[Homeroast] OT: Vacuum Problem PV=nRT

Dave Huddle 137trimethyl26dioxopurine at gmail.com
Mon Dec 21 14:49:15 CST 2009


Before leaving home this AM, I filled my Nissan Leak-Proof Travel Mug
about 2/3 full and closed the lid tightly as I knew I wouldn't be
opening it till after my dentist appoinment.

About 2 hours later, I was unable to open the mug since the coffee had
cooled a little and formed a partial vacuum inside.  I tried using
turning the lid with both thumbs, tried wacking it at the proper angle
with a strong stick, but it was stuck tight!

Then I remembered the ideal gas law, PV=nRT.  Rewriting the equation
as P=nRT/V.  In my situration n, R and V were constant, so P is
directly propotional to T.    I placed my mug on the perimeter heater
in my office and in about 5 minutes both T and P had increased and  I
was able to open it and enjoy my still warm Sumatra Blue Batak.

Next time, I reckon I should fill the mug to full capacity or not
close it so firmly.  (Of course, If I'd filled the mug this AM, it
would have deprived my wife of most of her 2nd cup and caused me a
different problem.)

Dave



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