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B Current Flow Through a Common Part of a Circuit (batteries and light bulbs)

  1. May 16, 2018 #1
    Hello,
    I have a simple question regarding current flow through a common part of a circuit. I encountered this as I am reading "Code: The Hidden Language of Computer Hardware and Software" by Charles Petzold. The images I've attached display what I'm talking about. I'd like to know if there's a mistake in the book with the drawing depicting the electron flow in red when both switches are closed. I've included the original image as well as what I think it should be. I don't see how the book's depiction makes sense. Shouldn't it be that the electrons are flowing from the anode (where oxidation occurs) to the cathode (where reduction, or electron gain, occurs in the redox reaction). I don't see how the electrons are flowing to the positive terminal on the leftmost battery.
    Thanks!

    1.1.JPG
    2.2.JPG

    3.3.JPG

    I guess, while I'm at it, I'll also ask: is the term current flow incorrect/redundant?
    Also: Is there any real direction of the flow of electrons, or are they present through all parts of the wire at all times, and closing the switch activates this flow? (Sorry if this question doesn't make sense.)
     
    Last edited by a moderator: May 20, 2018
  2. jcsd
  3. May 16, 2018 #2

    BvU

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    Hello rugerts, :welcome:

    I agree with you.
    Again !
    There is an average velocity. They are present all over the wire.

    You can do some calculations to find the number of electrons in e.g. 1 kg of copper wire and the number of electrons moving in and out when the current is 1 A.
     
  4. May 16, 2018 #3
    You can think of the electrons as a large crowd of people occupying a tunnel. With no electrical power source they remain in place. When a power source as a battery is introduced they begin to move simultaneously, en masse. A battery's purpose is to provide a force to move the electrons and in doing so collect them at the anode and replace them at the cathode via a chemical process. This train moves remarkable slow i.e., a fraction of a mm/sec. It is the shear number of electrons that pass a given point that results in size of the current.
     
  5. May 16, 2018 #4

    Mister T

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    It should be charge flow, because current is, by definition, a flow of charge. It's very common to speak of current flow, despite the grammatical error. It's common usage.

    The electrons are already present and their motion is random. Close the switch and the electrons "drift" through the wire at a very leisurely pace, despite their otherwise random motion. Do a google search for "drift velocity".
     
  6. May 16, 2018 #5
    Thanks for you reply. Does that mean the electrons are always drifting from anode to cathode and the closing of a circuit organizes this? Also, can I take it that you agree with me on the original question, which involves the pictures?
     
  7. May 16, 2018 #6
    So, the electrons are always moving from anode to cathode, but the closing of a circuit just organizes this motion?
     
  8. May 16, 2018 #7
    Electrons being negatively charged move from the cathode(-) to the anode(+). Closing the switch introduces the force in the conductor that causes the electrons to move as a unit as a parade of soldiers in lockstep.
     
  9. May 16, 2018 #8

    jbriggs444

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    The electrons are normally moving around randomly, neither toward nor away from anode or cathode. The closing of the circuit provides a path for an average movement from cathode to anode. Due to a historical accident, the direction of the conventional (positive) current is taken to be opposite to the direction of the average movement of the (negatively charged) electrons.
     
  10. May 16, 2018 #9
    Wow, interesting. So, there’s a difference between electron movement and current flow convention? That doesn’t seem right. Here’s an article on voltaic cells (https://en.m.wikipedia.org/wiki/Anode) which I believe says electrons flow from anode (-) to cathode (+) due to oxidation and reduction. Is the confusion here? So, positive current convention is opposite of the actual movement of electrons?
     
  11. May 16, 2018 #10

    jbriggs444

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    There certainly is confusion.

    https://en.m.wikipedia.org/wiki/Cathode_ray

    "Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube."

    And yes, electrons flow in the opposite direction to the current that their net movement constitutes.
     
  12. May 16, 2018 #11
    Is this why everyone is saying the opposite of what I am saying? More specifically, everyone seems to be saying "from cathode to anode" whereas I'm saying from anode to cathode. You are all correctly talking about convention for current, and I am correctly talking about convention for electron movement?
     
  13. May 16, 2018 #12

    Nugatory

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    https://xkcd.com/567/ - posted here before but always worth reposting
     
  14. May 16, 2018 #13

    sophiecentaur

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    Haha yes - strictly - because current is charge flow. Too late to put that right now, though. We all know what we mean by Current Flow. :smile:
    It is not much use trying to discuss flowing electrons in circuit problems. We use Conventional Current and have done since way before anyone discovered the electron so, as above, why change? Nothing is 'wrong' and the Book seems to be using Conventional Current too; electrons are only mentioned once and that is once too much afaiac. Direction of Current Flow is assumed to be from + to - battery terminals. (Whatever name you choose to give them)
    The possible confusion about the circuit is that there are actually two totally independent circuits. You can re-draw it with the - terminal of the left hand battery connected directly to the right hand lead to the right hand bulb. That is then more of figure of eight - still keeping the function of the circuit exactly the same and showing you that you have two totally independent battery - switch - bulb circuits. It is often possible to re-draw circuits so make their function more obvious.
     
  15. May 16, 2018 #14
    Thanks for the reply! I think my problem with the author's image is that I believe there should be an additional coloring of the wire to be red for the rightmost lightbulb, as indicated by the edited image I provided. Did the author make a mistake in coloring? Or am I wrong?
     
  16. May 16, 2018 #15

    sophiecentaur

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    The thread has got diverted into electrons vs current and the names of the battery terminals - best to keep with current and just use + and - terminals imo.
    Current must flow through both batteries so your correctly added red line must carry current. Basically you have two batteries connected in series with two bulbs and there is no PD across the remaining black length. Another way of looking at it is as a balanced Bridge Circuit.
     
  17. May 17, 2018 #16
    Yes, the red indicates flow of electricity. Can it be said that the electrons flow from the left battery's negative terminal, through the left bulb, sweep around (moving to the right), through the right battery's positive terminal, through the battery itself, and through the right bulb, back to the left battery positive terminal, repeat? Please reference again the picture I've edited.
     
  18. May 18, 2018 #17

    Tom.G

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  19. May 20, 2018 #18

    NascentOxygen

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    Yes, you can trace that out as the complete path, and this confirms your original query concerning the need to color in red that short length of conductor joining the right bulb and right battery's negative terminal. Note that the situation described, where the [black] wire between left battery and right bulb is present but carries no current, holds true only while both lights are glowing (and, incidently, needs identical bulbs and identical batteries otherwise its current won't be exactly zero). In the case where only one light is glowing the wire between left battery and right bulb is required to carry that light's current.
     
  20. May 20, 2018 #19

    NascentOxygen

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    This discussion has been compressed to bring it back on track. Thanks to all contributors.

    Thread is CLOSED to further comment.
     
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