Wednesday, January 20, 2010

Irodov Problem 2.7

An example of the pumping process is described in detail in the figure. The pump comprises of two chambers - a lower chamber of volume V and an upper chamber of volume . The upper and lower chambers are separated by a valve (valve 1) that can be opened and closed. There is also another valve - valve 2, above valve 1, that can not only be opened and closed but also moved up and down along the upper chamber. The goal of the pump is to reduce the pressure in the volume V under the lower chamber.

Initially, in step I, valve 1 is open, valve 2 is in the lower position and closed. Say the gas in the lower chamber has a pressure and volume of P and V.

In step II, valve 2 is moved to the upwards position and so the gas expands and fills up the entire volume of comprising both the chambers. Due to this expansion the pressure of the gas reduces to

In step III, valve 1 is closed, and the chambers are isolated from each other.

In step IV, valve 2 is opened and the air from outside environment flows into the upper chamber.

In step V, valve 2 is moved to the downward position to the bottom of the upper chamber.

In step VI, valve 2 is closed and valve 1 is opened.

Now the valves of the pump are in the same situation as the beginning of step I, except that the pressure in the lower chamber has decreased to P'. Thus, by repeating steps I to VI again and again one can keep reducing the pressure in the lower chamber. At each cycle the pressure in the lower chamber reduced by a factor of ( as seen in Eqn (1)).

Suppose that after n cycles, the pressure is Pn. Then we have the recursive relation,

Hence to reduce the pressure by a factor of times we have,


  1. Hi,

    Thanks for sharing this post it’s very helpful. Great advice for home remedies. This post is very informative related to interview and achieving goals. We are into the same business for home remedies for gas problem, constipation cures, indigestion relief, bloating remedies and irritable bowel syndrome symptoms. Thanks for sharing this story. Visit my link Gas Problem


  2. You have explained each and every problem very nicely. Thanks.

  3. This solution is basically based on the assumption that the change is pressure is solely compensated by the change in volume
    But in the process of pumping the number of moles of gas in the chamber reduces therefore PV is no longer constant

    This is just my thought correct me if I'm wrong

    1. The formula is applied within the constraints. Think carefully,you will definitely understand. I appreciate your fundamental style of thinking.