1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hydraulic resistance's unit ?

  1. May 1, 2012 #1
    For better understanding of it's principles, electrotechnics is often compared to hydronics (and, probably, vice versa).
    But, why having the electrical resistance to a flow of charged particles - with it's basic unit "Ohm", there in general Physics is no unit of measurement of the hydronic resistance to a fluid flow ?
    Last edited: May 1, 2012
  2. jcsd
  3. May 1, 2012 #2
    If you consider the analog version of ohm's law, you can quickly define a resistance's units.

    If you allow pressure (N/m^2) to represent voltage, and volume flow (m^3/s) to represent current flow, then you can find an analog resistance as:

    [itex]R = \frac{V}{I}\;\;--->\;\; R_{hydraulic} = \frac{\frac{N}{m^{2}}}{\frac{m^{3}}{s}} = \frac{N*s}{m^{5}}[/itex]
  4. May 1, 2012 #3
    Your formula implies flow, implies pipe's resistance change with the flow. But, as is known, electrical resistance is constant (so far as there takes place no essential changes in temperature). Such analogy for hydraulic resistance, seen in this respect, seems not to exist there.
  5. May 1, 2012 #4
    No, it does not imply the pipe's resistance changes with flow. It implies for any two given constant variables, you can know what the third is.

    If I define electrical resistance the same way, then you would have to argue that it implies resistance changes with current flow, which is not the case.
  6. May 1, 2012 #5
    We know aforehand the resistance of, for instance, a 1 m long 1mm diameter copper wire as 0.0263 Ohm, without resorting to other variables.
    But we do not know resistance to flow of, for instance, a 1 m long 1 mm hydraulic dyameter copper pipe, without referring to pressure.
  7. May 1, 2012 #6
    How do we know the resistance of the copper wire beforehand?
  8. May 1, 2012 #7
    The resistance to direct current of a 1 m long 1mm diameter copper wire is 0.0263 Ohm, independent of a current flowing through it or a voltage applied, so far as its temperature does not change.
  9. May 1, 2012 #8
    Yes, but how do we know what that resistance is? Someone who sells the copper tells us what it is?
  10. May 1, 2012 #9
    An electrician knows, and choses the wire correctly.
    (I am not telling a plumber does not know his business, I am just wondering there is no unit in qwestion).
  11. May 1, 2012 #10
    How does the electrician know?

    Edit: To cut to the point, do you know how electrical resistance is measured?
    Last edited: May 1, 2012
  12. May 2, 2012 #11
    I can see now: there is no "qualitative" difference among "imaginary" electrons flows, but countless ones among "real" fluids flows. So would have been the "resistances" - in the latter case.
    Thank you, DragonPetter for making me to think for a while.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook