# GaS physics

I’m in a well control class and.the teacher just said that a gas particle cannot move faster then the speed of sound at sea level, 1080 fps. This doesn’t.make sense to me since others things can move faster than that. The only explanation he is giving is that it’s a fundimental law so I’m turning to you guys. Any ideas?

The speed of sound is the speed at which bulk pressure changes can propagate through a gas. It’s not a direct limit on how fast individual gas particles can move, but the two concepts are related.

The distribution of velocities of particles in an ideal gas is given by the Maxwell-Boltzmann distribution. Now, beyond this my thermodynamics is pretty damn rusty, but if you want you can take that equation and find the probability that a given particle has a specific speed. And there’s nothing in that equation that means that, above some speed of sound, the probability is zero that a particle has that speed.

Someone will be along shortly with more, I’m sure…

Perhaps there was some miscommunication between the teacher and you. An individual gas molecule can indeed move faster than the speed of sound (and in fact, a certain percentage are at any given time), but in general no interesting change can be transmitted by the gas at a speed higher than the speed of sound. So from an engineering point of view, you could say no piece of air can move faster than the speed of sound and be correct. I’ll add that the engineering point of view is probably most relevant in a well control class.

Ohhh… well control. I thought it was an odd reference to a classroom environment where the teacher tolerated no dissent. :smack:

In fact, there must, at any given moment, be some molecules that are moving faster than the speed of sound, because sound requires those movements to propagate.

I didn’t have time to read the wiki but I understand the difference between the bulk flow rate and a particle. While I’m sure it’s explained in the like can I get a summary why no matter the force behind the flow does the acceleration go to zero as you approach the speed of sound. Down the gas begin to change phase?

I’ll catch the link tonight if it can’t be explained better.

Speed of sound is determined by the specific heat ratio of the gas in question, and its temperature. Warm a gas up, and you can definitely get it to move faster than the speed of sound.

And that’s without even generating a shock wave. If you don’t mind generating a shock wave, then the sky’s the limit; you can get gas particles moving pretty damn fast.