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Re: [Phys-l] Work done on tyre



On 11/03/2007 08:39 PM, carmelo@pacific.net.sg wrote:

(a) A car tyre has a fixed internal volume of 0.0120 m3. On a day when
the temperature is 25 degC the pressure in the tyre has to be
increased from 2.62 X 10^5 Pa to 3.23 X 10^5 Pa. Assuming the air is
an ideal gas, calculate the amount of air which has to be supplied at
constant temperature.
(b) Apply the first law of thermodynamics to the pressure increase in
(a) in order to calculate the amount of work which has to be done to
increase the pressure.



The way I see it:
-- Part (a) is the ideal gas law plus some high-school-level algebra.
-- Part (b) is the ideal gas law plus evaluating the integral of (1/V)dV.
Not very tricky.

I found it convenient to organize part (b) in two steps: compressing
the "new" air from P0 to P1, and then compressing all the air from P1
to P2.

Is this a thermodynamics problem? I'd say not, since it doesn't
involve the second law. For that matter, it doesn't really depend
on the first law, either; a high-school-physics notion of
work = force * distance
suffices, doesn't it?

What am I missing? Is there a tricky trap here that makes this
harder than it looks? If so, I fell into the trap ... please
explain.