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



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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