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Volume question

  1. Nov 12, 2003 #1
    hi :

    i have this question, which it reads pretty simple but I am trying to see if someone can walk me through the answer...
    A gas cylinder has two vents, V1 and V2, for
    discharge. The cylinder empties in 7 minutes when
    only vent V1 is open. The cylinder empties in 3
    minutes when both vent V1 and vent V2 are open. About
    how many minutes will the cylinder take to empty if
    only vent V2 is open?

    the answer is in actual minutes....
  2. jcsd
  3. Nov 12, 2003 #2
    obviously it doesn't matter what the vessel's volume is. So you could just assume it's 100 litres or something, and calculate how many litres per minute go thru each vent.
  4. Nov 12, 2003 #3


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    Homework Helper

    You have to know how (why) it empties for the conditions of the vents being open. Is there a piston pushing the stuff out? Is it in an infinite negative pressure reservoir?
  5. Nov 12, 2003 #4

    Doc Al

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    Staff: Mentor

    turin is right, but my guess is that this is meant as an algebra problem, not a physics problem. Is that correct?

    In that case, the rates are:

    V1 = 1/7 (tanks per minute)
    V1 + V2 = 1/3

    Solve for V2 and invert to get the time for Valve 2 alone.
  6. Nov 12, 2003 #5
    thanks every one for your replies... i found the answer and here it is...Think about it like this. For a given cylinder, the product (time to empty) x (total vent size) should be constant. Basically, if you open two vents of the same size, it should take half the time to empty. So let's say V1 and V2 are the sizes (areas) of vents 1 and 2. Then,

    7 * V1 = 3 * (V1 + V2) = T * V2

    From the first part of the equation, you can solve for V1 in terms of V2. That will then allow you to solve the second half of the equation for T, which is your answer.
    so using 7 * V1 = 3 * (V1 + V2) = T * V2, i am solving for the first part of the equation....
    7 * V1 = 3 * (V1 + V2)............ 7V1=3V1+3V2.....V1=3V2/4.....now u got to place 7 * V1= T * V2 to obtain T value
    7*3V2/4= T*V2........V2=7*3V2/4/V2 which V2=7*3/4=5.25

    this was a gre practice question.... pretty interesting ehhh???!!!
    thanks again
  7. Nov 13, 2003 #6


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    Homework Helper

    Why? Did you find this in the solution guide or something? The rate that the cylinder empties is a lot more intimately related to why it empties, than how it empties.
  8. Nov 13, 2003 #7
    I assume that the tank empties because the gas inside is at greater than atmospheric pressure. When the valves are opened the tank pressure changes until the pressure in the tank is the same as the pressure outside the tank. This way it doesn't matter how the tank empties as long as the starting tank pressure is the same each time.

  9. Nov 13, 2003 #8


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    Homework Helper

    If inactive pressure equalization is what causes the tank to empty, then it will never be empty (unless you assume that the ambient can be at a negative absolute pressure). Even if we assume that what is meant is pressure equalization with the ambient, then this will still take an infinite amount of time according to the proposed mechanism of strict baromotivation (I think it is a decaying exponential).
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