Are we allowed to use helium?

[DMetalKong]

Originally Posted by Al Skierkiewicz

Alan,
As to the 1 atm, will someone please explain how a deflated mylar balloon would magically fill up if the pressure inside and outside the balloon are the same? Assuming that gravity and temperature are ignored, convince me that helium gas would magically move to the interior of the balloon and fill it with no other force acting on it. And if that is the case, why would the balloon deflate when it is opened? (Ignoring of course the lighter than air helium gas)

By changing temperature. Helium is pretty close to an ideal gas as far as approximations go, so it will follow the law PV/T = constant. Since the P is being held constant, when the volume increases, the temperature will decrease, assuming the process is conducted instantaneously with no heat transfer. Since the balloons conducts heat, and because the process takes time, you won't see as significant a temperature drop as the naive law predicts because heat will start transferring into the gas as soon as there is a temperature differential.

The force is generated from the difference in concentration of the gases. http://en.wikipedia.org/wiki/Diffusion.