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Re: Fire syringe?



On Thu, 14 Aug 1997, Leigh Palmer wrote:

I've heard of that device, but I can't find it in the REI catalog.
I can't imagine that it is more convenient than matches filling a
container of equal size and weight, so why bother? It is true that
the air gets hot when it is pumped into a bike tire (and the pump
body becomes uncomfortably warm if one "dynamites" one's tires, as
I often do). The effect could, no doubt, be mechanized to reach
the ignition temperature of thin wood shavings, but where is the
utility in that; in what way would this device be superior to the
aforementioned matches?

While we're on the topic, here's a question for the teachers:

The ambient temperature is 20 degrees Celsius and the pressure is
one atmosphere. A automobile tire is inflated to a guage pressure
of two atmospheres and is in thermal equilibrium with the
surroundings. If the valve core is depressed the tire starts to
deflate. Is the air that escapes from the valve at a temperature
higher than, lower than, or the same as the ambient temperature?

For some reason many teachers seem to think the answer to this
question is obvious. I think that a careful experiment to measure
the exhausting air temperature is in order here.

Leigh


Leigh,

I don't know anything about the fire-maker, but I can comment about the
tire problem. If you can use a model in which air behaves as an ideal
gas that goes from a container at 3 atmospheres to a container a 1 atmos-
phere and the process is adiabatic; I would guess that temperatue would
be unchanged. But, in the real world none of these hypotheses are probably
fulfilled. So, I would say make your accurate measurements and see what
happens.

So many times when physics teachers think something is obvious it means
that they are placing too much emphasis on a simple model which may or
may not apply to the situation they are describing. Air is a complicated
mixture of molecules and is only approximately "ideal."

W. Barlow Newbolt 540-463-8881 (telephone)
218 Howe Hall 540-463-8884 (fax)
Washington and Lee University newbolt.w@fs.science.wlu.edu
Lexington, Virginia 24450 wnewbolt@liberty.uc.wlu.edu

"It is often stated that of all the theories proposed in this century,
the silliest is quantum theory. In fact, some say that the only thing
quantum theory has going for it is that it is unquestionably correct."

Michio Kaku, 1995