When does water analogy to electricy break down?

Discussion in 'General Electronics Chat' started by tpny, Oct 9, 2012.

  1. tpny

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    May 6, 2012
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    I find it helpful to think electricy flow using the water flow analogy. Such as: voltage difference across two points creates current flow vs. water pressure across two points creates water flow; and branches along the path divide the current as they do water flowrate, etc...

    However I think the analogy breaks down at some point.. Or does it.. Any examples when the water analogy doesn't apply to electricity?
     
  2. Sensacell

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    When it gets on your pants

    (I could not help myself)
     
  3. nsaspook

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  4. THE_RB

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    "When does water analogy to electricy break down?"

    When the water has salt in it? ;)
     
  5. Papabravo

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    When you try to imagine an AC water circuit.
     
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  6. crutschow

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    Ignore the jokesters. Engineers can't resist lame jokes. :rolleyes:

    The water analogy works ok for DC circuits, where resistance can be viewed as a restriction or valve to the water flow. It becomes more difficult for simple AC circuits but you can view inductance as inertia (from a weighted piston in the pipe for example) and capacitance as a chamber with an elastic membrane between the input and output.

    But it doesn't work when you want to consider electromagnetic effects and radiation, or the operation of active devices such as transistors.
     
  7. cabraham

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    Oct 29, 2011
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    My advice, which will not be agreeable to some, is to discard water analogies entirely, even for dc. Learning circuit theory is challenging at first, but if you learn it according to the proven basics, you will eventually grasp it. Then you can move on to more advanced topics like e/m fields, rf, power & machinery, etc. With water analogies, you can be stuck in a rut & not advance. Water analogies simply do not do justice to circuit theory. Avoid water analogies entirely right from the beginning, learn the basics, then you will progress much further than you would otherwise. Trust me, those analogies are pure trouble & an impediment to learning.

    Claude
     
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  8. crutschow

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    Is that opinion based upon personal experience? ;)
     
  9. tpny

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    I just think it's easier to imagine current distribution among different paths as water parting thru different branches. The bigger branch (less resistance) gets more water flow (more current) and smaller branche gets less flow/current. And if on the other side, all the branches regrouped together you add all the water bodies together and its equalled to what you started out with at the starting point, the current law or something. I don't know, this helps me comprehend the motion of distribution better and imagine the currents as all trying to flow to an equilibrium. I'm ok with that analogy for what it's worth. But I'm aware it could break down when you get into the more hairy stuff, that's when I lose intuition and begin to proceed very slowly..
     
  10. cabraham

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    Oct 29, 2011
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    Not on my own personal experience because I learned basic theory at the uni level majoring in EE. But I have seen 1st hand numerous people stumble & struggle with circuit theory after having relied on water analogies.

    The water analogy is so limited, it can only help with simple dc networks, & as soon as the student enters the ac domain, they cannot use it. It cannot provide insight into e/m fields, or electronics, or optical electronics, etc. People who rely on water analogies have never been able to relate it to anything other than the most simple dc networks. I have seen that 1st hand, so that is an answer of yes to your question.

    Claude
     
  11. nsaspook

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    My experience has mirrored yours. There are good alternatives that stay within the realm of EM field theory that can be used instead.

    Part of the problem is that the measurement of current flow (Amps) is not a fundamental unit but is derived from the movement of charge (Coulombs). The water analogy reinforces this in exactly the wrong way.

    http://amasci.com/miscon/fund.html
     
  12. crutschow

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    I fail to see the problem. The water in the pipe is equivalent to the electrons (charge) in the wire and the movement of the water is equivalent to current. :confused:
     
  13. nsaspook

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    In a fundamental way it's different. The mass of water contains the potential or kinetic energy of the system but the electrons in a electronic circuit do not. The kinetic energy that does work in electronics is contained not in the carriers as in water flowing downhill but in the fields that surround the wire and the electrons inside. SO the movement of water is not equivalent to current because current (the flow of electrons) is not kinetic energy. :D

    When we measure current do we actually measure electron flow, no. With a shunt we are actually measuring the E field generated from the difference in charge across the resistor as it flows in a circuit. With a Hall sensor there is no physical contact with the wire or any electrons inside it, we measure the magnetic field generated from the movement of charge.
     
    Last edited: Oct 11, 2012
  14. crutschow

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    The mass of the water can assumed to be zero for the water analogy. It's the pressure of the water times the flow rate, not the kinetic energy, that carries the power. For the analogy the water pressure is provided by a pump (voltage) source in a closed-loop connection, not gravity.

    A shunt is used to measure the voltage drop across a resistance, just as you would measure a pressure drop across a restriction in a pipe. No need to get the E-field involved for that analogy (and you're the first one I've heard use the E-field as the source of the voltage drop across a shunt. ;)).

    Of course there's no water analogy for a Hall sensor. As I previously stated, the analogy can't be use where considerations of magnetic or electric fields are needed.
     
  15. nsaspook

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    This is what I mean :(, we now have the Mass-less Mass of water as an attempt to twist reality.

    Just because it's new to you doesn't mean it's not true. :D

    So as soon as the student gets pass a circuit with a battery, a resistor and a wire it's useless. :rolleyes:

    I think that's the answer to the OP's question.
     
  16. electron_prince

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    I agree. It is actually like using a proxy server to browse the web.

    Our study on electricity starts with study of "charges" which is quantized. Study of Fluid mechanics probably start with "mass" which is not quantized (not yet). So when i imagine current flow i visualize discrete charges flowing and when i imagine water flow i visualize a continuous stream of water.

    And when it comes to semiconductors, i cannot trust of water analogy.

    flowing charge produce magnetic field. flowing water does not.
     
  17. nsaspook

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    Teaching students that charge is fundamental to electricity in DC circuits instead of current (and water analogies of current) and that current is a proxy for flowing charge is advantageous to the understanding of energy transmission via fields when the subject of AC power and EM waves are being explored.


    It's much easier to visualize EM fields generated from charge movement, density and changes in density if you understand (at a basic level) that movement and density at one point in space generates magnetic force B-field lines and a difference in charge density from another point in space generates electric potential E-field lines between them.
     
  18. crutschow

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    I didn't say it had to be massless (any more than electrons are required to be massless). It's just that it's not necessary to consider the mass of the water or kinetic energy in the analogy (as you implied in your previous post).
    The problem is, you have it backwards. It's the voltage that generates the electric field not the field that generates the voltage. :rolleyes:

    ................................
     
  19. nsaspook

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    You can't get it backwards if it's the same thing. The both exist yin-yang as they rise out of charge. Field lines always exist from a point charge, it's only when another point charge is near that a difference in charge (voltage potential) between the two points exists.

    A little classroom: http://www.physicsclassroom.com/class/estatics/u8l4c.cfm
     
    Last edited: Oct 13, 2012
  20. THE_RB

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    Oh I just knew this would start up again. :(
     
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