### Re: When a vacuum is created in a glass, why doesn't the glass collaspe?

Date: Thu Apr 19 11:52:30 2001
Posted By: Jeff Yap, Materials Engineer
Area of science: Engineering
ID: 985905217.Eg
Message:
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Hi Ethan,

You've asked a very good question, and it shows that you have a pretty
solid understanding of how vacuum and atmospheric pressure behave.

The short answer to your question is that the glass jar didn't break
because it was strong enough to withstand the kind of pressure that Boyle put
on it.

Why the glass was strong enough is a bit more complicated, but still
pretty cool.  Glass typically has a tensile strength of about 2000 to
25000 pounds per square inch (psi).  The compr
essive strength is normally much higher than that.  Atmospheric
pressure (On average, at sea level, at room temperature) is 14.7 psi
(101.3 kPa or 760 mmHg).  So at full vacuum, the jar would have 15 psi
compressing in on it from all directions, which is considerably less than
the pressure needed to crush it.  Additionally, the air pump used, (built
by Hooke, who also did a lot of cool science stuff)
probably wasn't able to suck out all of the air from the jar.  Even modern
vacuum pumps like Turbopumps and Cryopumps can't suck ALL of the air out
of a container...

In order to crush (also known as "implode" because it catastrophically
fails inward) a glass jar using vacuum, (I highly recommend AGAINST doing
this...) you'd have to do one or more of the following:
1) Make the walls of the jar very very thin.
2) Add scratches, chips, and dents to weaken the glass.
3) Apply extra atmospheric pressure.
4) Apply a very high vacuum in the jar.
5) Make the jar non-cylindrical.  (Cylin
drical structures are very strong because of their shape.  To see this
better, get a two liter soda ("Pop" for most people in your region...)
bottle, and try to collapse it by sucking the air out of it.  You'll
notice three things: 1) it collapses by having the sides cave in (also
known as "buckling"), 2) it's easier to do if you dent or bend the sides
with your hands before pulling the air out of it, and 3) it's hard on your
lungs...)

I hope this helps!  Let us know if you have any more questions.

Jeff Yap

Cool sites:
Encyclopaedia Britannica Online
Glass
Encyclopedia - American Glass

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