Date: Wed, 27 Jan 1999 16:24:31 -0800 (PST)
From: Jeff Erickson <jefferick@yahoo.com>
Subject: copper patina and air pressure demo
To: Pinhole Listserv <pinhole@exploratorium.edu>
I'm interested in finding the BEST way to put a green patina on copper
which will stay on. I've tried some acids with mixed results...
Also, remember the air pressure demo where you amaze your students by
putting water in a cup, covering it with a card, inverting the whole
set-up and then watch the card remain pressed against the cup...with
no leakage of water. My understanding of the explanation was that the
total force on the top of the card was less than the total force on
the bottom because the weight of water in the cup exerted much less
than the 14 psi exerted on every other part of the card. Wouldn't
this only be true if there was a vacuum above the water in the cup?
If there is any air in the cup, wouldn't it be at 1 atm, and add to
the pressure exerted by the water...resulting in a wet counter and/or
students? Is there another explanation?
==
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