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Unread 30-03-2013, 10:36
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Re: Are we allowed to use helium?

Quote:
Originally Posted by Al Skierkiewicz View Post
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)
I'll try

Pressure is the sum of the forces from all gas molecules hitting the container walls, divided by the walls' area. Thus, it is proportional to the number of collisions per unit area, and the velocity of the particles. The latter is temperature, which we're ignoring. The former is based on the container surface and the number of gas molecules. In elastic balloons, these two do not rise proportionally, because the internal pressure is balancing not just the external pressure, but also the the elastic potential energy of the balloon. In a mylar balloon (not overly inflated), the internal pressure is not compensating for elasticity, so the two pressures are in fact equal--the increasing volume exactly compensates for the increasing number of gas molecules. So it's not that there's no force acting on it, it's that the force in P=F/A is increasing exactly proportionally to A.


...Maybe.
<Is about to be killed by Ether>


EDIT: Hey, hey! It's seems that someone actually did this experiment. See page 3, it is indeed constant at 1atm. Who'da thunk? I was not expecting to get that right.
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Last edited by Siri : 30-03-2013 at 10:42.
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