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Visual model for voltage

  1. Nov 27, 2007 #1
    I've always had trouble visualizing what voltage is, and how it relates to other electric concepts that are easy to visualise such as the colomb or the amp. The best I can guess is that voltage is the difference between two charge densitys, therefore voltage is measured in charge/volume but I don't think that's true. Can anyone help?
    I could post a dump of my mind's rambling internal dialogue on this problem but I hope that's not necessary to clarify what I mean.
    Search engine for "charge density voltage" haven't helped me much.
    This is in the atomic forum rather than classical because I find it useful and preferable to visualise at the atomic level in case you need a reason.
     
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  3. Nov 27, 2007 #2

    Gokul43201

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    Forget the atomilism. You do not need that to visualize voltage.

    Perhaps the analogy to mechanical systems will help:

    Imagine a ball. Let it go from a height h - it falls to the ground. This is analogous to having a charge at a point is space with a potential (or voltage) V - release it, and it "falls" towards a lower voltage.

    The mass of the ball is analogous to the charge, and the height is analogous to the voltage.

    To crudely model potentials and fields (potential gradients) in space, put your ball on a sloping ramp. The steepness of the ramp can be thought of as analogous to the electric field, while, as before, the height of a point from the bottom is the voltage.
     
  4. Nov 27, 2007 #3

    robphy

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    I saw a nice visualization in an intro textbook (was it Hecht? or Ohanian?) a few years back.

    Fu-Kwun Hwang has a nice applet
    http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=30
    (as part of a huge collection of physics applets).

    Click on the desired circuit element, then click on a branch.
     
  5. Nov 27, 2007 #4
    I don't like mechanical analogy. It conveys the mathematics, but not the physics.
    The maths of simple gravity and simple electromagnetics are linear relations e.g. E=hmg, V=IR, but those linear relations are arrived at by very different physics, differences that aren't always exposed by analogy between 2 system.
    For example,
    In your ball analogy, you've got voltage as height, and go onto describe the effect of height on a ball.
    What I'm interested in is why does voltage have that effect on charge? ( Why does height have that effect on a ball? )
    Your analogy says nothing on this.- For the ball, the height formula is a simplification of the inverse square gravity formula. Ohm's law is derived with quite different maths.

    In principle, visualising at the atomic model should work, and should reveal any subtlties left out by other models.
    To phrase my original question better - if you could get a graphical\spacial\real\physical ( abit like under an electron microscope ) representation of the atoms and their electrons, of the same material in two states, one at high voltage, one at low, what would be different?

    Thermodyamics is a useful analogy to electromagnetism.
    Heat will travel\disperse through a heat conducting material with the same pattern voltage and charge disperse through an electric conducting material.
    I would find it an interesting exercise to get the metrics of each system paired.
    e.g.
    Resistance, voltage, charge, current with heat, heat transfer, heat capacity of the material, and heat condutance.
    Can heat be compared to charge or voltage in this analogy? If so, is it a meaningful analogy? Heat itself is composed of things like internal KE, internal pressure. What aspect of heat is key to it's analogy to voltage, and why?
     
    Last edited: Nov 27, 2007
  6. Nov 27, 2007 #5

    Gokul43201

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    Those 2 equations are not analogous to each other.

    In any case, looking at atomic models is completely orthogonal to understanding potentials. Best of luck, anyway. Perhaps someone else can pick up the ball on this.
     
    Last edited: Nov 27, 2007
  7. Nov 27, 2007 #6
    You can think of voltage as a measure of charges being crammed together. The higher the voltage, the more charges are present to provide potential energy. Therefore together as a group, the charges will exert a greater force on other charges.
     
    Last edited: Nov 27, 2007
  8. Nov 27, 2007 #7
    In that case, for the sake of clarity, would you state the two equations you were using in your analogy in your previous post, because it looks like you were comparing (at least some form of) E=mgh to V=IR.
     
  9. Nov 27, 2007 #8
    Well that suits me completely waht. It seems the most intuitive likelyhood to me. So, voltage == charge density like I hypothosized in my 1st post. I really hope it's that simple.
     
  10. Nov 27, 2007 #9
    If you consider how a Van de Graaff works, a belt continuously carries charges to a metallic sphere, as they accumulate the voltage on the sphere can be build up to hundreds of thousands of volts. This is simply a result of depositing the charge on a conductive surface. The charges can't go anywhere, so they cram together to accommodate more charges that are forced in. Voltage is J/C (Joules per Coulomb) not per area. So if you were to change the radius of the charged sphere, the voltage would still be the same. But the same amount of charges would redistribute themselves differently on the surface, always such that a potential difference between the charges themselves is zero.
     
  11. Nov 28, 2007 #10

    Dale

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    For circuits I tend to mentally use a "plumbing" analogy. The voltage is like the water pressure, and the current is like the flow.

    This analogy works pretty well for circuits, but is not so useful for things like charges in free space etc. For charges in free space I tend to think along the lines of Gokul's voltage -> height analogy.
     
    Last edited: Nov 28, 2007
  12. Nov 28, 2007 #11
    The rigorous approach is to say that voltage between two points is the work you need to move a charge from one point to the other point.
    Work = force * distance.
    So, at atomic or macroscophic level, if a charge feels a big force there is a big voltage per distance ( this is E ). If there is a long distance there is a big voltage.
     
  13. Nov 28, 2007 #12
    one analogy given to us in class was using water. we had 2 big containers filled with water, and there was a tube from the bottom of one to the top of the other. Then if the first container was lifted up, the air pressure would act on the surface of the water and push the water down through the tube into the second container. So the water particles wanted to move to the second container, to a state of lower potential energy. So you can almost visualize the vertical displacement as the potential drop. It never really helped me but maybe it works for you :)
     
  14. Nov 28, 2007 #13
    I think of voltage and current like a hydro electric dam.

    The higher the drop, the bigger the voltage. The current is the flow through the sluice gates.

    No flow. no current. It doesn't matter how high the dam is, with no flow it's all potential.

    Conversely a dam of no height and a big flow is still useless because while current is high, there is no way to capture it.
     
  15. Nov 29, 2007 #14
    Agree with meemoe that the voltage is somewhat similar to 'charge density'. If you have the same amount of charge particles and you want to pour into an object. The smaller the object, the higher the voltage (and the 'pouring work' is higher). It's also like you pour 1 litter of water into a cylinder. The smaller the cylinder in diameter, the higher the water level (and higher potential).
    This also true that in a conductor, the voltage is the same everywhere, because the charge density is leveled just like the water surface.
     
  16. Nov 29, 2007 #15

    Integral

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    Watch out with your charge density analogy. Consider that a dead battery will have the same voltage as a fresh battery. The voltage of a battery is a result of the chemistry, not the available charge. This is the reason you cannot check a battery with a high impedance digital voltmeter.
     
  17. Nov 29, 2007 #16
    In batteries, the voltage is created by chemical potential. If a battery is flat, that means there's still some charges to make potential, but the rate of reaction is not high enough. When you use it, the charges created by chemical reaction are not enough to compensate for the deficit, that's why you can measure it in volts, but you can not use it.
     
    Last edited: Nov 29, 2007
  18. Nov 29, 2007 #17
    My idea is this: consider there are objects just like 1 liter balloons. The charge is something like air. If you pump 1 liter of air in a balloon, there is no potential (no pressure). But the pressure will build up if you pump in more than 1 liter of air. The more air in it, the higher the pressure. A very high pressured balloon will be cabaple of making more strong jet of air if there is a leak, and of course the work done will be higher as well.
    That is exactly similar to charge-voltage and objects. If you connect two (or more) balloons with pipes, there will be rearangement of gas and finally they will have the same pressure (or potentiall) no matter if they are different in size.
     
  19. Mar 13, 2008 #18
    So, a visual representation at the atomic scale for higher voltage would be higher energy electrons. This energy can be either velocity - electrons bouncing around fast, or potential - electrons squashed together.
    This seems apparent from the van de graf scenario.

    What intergral says is interesting. Voltage without charge. J/C without C? What does that look like? It has to be entirely potential energy. But not the spring loaded potential of electrons squashed together. Sort of like a slide but without a kid to ride the slide. What is the atomic equivalent of a slide? I guess it's an array of materials, the volta pile, which propel an electron from one end to the other of the pile.
    Add this to the visualisation and we've got velocity + electro-static potential + <slide> potential.

    Right?
     
  20. Mar 13, 2008 #19
    The best way to understand electrical concepts is to study from peer approved textbooks that deal with the fundamentals. An EE electromagnetic fields text is a good reference as well as a physics text.

    In a nutshell, voltage from "a" to "b" is the work per unit charge incurred transporting that charge from "a" to "b" along a specific path. It has units of energy per charge or joules/coulomb. Since charged particles naturally exert forces upon one another, i.e. "Coulomb force", work must be done to move charges. It can be computed by evaluating the line integral from a to b of the electric field dotted with the incremental path length.

    Again, forgive me for restating this, but good peer reviewed textbooks used in university physics and EE curriculums are the best source of info. A university library is a great resource.
     
    Last edited: Mar 13, 2008
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