Wednesday, January 30, 2008

Boyle is Serious About His Law

So I waged an epic battle against Boyle today. Guess what, I lost.

I put boiling water in my Thermos before shabbes last week, but did not have reason to make tea over the course of the weekend. There the insulated water sat until today, when I tried to pry the top open. Turn as the cap would, it came no closer to coming off. Heating and cooling were of no avail (particularly cooling which was only last ditch) as the thing is a Thermos for God sakes! Its entire purpose is to insulate! I did learn that the cap comes apart and I was able to poke a small hole in the cap with a heated nail and POP, the cap came right off.

As I struggled with the bloody cap for neigh a half hour (using rubber things and clamps to get a tight grip) I realized: P=VT. The volume is constant so P:V. As the contents of the Thermos cooled from say 100 degrees C to 30 degrees C the pressure decreased to 3/10 the original, creating a dang tight vacuum. Let that be a lesson to all those lazy Thermos users out there! Oh, and don't put the boiling hot contents of the Thermos in the freezer either just to test how well it works. It might be your last Thermos adventure for a while.

Boyle 10^big number : Zev 0


Josh M. said...

In an invariant single phase system (approximated by the ideal gas law, P=nRT/V), the pressure would have only dropped to 303/373 (~81%) of the original. However, you had a two-phase system.

The vapor pressure of water at 100C is 101.3 kPa (atmospheric pressure), while the vapor pressure at 30 C is a miniscule 4.24 kPa. Hence, if there was no air in the system, cooling the thermos down to 30 C would have reduced the pressure to 4% of the original. Lowering the temperature any further would have a negligible effect.

Zev said...

Wow, that was really informative.

I only tried cooling it on the hunch that the plastic cap might contract more than the metal thermos, but I was not really that sanguine.