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Electrical circuits don't flow in a circle?

  1. Jul 19, 2011 #1
    So I came across http://amasci.com/miscon/eleca.html#circle" on W Beaty's site and it's not sitting well with me.

    Here is a quote:

    This doesn't seem right to me. My impression has always been that the charges flow from one of the slots (hot) in the wall socket and return on the other slot (neutral) in the socket. Then the charge flows back from that slot (neutral) to the first slot (hot) in the wall socket... This goes back and forth and repeats itself 60 times a second (here in the US anyway). I have never heard anyone claim that these charges are simultaneously (hot and neutral) leaving the socket at the same time and diving into the filament.

    I understand about drift velocity and I get that the wave, or impulse, or whatever you want to call it, is what travels along the wire at near light speed and not any individual electron... that's not what I'm having an issue with.

    I just always thought that this wave traveled from one side of the source and to the other side of the source. Never have I heard anyone claim that this wave travels simultaneously out of both sides of the source to plunge into a load... I think this notion will only confuse people.

    EDIT: It just occured to me that he's talking about energy and not current. An important distinction... yet I tend to think it would still cause a great deal of confusion to students if this difference is not heavily stressed. any thoughts?
     
    Last edited by a moderator: Apr 26, 2017
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  3. Jul 19, 2011 #2
    I agree with you...that analogy is not good at all....even from the energy balance point of view...I mean, what if you have devices in series...how is he going to explain energy flowing to two different lamp and the energy coming from both wires at the same time...where is the energy coming from for the wire between the lamps? from the middle of it? magically?

    I think that if you are going to explain something, you need a good analogy or your audience is not ready for such concept...but don't confuse them.

    And you are correct, too, in thinking that in AC the electrons are moving back and forth inside the wires.
     
  4. Jul 19, 2011 #3

    Drakkith

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    I personally don't like the phrase "energy flow". It doesn't describe what is happening in the circuit at all. I mean, he is correct in saying that energy doesn't flow back and forth in the wires. That is current. But it is the current and all of the forces and effects involved that causes something to happen. Saying energy is flowing from A to B isn't telling anyone anything.
     
  5. Jul 19, 2011 #4

    HallsofIvy

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    For AC, alternating current, neither of those is true. What happens is that the electrons in the wire move, very rapidly, back and forth.
     
  6. Jul 19, 2011 #5
    Energy indeed flows down the outside of the pair of wires to the filament where it is converted into heat.

    The current flows back and forth slowly, not rapidly. The energy delivery (Joules per second) to the filament pulsates at 120 Hz (not 60 Hz like the current) in the US as [tex]P = 169^2 sin^2(2\pi*60t)/R[/tex] Watts where R is the resistance of the hot filament in Ohms.
     
  7. Jul 19, 2011 #6
    Hmmmm... Still having a hard time digesting a"flow" of energy is what's taking place here... Electrons are flowing for sure. When they meet a resistance heat is created... Does that mean energy "flowed"?
     
  8. Jul 19, 2011 #7

    Drakkith

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    The current switches direction 120 times a second, with corresponds to 60 hz. The "energy" you refer to is the various forces in the circuit producing work. Energy is not a thing, it cannot flow anywhere. It is a measure of the ability to do work. I see it as similar to Beaty's own example on his site on what current is. As he says, current cannot flow, only charges can. The propagation of forces through the circuit is not the same thing as energy, though it is used like that alot.
     
  9. Jul 20, 2011 #8
    Electrical energy is very much a thing. It has a spatial density which can be integrated over space; this energy density gravitates, has inertia, and therefore an equivalent mass. It flows from place to place with the precise flow governed by Maxwell's equations. If that's not a thing, what is?

    Not sure what this means. I go by the definition of current, which is the flow of charge. It's common to say that current flows but it's more correct to say that there is a current which is a flow of charge.

    When EM energy propagates in free space there is no force. When EM energy propagates along a wire there are forces but they are perpendicular to the motion of the charges so no work is done (no energy is lost/transformed.)

    Only when the energy does work on a charge does a force and a parallel motion exist. This doesn't happen in ideal wires which is why they don't heat up. It does happen in resistors where the energy become heat or in a motor where the energy becomes force x distance.
     
  10. Jul 20, 2011 #9

    Drakkith

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    No, that is not electrical energy. It is an electromagnetic field which has mass and can do work. How much work it can accomplish is measured by the term energy.


    That is what I'm talking about. Current is a flow of charges. Current itself cannot flow since it is describing something that IS flowing, the charges. Similarly, energy is the capability to do work. It is not flowing anywhere, as it cannot flow.

    Energy does not do work on a charge, forces do. Those forces are "created" for lack of a better word by particles and EM waves, not by energy.
     
  11. Jul 20, 2011 #10
    Here is some http://amasci.com/miscon/ener1.html"...

    Money quote:

     
    Last edited by a moderator: Apr 26, 2017
  12. Jul 20, 2011 #11

    Drakkith

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    I agree completely Evil Bunny.
     
  13. Jul 20, 2011 #12

    Dale

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    Hi Drakkith, you and I generally seem to agree on the endless debates about energy, but I think you may be taking this a bit too far here. Clearly there is a quantity called an "energy flux" and I am pretty sure that "flux" is a fancy word for flow. So I think talking about energy flowing is probably reasonable.

    In fact, I can think of energy flows, mass flows, charge flows. It seems to me that many conserved quantities can be considered to flow.
     
  14. Jul 20, 2011 #13

    Drakkith

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    Dale, I spent about 10 minutes writing and re-writing a response trying to explain my reasons, but I cannot seem to get it down correctly in a way that explains it well. It is very difficult to get my thoughts out of my head and down onto paper, text, or conversation correctly. So frustrating...

    I agree with you, I simply think it is inaccurate in a similar way as saying Current flows.
     
  15. Jul 20, 2011 #14

    Dale

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    Perhaps the confusion and frustration comes from a slightly faulty analogy? I would think that a better analogy would be between charge and energy, not between current and energy.

    After all, both charge and energy are conserved scalars in Newtonian mechanics, while current is a non conserved vector. And in relativity both charge and energy are the timelike components of a vector, while current is the spacelike part.
     
  16. Jul 20, 2011 #15

    Drakkith

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    Energy, Charge, and Current are all different things, so saying one is more or less faulty is kind of meaningless in my opinion. To me its as simple as this: Current is the flow of charges. Energy is the capability to do work. For whatever reason, energy apparently means more than that however. At least to most people. I don't know why. But I'm sure I'm wrong somehow.
     
  17. Jul 20, 2011 #16

    Dale

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    Agreed completely. But I don't think that the capacity to do work is permanently stuck in one location, which means it can move, which is all that is meant by "flow."
     
  18. Jul 20, 2011 #17

    Drakkith

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    I agree. Just when discussing something like the operation of a circuit, saying the energy flows here and there doesn't really tell me much.
     
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