Ratio of Flow Rates.

  1. Jan 15, 2010 #1


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    1. The problem statement, all variables and given/known data
    Two tubes carry the same incompressible fluid with viscosity 1.5 Pl. They have lengths L1 = 6 and L2 = 22 m and diameters d1 = 1.2 and d2 = 4.5 cm. What is the ratio of their flow rates F1/F2?

    2. Relevant equations
    Poiseuille's law: 8nLI/(pi*r^4)
    while n is viscosity
    L is the length
    R is radius

    3. The attempt at a solution
    F1= (8*1.5*6*I) / (pi*(1.2/2)^4)
    F2= (8*1.5*22*I) / (pi*(4.5/2)^4)

    The ratio F1/F2 is:
    (8*1.5*6*I) / (pi*(1.2/2)^4)* (pi*(4.5/2)^4)/ (8*1.5*22*I)

    F1/F2= 46.296*1.1649= 53.932.

    And this is wrong, can u help??
  2. jcsd
  3. Jan 15, 2010 #2


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    Homework Helper

    You need to write out your steps more clearly. 8nLI/(pi*r^4) is not Poiseuille's equation; Poiseuille equation is ΔP=8nLI/(pi*r^4), where I represents flow rate. You now have 2 equations:


    If you assume the two ΔP's are the same and divide one equation by the other, you'll see your mistake.
  4. Jan 15, 2010 #3


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    Thank you alot :)
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