from first principles


The temperature of the gas depends upon the kinetic energy of the gas or say on the speed of gas molecules. So the expansion of volume does not affect temperature, and the same goes for compression of volume.

But the pressure of gas is energy density and density decreases with an increase in volume and vice versa, so pressure inversely depends upon volume.

If a gas does not expand the piston by itself then the temperature remains the same, because the energy of the molecules do not lose in collisions. One thing to be noted is that temperature cause speed, not the speed causes temperature and high speed or vibration of molecules is to cool down.

In air conditioners or high speed jets, speed causes condensation or cooling down. Physics has many contradictions like it. It is said that the expansion of the universe cools it down, but thermodynamics says expansion without work doesn't lower energy of the matter.


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