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[Phys-L] Re: The power of compressed air



The V of the container doesn't change but the V of the
air (starting from stnd conditions) changes a lot.
That's why a rigid container with air @ 100psi is
potentially much more dangerous than the same
container with water @ 100psi. Lots of energy stored
in the compressed air, almost none in the water under
pressure.
John Barrere
Univ HS
Fresno
--- Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU>
wrote:

1) V does not change by as much as P, when a lot of
air is added to a
nearly rigid container. How much energy
would be, approximately, in a perfectly rigid bottle
(say V=4 liters)
by changing the air pressure from 1 to 10
atmospheres?

2) Students often have no feeling about kilojoules.
To create such
feeling one may ask them to express the kinetic
energy of a small
cannonball (say m=10 kg and v=100 m/s) in
kilojoules. The answer, as
they may be asked to verify, is nearly the same as
0.014 kWh. They
should also be aware that the cost of electric
energy is still close to
10 cents per kWh. A two-step problem about the
energy cost of an
electric cannonballs thrower can easily be invented.

Ludwik Kowalski, who also does not know what the CYA
stands for.
Let the perfect not be the enemy of the good.


On Sunday, Jul 24, 2005, at 08:57 America/New_York,
John Barrer wrote:

Cover your tuckus (sp?), Herb.

Leigh's comment about "larger volumes and
pressures"
brings to mind the benefit (for conceptual
understanding) of presenting pressure as energy
per
unit volume. Or, more precisely, as the integral
of
PdV. Over "inflating" a truck tire with water
presents
very little hazard b/c the dV is infinitesimal,
but
over-inflating with air creates a bomb of sorts.
Not
much energy stored in the water case, LOTS in the
air
case.

John Barrere
University HS
Fresno, CA

--- Herbert H Gottlieb <herbgottlieb@JUNO.COM>
wrote:

On Sat, 23 Jul 2005 22:55:41 -0700 Leigh Palmer
<palmer@SFU.CA> writes:
People have been killed by overinflating tires
with air from gas
station compressors. The problem is exacerbated
in
the case of
tires with larger volumes and pressures, truck
tires, for example. Are
there CYA signs on air hoses now in the US?

Leigh

Just what is a "CYA sign" ? On what part of the
airhose does it appear?

Thanks.

Herb



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