Friday, March 31, 2006

A little about the physics of gas

I just listened to a news bulletin with a news that scared me after making me laugh my ass out. They talked about how using Nitrogen instead of regular air would reduce your gas consumption and help your car performance. Well first thing first the air is 78% nitrogen so you where (with "regular" air) mostly using nitrogen. Now their argument why it is reducing your gas consumption is as follow: "Nitrogen is a dry gas, so when it get colder or hotter the pressure of your tire doesn't change and that translate to a smoother ride and less consumption". Now, that is complete and utter bullshit, the expansion of gas is not due to water in it or has any other relation to the present or absence of water vapor in it. No matter the kind of gas (well not exactly their are variation but many if not most gas can be approximated to this) we can use a simple equation to tied it fundamental propriety (p for pressure, V for volume, T temperature as well as n the number of particles) :

pV=nRT (where R is a physical constant)

now if you put the condition for a tire into this equation, you find: volume is fixed (it is the volume of the tire), R is fixed (it is a constant), n is fixed (you do not had or remove gas particles from your tired) so you are left with two variable... p and T now notice that since they are on opposite side of the equation when one goes up the other follow the same comportment (if T increase p increase also). Notice that even if n would change because of different type of gas the fact that we are not changing what in the tire between each change of temperature it becomes a constant and as such not a factor.

In short, if anyone telling you to change the air in your tire for some other gas is bullshitting you and is probably trying to steal your money. So next time you see a claim like that on TV or somewhere else contact your local physicist and see if the claim is at least possible realistically. Or well just get information about how things work and most of the time you'll have question about how it is possible and when you start having question about if something is possible or not, well most of the time it isn't.