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Air pump physics problem

by Yegor
Tags: pump
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Feb9-05, 03:47 AM
P: 147
Air pump is pumping out a gas from a vessel (vessel's volume is V).
After each cycle i it pumps out dV.
Gas is ideal. T=Const.
After how much cycles pressure P will decrease k times?

Here is my work.

PV = nRT
Initially n=n(0)

After each cycle n decreases (1-dV/V) times.
So, after x cycles n(x)=n(0)(1-dV/V)^x
So x =ln(1/k)/ln(1-dV/V)

But the answer is x=ln(k)/ln(1+dV/V)

As I understand, they assume that after each cycle n decreases 1/(1+dV/V) times. In this way i really receive x=ln(k)/ln(1+dV/V)
I don't understand it. Help me please.
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Feb9-05, 01:05 PM
P: 147
Has anybody any ideas?
Feb9-05, 01:36 PM
Gamma's Avatar
P: 333
They have done some mathematics with x =ln(1/k)/ln(1-dV/V) to arrive at a more nice looking answer.

x= -ln (k) / ln (1- dV/V)
= ln (k) / ln [(1-dV/V)] ^-1

now, [(1-dV/V)] ^-1 = 1 + dV/V (approximately)

so, x = ln (k) / ln [(1+dV/V)

Feb9-05, 02:05 PM
P: 120
Air pump physics problem

Each cycle corresponds to a T=const. process. (in another version of this problem the pumping process is very fast and a the transformation would correspond to an adiabatic process - all you have to do is to add the exponent [tex]\gamma[/tex] to the equations below). After the first cycle the air volume increases from V to V+dV and the pressure decreases at [tex]p_1[/tex]. You have
[tex]p_0\cdot V=p_1\cdot (V+dV)[/tex]
After the first cycle, the pump takes out the air from dV, and you get again
[tex]p_1=p_0 \frac{V}{V+dv}[/tex]
In the second cycle,
[tex]p_1\cdot V=p_2\cdot (V+dV)[/tex]
[tex]p_2=p_1 \frac{V}{V+dv}=p_0 \frac{V^2}{(V+dv)^2}[/tex]

and so on.....

[tex]p_n=p_{n-1} \frac{V}{V+dv}=p_0 \frac{V^n}{(V+dv)^n}[/tex]

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