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Question about hydraulic analogy!

  1. Aug 16, 2013 #1
    Hello there; I am having a conceptual problem with the hydraulic analogy (a collection of similarities between electrical circuits and piping systems). It specifically has to do with the topic of pressure and voltage drops.

    The hydraulic analogy claims that pressure drops act like voltage drops, and that constrictions to flow act like resistors. I know from basic circuit analysis that there is a potential drop across a resistor (Ohm's law). By the hydraulic analogy, there should be a pressure drop across a constriction in the bore of a pipe. However, Bernoulli's equation says that (in a closed loop piping system) once the bore of the pipe returns to its pre-constriction magnitude, the pressure in the pipe returns to its former value as well. This implies that there is actually no pressure drop across the constriction, directly contradicting the hydraulic analogy. I am sure that there is a clear error in my logic; please help me find it!

    Thanks for your help and time,
    F2
     
  2. jcsd
  3. Aug 16, 2013 #2

    nsaspook

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    The hydraulic analogy is a pipe dream, don't waste your time trying to make it work. It detracts from the idea that charges have fields that transfer electrical energy which is what you really need to understand when learning circuit theory.
     
    Last edited: Aug 16, 2013
  4. Aug 17, 2013 #3

    sophiecentaur

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    Absolutely. There is no point in examining the hydraulic theory for more than a few seconds. It is full of holes because it confuses units of energy with force, it assumes that the KE of electrons is relevant etc. etc.. Steer well clear of something that used to the taught to 'squaddies'. often by people with no depth of knowledge of Science. 'Electricity by Numbers',. . . .hup two three . . . . ., . . . . hup two three. . . . ., is not the way to 'understanding' anything.
     
  5. Aug 17, 2013 #4
    There is no point in examining the hydraulic theory for more than a few seconds. It is full of holes
    I completely agree with this statement. A hydraulic analogy full of holes is no use whatsoever.
    An analogy without holes is of some very limited value in the very early stages of education.
    Ideas about continuity of flow can be introduced and must have proved useful, after all we use words such as 'current', 'circuit' and 'capacitance' freely in describing electrical behaviour.
    Beyond that there is not much more to be got from the hydraulic analogy.
     
  6. Aug 17, 2013 #5

    OmCheeto

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    I'm not trying to defend the use of analogy, but the above struck me as wrong, and was curious where you read such a statement. So I googled it:

    As a squaddie, I'm not an expert in Bernoulli's Principle. But I have a feeling the above analysis is correct.

    On second thought, I'm going to contradict all the experts here, and say that this analogy is perfect!

    A closed loop hydraulic system without a pump is a near exact analogy of a single loop circuit with resistors without an emf source.



    ---------------------------
    And thank you for asking the question. The second google link provides me with something I didn't even know I was looking for.
    Bernoulli’s equation can be used to determine pump head required!
    I would query if anyone knows what the friction loss of garden hose is, but I believe that would be off topic.
    /me scratches head.
    Ah ha! I know how to measure it!
     
  7. Aug 17, 2013 #6

    sophiecentaur

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    No disrespect to squaddies but they are not required to appreciate the subtleties of circuit theory. You have clearly advanced beyond the basics of hit it till it works, Om Cheeto.
    I don't see how a closed hydraulic system without a pump is in any way relevant to an electric circuit with an energy source in it (the only one of much interest to anyone but a still life sculptor haha).
    Rather than bringing up red herrings which will confuse the OP, why not just agree that the water model sucks for explaining pretty much every problem in electrical engineering? There are so many threads on PF in which some poor soul comes in with a query about one or other lame analogy and regular members take up cudgels about it because they can find some loophole through which to defend it. If I suggested a model of the Solar System, based on the medium of Dance, I am sure I would get a few takers. I have actually tried this in a class of 13 year olds, to explain the difference between a solar day and a sidereal day, when I discovered we had one girl in the class who was a dancer and proud of it. It was a very nice interlude and she was pleased to show the class what she could do (pirouettes on the move) but I reckon my message was lost on most of them!
    Bernoulli is a fascinating topic and so is the garden hose but wouldn't it be better to discuss those in their own thread? (And why does my almost new Hoselock connector randomly throw itself off the tap and spray water all over the patio?)
     
  8. Aug 17, 2013 #7

    nsaspook

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    It's also a near exact analogy of a pet rock.
     
  9. Aug 17, 2013 #8

    OmCheeto

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    It would appear that we are all in agreement here. :thumbs:

    That's why I went with cars.

    To this day, I have no idea why I didn't get a perma-ban for that post.
     
  10. Aug 17, 2013 #9

    CWatters

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    The hydraulic analogy works just fine as long as your knowledge of hydraulics is as basic as your knowledge of electronics. Once you get much beyond the initial stage with either it doesn't really work well.

    Anyone fancy recommending an alternative analogy?
     
  11. Aug 17, 2013 #10

    sophiecentaur

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    I reckon a bicycle chain is at least as good if not better, actually. You have power transfer and a slow moving medium (as with the electrons) and it doesn't work when the circuit breaks. A belt with series take-offs even gives you a proportional change in tension as you go past each take-off so there's your potential divider. Look out squaddies!
     
  12. Aug 17, 2013 #11

    nsaspook

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    I explained basic electricity with charges and fields to my 8yo with a sheet of paper with a line graph and bar magnets sliding on that graph to demonstrate attraction, repulsion and how fields transmit force once she understood the parts of a atom have charge. It took her a while to develop a mental picture of power from a AC socket but now she has a background to understand and build from without saying "well, it really works like this" as questions become more advanced. I think sometimes we forget the (sometimes negative) ability for a young mind to absorb analogized information as facts and develop impressions that last a lifetime.
     
    Last edited: Aug 17, 2013
  13. Aug 17, 2013 #12
    Did you use the solar system analogy of an atom in this exercise?
     
  14. Aug 17, 2013 #13

    nsaspook

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    No, I explained that the atoms are not like solid balls or orbits, I used a simple version of the Quantum Mechanical Model like electron clouds where there is likely to be an electron. We played with cotton balls to give a mental image.
     
    Last edited: Aug 17, 2013
  15. Aug 17, 2013 #14
    The problem is that the Bernoulli equation is not the appropriate relationship to use when discussing the hydraulic analogy. The Bernoulli equation assumes that inertial (kinetic energy) effects and gravitational effects (potential energy) are of primary importance, and that viscous drag (friction) is either secondary or negligible. The tacit (typically unstated) and implicit assumption made when invoking the hydraulic analogy is that inertial effects and gravitational effects are negligible, and that the pressure variations are dominated by viscous drag. Do situations like this often exist in practice? As an engineer with extensive experience in hydrology and fluid mechanics, I can assure you that such situations are abundant, particularly in the polymer processing industry, where one is dealing with highly viscous polymer melts. But, aside from that, even for laminar flow of a fluid in a straight pipe of constant cross section, the fluid mechanics prediction is that the pressure drop is directly proportional to the volumetric flow rate (Hagen-Poiseuille). The Bernoulli equation (in its usual form which neglects viscous friction) predicts that the pressure drop will be zero. As long as inertial and viscous effects are negligible, the pressure variations are directly proportional to the throughput rate, just as in circuits with voltage variations through resistance networks, where the voltage variations are directly proportional to the current. The qualifying statement which is always omitted from the hydraulic analogy is that the pressure variations from fluid inertial and gravitational effects must be negligible compared to the pressure variations from viscous drag, and the flow must be laminar (so that there is a linear relation between flow and pressure drop). These conditions can always be attained by using a fluid of a high enough viscosity.
     
  16. Aug 17, 2013 #15
    I don't get this!....it is an analogy too far. I think an analogy is best used 'at the start' and is dumped as soon as possible.
    Weaknesses of the 'bicycle chain' analogy:
    Continuity of flow requires 2 components...a medium and a path for the medium ( water in pipes)
    Could you demonstrate a parallel circuit easily?
    Could you demonstrate a series circuit of different 'resistances'
    The 'medium' ('electrons') and the route are one and the same thing....the chain...a 'break' in the circuit is due to what? The electrons or the route?
    Once you have a good understanding of the basics it could be useful to elaborate on power transfer but I would expect that at that stage of understanding there would be less need for 'analogy'
     
  17. Aug 17, 2013 #16

    sophiecentaur

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    I could easily have two parallel chains, driven by two drive sprockets on the same driving crank. The two loads would share the power according to the gearing or load (Kirchoff 1). Two driven sprockets with different loads or diameters on the same chain would be the equivalent to two series resistors (Kirchoff 2). A broken chain would stop the links circulating.
    That's no worse than water in pipes which, in the usual model, don't actually do anything but dissipate energy due to turbulence and the kinetic energy of the water (unlike the chain links - which could have zero mass like electrons and the model would still apply). It's a rubbish analogy but not as rubbish as the water one.
     
  18. Aug 17, 2013 #17

    nsaspook

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    This thead prompted me to ask the little one about atoms and electricity again. She remembered what we had talked about before and even asked a few questions about how light, tv and radio worked. At the end she smiled and asked if a Scientist worked on these things. I said sure and then said she wanted to be a Scientist when she grows up. That made my day!

    My personal belief is that analogy is useful but should be mainly used as a means to express already developed abstract knowlege between different overlapping subjects like telling my kid that a TV is 'like' a radio for pictures and sound instead of just sound.

    For people who don't understand basic electromagnetics or fluid mechanics they just don't know when it's useful or harmful to take the analogized information as factual.
    http://www.lhup.edu/~dsimanek/scenario/analogy.htm
     
    Last edited: Aug 17, 2013
  19. Aug 18, 2013 #18

    sophiecentaur

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    What a joy. I find that kids may as well be given the real deal, at least, initially. The use of analogies often reflects the limitations of the teacher (or institution) rather than the pupil.
     
  20. Aug 18, 2013 #19

    OmCheeto

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    That reminds me of Mr. Mansplainer, and Mr. Notaclue, from last weekend. Neither had a clue about either hydraulics, nor electronics.
    I enjoy the hydraulic analogy, as it creates some very weird quandaries. A while back someone posted a question about a charged capacitor being hooked to an uncharged capacitor, and why the system lost half its energy. I'd never heard of such a thing, and had to mentally draw a picture. I figured out that the energy was lost because the charges were oscillating back and forth, and losing energy by dissipating electro-magnetic fields into space. Or something like that.

    I decided that the hydraulic analogy would have been more complicated than the electrical one, as it involved infinite numbers of water powered massless torque wrenches attached to the perimeter of two infinite sets of bicycle ratchet type gears. Except the torque wrenches don't really turn, and they expand at the speed of light, with a force circular to and in a particular direction, depending on which way the fluid is flowing. I think I stopped when I decided the time varying infinite numbers of water powered massless torque wrenches attached to the perimeter of two infinite sets of bicycle ratchet type gears required another slightly different set of infinite numbers of water powered massless torque wrenches attached to the perimeter of two infinite sets of bicycle ratchet type gears.

    I don't think I even stopped to think about how the energy was carried away, as this imagery hurt my brain.
     
  21. Aug 18, 2013 #20
    A while back someone posted a question about a charged capacitor being hooked to an uncharged capacitor, and why the system lost half its energy.

    This is one area where the hydraulic analogy works quite well. You can experiment in the kitchen with 2 plastic bottles. Fill one bottle, connect it to the empty bottle (not using bicycle chains !!). See the water flow from one to the other...look at the starting height, look at the final (same!!) height...can you see it in the 'capacity'....do you see any oscillation, swirling (energy dissipation)....you will see charge conservation (assuming your hydraulic analogy does not have 'holes in it'), loss of energy due to transfer of charge, some of the lost energy as KE, an equality of PE (voltage) at the end. You may even pick up some indication of the time aspect in the process.....time constant??
    Whatever is available....use it....do it with bicycle chains if you have nothing else.
     
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