Longer conduit Impedes flow? What?

  1. Hi all,

    I am studying for the MCAT and a section in the book lists the equations

    dT= IR
    dP=QR
    dV=iR

    For thermal, fluid, and electricity flow respectively. It then goes to state that a thicker conduit allows for greater flow. That part makes sense to me because a larger diameter pipe has relatively more volume compared to its surface area, which gives it relatively less resistance compared to a pipe that has a smaller diameter.

    However, it then goes to say that a "LONGER conduit impedes flow". This part I don't get because I can't see how a longer pipe would in any way increase resistance or slow down the flow rate. For example, wouldn't a 1m pipe of 5m diameter have the same flow rate as a 100m pipe with a diameter of 5 if the pressure difference between two openings of both pipes were equal?

    Is my book BSing me right now??
     
  2. jcsd
  3. nsaspook

    nsaspook 1,218
    Science Advisor

    I don't think so as there is flow resistance/friction in any real piping system that increases with length so you can't expect the pressure to be the same at both ends for all flow rates just like you can't expect the voltage to be the same on both ends of a resistive electrical transmission line under load as the length increases.
    http://en.wikipedia.org/wiki/Darcy–Weisbach_equation

    We normally use piping tables to calculate pressure losses with cooling systems on the job.
     
    Last edited: Aug 30, 2014
  4. I see.

    Edit: nvm you are correct. I had forgotten that length of resistors are a factor in determining resistance. Oops
     
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