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Why does EVERYONE claim voltage is electrical pressure?

  1. Jul 15, 2005 #1
    Why does EVERYONE claim voltage is "electrical pressure?"

    Nearly everyone I talk to who says they know "a lot" about electricity say voltage is electrical pressure.....

    However, I fail to realize how voltage has anything to do with pressure.....in fact, I am quite certain it has NOTHING to do with pressure, and everyone who claims the contrary is completely wrong.

    Can someone provide some insight that possibly validates the claim that voltage is like electrical pressure? From what I understand, voltage is the potential energy (force exerted through a distance) per unit charge a given charge has due to an electric field. The force would depend on the charge producing the e-field.
  2. jcsd
  3. Jul 15, 2005 #2
    ok, I guess I can see why voltage is like pressure....it just seems like a half assed way of thinking about it, and I don't like it.
  4. Jul 15, 2005 #3
    and if someone else uses the analogy of water flowing through a pipe to explain electrical flow, I am going to flip.
  5. Jul 15, 2005 #4
    When you have something under pressure, you can make a hole in the tank and let the water flow out of the hole. This flow can be put to use, (turn a paddle wheel, run a generator). The more pressure you have, the faster the wheel or generator will turn.
    Likewise, the more voltage you have, the more work you can possibly do. Its just an analogy so that you can have a tangible understanding of it.
    Last edited: Jul 15, 2005
  6. Jul 15, 2005 #5


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    You need a pressure to push the electrons in a conductor. If not, what will force the electrons(current) to move? With no pressure(electro motive force or potential difference), the electrons will simply idle around at a small pace.

    Don't flip- But, think of the Ocean wave as the Voltage and the water as the current.

    So, if you take a voltage source, then there has to exist a difference between the two points in order for a voltage to be generated. Potential difference is defined as how much work is needed to move a charge from one point to another. You can integrate the Electric field from one point to another to obtain the Voltage.
  7. Jul 15, 2005 #6
    I'm aware of what voltage is defined as and I know you can integrate the electric field over a given distance to get voltage. Since e-field is N/c, you can integrate that with respect to distance to get J/c. This wasn't really my question.

    I guess I understand how electrical potential is sort of related to water pressure, but it is a poor way of thinking about it....it seems so half assed that it is just seems wrong...

    Sorry if I seem kinda rude, but this just frustrates me.

    Thanks for the quick reply.
  8. Jul 15, 2005 #7
    I've heard that analogy before, but it still REALLY doesn't make a strong connection between work that CAN be done on a charge due to an E-field and the pressure of water hitting a paddle wheel....does it?

    A better analogy would be that voltage is kinda like the work the water does due to the force of gravity pulling the water a distance equal to the height of the tank after it is dumped PER unit volume of water. The voltage is just like this, except instead of the force being a gravitatial field, it is electric field, and instead of water, it is charge. Also, instead of the force being only attractive (as in gravity), it can be both attractive and repulsive.

    No physics professor I have ever had has used the "voltage is like pressure" analogy....because it is wrong.
    Last edited: Jul 16, 2005
  9. Jul 16, 2005 #8


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    I don't know why you're going to flip - the reason people compare these phenomena is because they really are exactly the same thing. A watt is a watt is a watt whether it's electrical or mechanical or chemical. If you want to calculate how much electrical energy you can get from a hydroelectric dam, for example, you calculate how much mechanical energy is released by the water flowing through it (and multiply by the efficiency of the turbines). It should not be surprising that the equations for describing what happens in the wires look a lot like the equations for what happens in the pipes.
    Last edited: Jul 16, 2005
  10. Jul 16, 2005 #9


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    I can kind of understand your frustration with that analogy. But it's not a totally useless analogy. In fact, it helped me understand how water flows through a pipe. :rofl:
  11. Jul 16, 2005 #10


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  12. Jul 16, 2005 #11


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  13. Jul 16, 2005 #12


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    Ohm's Law for cuurent in a conductor : V = IR
    V : voltage or potential drop across ends of conductor
    I : current (or rate of flow of charge)
    R : resistance of wire/conductor

    Power drop in a resistor = IV

    Effective resistance of conductors in series, R(eff) = R1 + R2 + ... + Rn

    Effective resistance of conductors in parallel, 1/R(eff) = 1/R1 + 1/R2 + ... + 1/Rn

    Flow of incompressible fluid through a pipe : P = Qr
    P : head or pressure drop across ends of pipe
    Q : fluid flow rate
    r : impedance of pipe

    Hydraulic power drop in pipe = PQ

    Effective impedance of pipes in series, r(eff) = r1 + r2 + ... + rn

    Effective impedance of pipes in parallel, 1/r(eff) = 1/r1 + 1/r2 + ... + 1/rn

    Well, do you now see the reason for drawing up the analogy ?
  14. Jul 16, 2005 #13

    Actually Professor Feynman used that analogy. He's a physics professor I think.
  15. Jul 16, 2005 #14
    Correction: *was*
  16. Jul 18, 2005 #15
    It's an analogy. Analogies by defination are not entirely exact and specific. I suggest you deal with it. I don't think anyone said "Voltage IS pressure", they said "Voltage is similar to pressure because the potential difference drives the current, just as the pressure drives the flow of water.....although they are different things"

    A lot of analogies work on a similar basis, although the more knowledgable physicists will of course know better. Analogies are just used to help people understand otherwise complicated things.
  17. Jul 18, 2005 #16
    The analogy can be made much deeper.

    If you measure the attractive force between the plates of a capacitor they
    are proportional to the voltage. (If you measure the repulsive force between the
    windings of an inductor they are proportional to the current in the inductor. The
    energy in the inductor is exactly analgous to the kinetic energy of moving water in a pipe.)

    Pressure is force/unit area.

    For a fixed geometry of a given capacitor attractive pressure on the plates will be related by a constant of
    proportionality to the voltage.

    For a fixed geometry of a given vertical pipe, outward pressure on a cross section of pipe will be related
    by a constant of proportionality to the height of the fluid colum in the pipe.

    While not exatly the same they are similar enough that you could for many
    problems solve the equations for one and get solutions to the other. That is
    by definiton a good and useful analogy.
    Last edited: Jul 18, 2005
  18. Jul 22, 2005 #17
    Read some of the history about electricity and you will find that Volta et al. thought of it as, literaly, a fluid/aether. Is it any wonder that they applied what they knew of hydraulics to describe it? No, the wonder is that the description actually works so well, within it's limits.
  19. Jul 23, 2005 #18
    Yes Kleinjahr. Good point.

    And to this day the transmission of electrical power is done over
    "high tension wires". They aren't talking about the mechanical tension of the
    cables but rather the "tension" means voltage in another analogy to pressure.
  20. Jul 24, 2005 #19
    Actually, there is quite some confusion on exactly how stuff like EMF, voltage,...should be defined. The link will give you an article of IOP, illustrating the various contradictory definitions. You will need a subscription to download the file so you should open it at your university-library. It really is a great article that also explains in a very easy way the quasi Fermi Level (though not really stringent)

    IOP Article

    Last edited: Jul 24, 2005
  21. Jul 25, 2005 #20
    I don't have convenient access to it. Can you give en example of one of the
    contradictory definitions for us?
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