I have recently been completely baffled by someone’s mention of “the velocity of hydrogen and helium” as if it were something everyone should know.
I was under the impression that atoms of helium and molecules of hydrogen can all move at different velocities, like a bunch of marbles being shaken in a jar. The higher the temperature, the faster the average atom or molecule is moving.
The contention this person was making is that these particles would all move at the same speed, and if they’re in some medium like air or water, that medium determines their speed.
I think that you’re reading too much into this person’s statement. In these sorts of situations, it is pretty much taken for granted that the average velocity is being discussed, even if this is not explicitly stated.
Strictly speaking, the average velocity of molecules in a gas (with no wind) is zero. We should be discussing the speed. The most probable speed for a molecule is the square root of 2kT ÷ m, where k is Boltzman’s constant, T is absolute temperature and m is the mass of the molecule. The speeds fall under a Maxwell distribution, a small proportion having near zero speed and a small proportion having a very high speed.
You’re right, at any one temperature, the molecules of a gas have a range of speeds given by a Maxwell distribution. The AVERAGE speed goes up with temperature.
However, for different gases at the same temperature, the AVERAGE speed of the molecules is higher for light molecules and lower for heavy molecules. Hydrogen and helium have high average speeds because they are the two lightest gases. (I make it roughly 1600 metres/second for hydrogen and 1100 m/s for helium at 25 degrees Celcius. Compare with 400 m/s for oxygen at the same temperature.)
In fact, their average speeds are high enough that a significant fraction are above escape velocity, and we lose them. This doesn’t matter with hydrogen since we have so much of it, but if helium wasn’t being constantly replenished by radioactive decay we wouldn’t have any, at least not in the atmosphere.
I see where your friend is coming from, and he’s either very confused or dealing with something very esoteric.
Photons travel at the speed of light, whatever that is in the medium they are in. So in a particular medium, photons DO all travel at the same speed.
from your OP, it appears the guy believes the same to be true of hydrogen and helium atoms/molecules. As far as I know, the only way hydrogen and helium can travel through a medium is by diffusion, which again allows a variety of speeds.
I’m willing to be corrected, but I think the guy was talking out of his arse, and you can quote me :).
Just a WAG, but he probably meant “diffusion speed,” which is higher for lighter gases and is constant for a given gas at a specific temp, pressure, etc.
