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What exactly is voltage?

  1. Jul 3, 2011 #1

    Edi

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    .. if we go by the plumbing analogy, ... pressure [difference]
    But, if same amount of water (amount of electrons for electric current) flows trough the same sized hose, but with different pressures, in two, or more, different samples, then water in the sample with the highest pressure will flow faster (thus the system would have more power AND total energy ) than water in the sample with the lowest pressure. - can we conclude that electrons/ wave of electrons moves faster trough a given sample if the voltage is higher? (that would account for the increased total energy.. but it, most likely, wont be true, as most of my interpretations tend to. So what EXACTLY does account for the increased energy of the flow and the system?)
     
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  3. Jul 3, 2011 #2

    rcgldr

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    If your using a plumbing as a comparson model, then a better analogy for voltage would be height, since height times the strength of gravity is also a potential.
     
  4. Jul 3, 2011 #3

    Drakkith

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    Staff: Mentor

    The key here is that the voltage determines the current, not the other way around. You say if the same amount of water flows through two hoses, but one has more pressure it will flow faster. But that isn't true, as there is MORE water flowing past a point in the high pressure hose and LESS water flowing past a point in the low pressure hose as a direct result of the increased water pressure.

    Similarly, applying more voltage to a circuit directly increases the amount of current flow. This increase results in more charges (electrons) actually flowing past a point in the circuit, not in an increase in speed.
     
  5. Jul 4, 2011 #4

    EWH

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    Yes, in normal materials (ohmic) the drift velocity of the charges is directly proportional to the voltage. The current is also proportional to the voltage. The signal propagation rate generally isn't, because the charges throughout the circuit move like a conveyor belt - all parts move at the same time. (Delayed by the speed of sound for the conveyor belt material, or light for the electric circuit.

    What causes the additional energy for a higher voltage across a given resistance? Well, the unsatisfying answer is that 1Volt=1 Joule/Coulomb. A more satisfying answer is that when there is a higher voltage, there is a greater electromagnetic field outside the wires carrying the current, (or electric field in the dielectric between the plates of a capacitor for the case of a displacement current) and this is where the energy is stored. (Mostly. There is a little inertia of the current itself, but virtually all the energy is in the fields outside the wires.)

    See Bill Beatty's detailed explanation of energy flow in a simple circuit http://amasci.com/elect/poynt/poynt.html". His article "What is Voltage? there may also be helpful.
     
    Last edited by a moderator: Apr 26, 2017
  6. Jul 4, 2011 #5

    Drakkith

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    For some reason that site makes me uneasy. I don't know enough to call it incorrect, but some of the things I've looked at there just don't quite add up to me.
     
  7. Jul 4, 2011 #6
    Where does EM energy flow......is just plain ridiculous !!!

    Simply awful....incoherent.....

    The worst misconceptions I think I have ever seen!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    rather funny though...


    I only skimmed the first half dozen paragraphs.....I would not even bother to go further.

    joules "flow"??? ...nonsense...

    "The field energy flows parallel to the wires, and eventually it dives into the lightbulb filament." silly....swimmers dive not electrical fields.


    "Electrons in the metal momentarily speed up before colliding with tungsten atoms."
    nonsense.

    Whoever wrote this apparently doesn't know that while individual electrons do move at only a few meters per second, the power moves almost instantaneously...analogous to a crowded highway where one car moves and another moves almost simultaneously miles away...but each is crawling along....
     
    Last edited: Jul 4, 2011
  8. Jul 4, 2011 #7
    I looked at "Bill Beaty's" link, and that's actually the way I understand circuits: the wires guide electromagnetic waves produced by a generator or battery to the load, where it gets converted to ohmnic heating. I didn't read the link except for the first couple of paragraphs, but skimming the pictures quickly, they seem all right.

    Here's a link to a picture of an electromagnetic wave being guided by two wires (figure 3-14 in the middle of the page):

    http://www.tpub.com/neets/book10/41d.htm

    One of the currents is going towards you, and another is going away from you. The fields flow down the circuit to the load.

    For more information (DC and AC wave being guided along two wires) see this page:

    http://www.tpub.com/neets/book10/41f.htm

    The e-book I believe is from the Navy, though I'm not sure. They like to draw the direction of the current as the direction of the electrons, which I think is characteristic of the Navy (makes more sense since electrons are really what's flowing)
     
    Last edited: Jul 4, 2011
  9. Jul 5, 2011 #8

    Drakkith

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    It's his terminology and apparent "Everyone else is wrong" attitude that makes me wary. That plus some obvious mistakes I thought I saw. For some reason the site is down right now for me, so I'll go back and take a look at it later.
     
  10. Jul 5, 2011 #9
    I just now saw that attitude. He seems to bash Feynman as if he had some sort of personal grudge. It's ridiculous really.

    However, you can calculate the magnetic field produced by a loop of wire, and the electric field, and if you take the Poynting vector ExB, then you'll get that energy is flowing into the resistor from all directions. That's what diagrams 7-10 show: those lines are lines of Poynting flux and they always point towards the resistor. The wires guide the waves to the resistor and prevent the waves from radiating into space.

    And this idea is not something that this Bill Beatty guy came up with. It actually comes naturally when you study transmission lines and antennas. When studying these things, there are two viewpoints. One is you can look voltages and currents. The other is you can look at the electric and magnetic fields. Usually you look at the latter for waveguides, and the former for two parallel wires. But you can use either viewpoint for either application.
     
    Last edited: Jul 5, 2011
  11. Jul 5, 2011 #10

    EWH

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    Naty1:"joules "flow"??? "

    Energy flows, and end energy is measured in Joules. I don't see any misconceptions there, although there are some figures of speech. Bill Beatty has written many meticulously correct articles which attempt to correct the misconceptions fostered by standard presentations of the material. He does try to make his presentation visualizable, but that is a good thing I think.

    Naty1: "Whoever wrote this apparently doesn't know that while individual electrons do move at only a few meters per second, the power moves almost instantaneously"
    You appear to have totally missed the entire point of the article. That is exactly what he is explaining.

    RedX: "He seems to bash Feynman as if he had some sort of personal grudge. "
    Er.. well Feynman did mess up there a bit. But B.B calls him "an honest free-thinker", so he clearly has respect for Feynman.

    The Poynting field is the ONLY correct description of classical EM field energy and transmission. The power absolutely does NOT flow primarily through the wires. The dielectric constant of the materials near the wires affects the transmission speed substantially, as well as the field strength outside the wires and the voltage of the wires; the dielectric need not be in contact with the wires to do so. The same is true of the magnetic permeability of materials near the wires and the specific geometry of the wires (e.g. coils, plates). Beatty may be belaboring the point a bit with the "speed of light in copper" criticism, since IIRC Feynman did present the drift velocity in the lectures, but Beatty's point was:
    "If the common misconception that 'energy flows inside wires' has had such a deleterious effect on an honest free-thinker, imagine the trouble a more conventional mind would have with it."

    Beatty is also an honest free-thinker, and no sort of crackpot. Though he does have a high affinity for finding out all the potentialities of all sorts of ideas, he also picks apart every little inconsistency in theories. He designs and builds electronic equipment for a university, he used to design and build exhibits for the Boston Museum of Science, and his website has been up since 1994.
     
    Last edited: Jul 5, 2011
  12. Jul 5, 2011 #11
    Voltage is energy per unit charge, especially the electric potential energy.Your analogy would be of a pressure potential. So googd analogy, I think.

    So you just see where does the potential come from. Pressure poetential in your analogy would be a pump which comsumes fuel. Chemical energy stored would trnsform to pressure. So the battery would be the "pump" for the circuit, which could move charges. Where charge accummulates has higher potential.
     
  13. Jul 6, 2011 #12
    Ref bold quote: not always. If a constant voltage source is connected directly across an impedance, then current is determined by voltage divided by impedance. But if a constant current source is connected to an impedance, then voltage is determined by current and impedance. It can work either way.

    A switching power converter is a prime example. Energy is stored in an inductor which tends to behave as a current source. It transfers energy into an output load through a sensing resistor, diode, & recharges the capacitor. The inductor forces a constant current through the sense resistor & diode & the voltage is determined by the impedance of the elements times the current.

    The output has a filter capacitor. A cap acts like a voltage source. Any load across the cap sees a constant voltage. So the current in the load is the voltage/load impedance. It can work either way. Voltage does not always determine current. It can, & vice-versa.

    Claude
     
  14. Jul 6, 2011 #13
    No, the problem is that Feynman is actually wrong in this instance. I point this out. (That's bashing? We're not allowed to criticize?!)

    And "personal grudge?" Don't be silly, I'm a huge Feynman worshipper, had "Surely" when it first appeared, and in fact put up one of the internet's early Feynman pages.

    Feynman apparently hates the Poynting Vector description of DC circuitry. (Somewhere he has a paper on the details of the difficulty, but I haven't found it yet.) Feynman's apparent mistake was in believing that DC energy sources radiate their energy outwards in all directions to infinity. Nope. The energy is located in the EM fields as always. If fields don't escape, then Poynting-energy doesn't escape either. Dipole fields such as with capacitors, inductors, and wire-pairs all keep their energy mostly local. The strong fields inside a coax cable are the actual location of the moving EM energy. Same thing with the bare wires of a simple flashlight circuit.

    Obviously not! But I've found only very few physicists ever teach it to students. One was Robert Oppenheimer (of Exploratorium Museum fame.) I've noticed something odd: quite a few physics students become angry when they encounter this discription of circuits, as if it's some kind of heresy. Probably the sticking point is that the description doesn't appear widely in undergrad texts. If it was this simple and useful, why aren't we taught about it in Halliday & Resnik, Sears Zemanski, etc.? It must be WRONG!

    :)

    Of course double-E students learn all about it in our fields/waves classes. But we're taught to apply it to waveguides. Nobody ever told us that a DC flashlight circuit constituted a valid "waveguide." There is no magical lower cutoff frequency. It works all the way down to DC.
     
  15. Jul 6, 2011 #14
    Sure, it's easier to see in a coax cable where the dielectric between inner and outer conductors affects the speed of propagation of the voltage/current to the load, but it applies to any two conductors.

    But, you can model a long circuit as an LC ladder, and explain the effect of the dielectric as belonging to the dielectric of shunt capacitance between hot and neutral. I believe the terminology is parasitic capacitance. Or in general, having distributed circuit elements rather than lumped elements.

    I'm visiting the library tomorrow, so I'll look at chapter 27 and see if Feynman made a mistake. I really enjoyed chapters 23 and 24 where Feynman talks about cavity resonators, transmission lines, and waveguides, and Beatty mentions that Feynman doesn't put everything together like Beatty's figure 7 does even though Feynman talks about Poynting vector on chapter 27. Well, maybe that's because chapters 23 and 24 are where he talks about these things, so to go back to chapters 23 and 24 to apply the Poynting vector concept might have been too much of a hassle.

    Anyways, here's another picture from a Navy training manual on how a parallel line turns into a waveguide using the fact that a quarter-wave short has infinite impedance, forming a metallic insulator:

    http://www.tpub.com/neets/book11/44b.htm
     
  16. Jul 6, 2011 #15
    PLEASE tell me about any mistakes you find. I'm always updating these pages.

    But beware: most of my articles involve common widespread misconceptions. Very often an "obvious mistake" is actually a situation where the entire education community really is wrong, and I'm attempting to debunk the widespread error.
     
  17. Jul 6, 2011 #16
    Perhaps you should consider writing a paper on this for one of the pedagogical physics journals such as The Physics Teacher. You're absolutely correct that it's rarely discussed in physics books. Even looking at the canonical graduate text by John D. Jackson, I can't find a mention of it. A website is good, but I'm not sure if instructors go on the web, so the way to get instructors to adopt and disseminate it to students might be through a journal (although I'm not sure many instructors read journals on how to teach physics, because a lot of them are pretty bad at it!).

    edit: actually, I think Jackson briefly mentions it in chapter 6.9, but as is typical of Jackson, it's incomprehensible.
     
    Last edited: Jul 6, 2011
  18. Jul 6, 2011 #17

    Drakkith

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    Actually I think I just understood what you were talking about in one of your articles that I wasn't sure was correct. Once I understood what you were saying I was like "Oh, yeah, i thought that was obvious" lol.
     
  19. Jul 6, 2011 #18
    If radio waves, light, as well as b-fields/e-fields all contain "electromagnetic energy," then we could be more nitpickingly precise, :) and instead say that "EM energy propagates" across a circuit.

    When the beam from a laser pointer zips across a room, that's a flow or propagation of EM energy. When RF waves are gathered in by an antenna and sent along a 2-wire cable to your 1950s antique television set, EM energy is flowing from the antenna to the TV receiver. And when a D-cell sends some joules to a flashlight bulb, EM energy is flowing from battery to bulb in the space around the connecting wires.

    If our descriptions must apply to all frequencies, then we don't want to say "light is flowing" or "RF energy is flowing." Instead, the generalized moving entity is "EM energy."

    But too bad we have no single word which means light, microwave, rf, and DC circuit energy. Grammar-engineering sometimes can greatly clarify physics. Speaking of which...

    I suggest "Wakalixes."

    Submitted for your approval: From this point in time forward, Wakalixes means "a certain quantity of EM energy occupying a certain volume of space."

    The word comes from "Judging Books By Their Covers:"

    "I turned the page. The answer was, for the wind-up toy, "Energy makes it go." And for the boy on the bicycle, "Energy makes it go." For everything, "Energy makes it go."

    "Now that doesn't mean anything. Suppose it's "Wakalixes." That's the general principle: "Wakalixes makes it go." There's no knowledge coming in. The child doesn't learn anything; it's just a word!
    http://www.textbookleague.org/103feyn.htm" [Broken]

    So Susie, what comes out of the front of this flashlight? "Wakalixes of optical wavelength, sir!" Correct. And Johnnie, describe the flashlight operation from the EM fields perspective. "The electrochemical cell emits Wakalixes which are guided by the pair of conductors. Some of it glides inwards to warm the conductors, but the major portion arrives at the bulb and dives into the surface of the tungsten filament."

    :)

    Oooh, I may regret this. Crap, what if the above meme catches fire!
     
    Last edited by a moderator: May 5, 2017
  20. Jul 6, 2011 #19
    Yeah, I was wrong about that. You have a a large Feynman page on your website that has some footage I've never seen, including Gell-Mann on Feynman:

    http://amasci.com/feynman.html

    So obviously you weren't bashing, and I misinterpreted your words
     
    Last edited: Jul 6, 2011
  21. Jul 6, 2011 #20

    Drakkith

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    Are we talking about an AC or DC circuit here? If it is DC then the impedance is simply resistance, correct? Then it just becomes Ohms law correct?

    What? The inductor causes a voltage that resists the change in current through it. This causes the circuit to have a steady current, but only because the voltage in the inductor is changing in response to the current, either resisting the increase or compensating for the decrease. Either way the current in the circuit is directly caused by voltage sources.
     
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