From: Paul Doherty (pauld@exploratorium.edu)
Date: Fri Nov 10 2000 - 17:25:12 PST
Message-Id: <l031107d3b6324c1d4f77@[192.174.2.173]> Date: Fri, 10 Nov 2000 17:25:12 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole superheating and boiling
Superheating
The pressure inside a bubble inside water is higher than the pressure in
the surrounding fluid. This is because the bubble has surface tension. The
extra pressure is inversely related to the radius of the bubble. Very small
bubbles have very high pressures inside them.
(Just like it is hard to start to blow up a balloon by mouth when it has a
small radius, but easier as the radius increases.)
So in order to form a boiling bubble the temperature of the water has to
equal the boiling point at the pressure inside the bubble. This means that
a temperature over 100C is needed, quite far over 100C for starting small
bubbles.
However, old ceramic cups have scratches in them which trap air when they
are filled with water. This trapped air acts like a starter bubble so water
with air bubbles trapped in it boils at 100C.
So assume you have no starter airbubbles in a cup, the cup contains
superheated water at 110C. You jiggle the cup and dislodge a piece of dust
from the inside wall which revelas a starter bubble of air attached to the
wall or dust. This bubble grows and as it grows it becomes easier for it to
grow faster and faster. (Positive feedback) The entire cup explodes into
steam and water.
To scald you water only has to be 60C.
So the water in the bulb need not be superheated, although it could have
been. The inside of the bulb was probably free of scratches.
The filament of the bulb is a tungsten resistor electric current flow from
one end of the resistor to the other normally heating it to incandescence,
surround it with water and the heating still happens, (even though some
current might flow through the water that electric current heats the water
too.) The filament heats up, and heats the water but doesn't reach
incandescence.
Paul Doherty
Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld
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