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How to talk about the direction of the flow of charge

  1. Jul 29, 2012 #1
    Hi all,

    I don't know much about physics, but I understand that when you turn on a light switch, electric charge flows to the lightbulb because electrons in the wire bounce into each other (that's right, isnt' it?). Unfortunately, the electrons are negatively charged, but Ben Franklin, who named the charges 'negative' and 'positve', falsely believed that the charge carriers were positively charged, and his convention of calling the direction of the current the direction of movement of positive charge has stuck. So we have to say that the electrical current flows from the lightbulb to the source of the electricity.

    If that's right, and I want to talk about the physical quantity that travels from the source of the electricity to the lightbulb, I assume that I should just say that electric charge travels from the source of electricity to the lightbulb. Is that the description of the situation that is simultaneously most physically correct and most intuitive?

    Thank you very much,
    Howard
     
  2. jcsd
  3. Jul 29, 2012 #2

    CWatters

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    You are slightly confused there. Firstly electrons move remarkably slowly..

    http://c2.com/cgi/wiki?SpeedOfElectrons

    Imagine them like a long queue of people. If everyone takes one step forward, a space appears at one end and a new person appears at the other - so it appears as if a person has travelled from one end of the queue to the other very quickly but in reality they only moved one place. Turning on a light is like giving the last person in the queue a shove. If each person bumps the one in front then the shove propagates down the queue quickly but the queue hardly moves.

    Positive current is generally considered to flow from positive to negative. However electrons actually flow from negative to positive. This is nothing to do with the source and the light bulb.

    Electricity doesn't travel from the source to the lightbulb and stop there. That would imply electricity somehow acumulated at the light bulb (or the source). Electric current allways flows around a closed circuit. That's why the light bulb needs two wires.
     
  4. Jul 29, 2012 #3

    Dale

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    I would say "electric current travels through the lightbulb" rather than "electric charge travels to the lightbulb". The lightbulb does not accumulate charge and become charged itself, and to me, that is what is implied by saying that charge travels to the bulb.
     
  5. Jul 29, 2012 #4
    Hi,

    Thanks very much. If I'm in a context where I just want to indicate the direction in space in which charge first begins to propogate, could I say something like 'Turning on the switch initially causes charge to flow from the wall socket in the direction of the lightbulb' or 'When charge first begins to flow, charge flows from the wall socket in the direction of the lightbulb'?

    Thanks again!
    Howard
     
  6. Jul 29, 2012 #5

    HallsofIvy

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    If the light bulb is attached to a battery, as in a flash light, you have "DC", direct current- the electrons go from the negative pole, through the bulb, causing, in a sense, "friction" to heat the filament, to the postive pole of the battery. But the light bulbs in your house are conect to "AC", alternating current- electrons surge first one way, then back, with no individual electrons going very far.
     
  7. Jul 29, 2012 #6
    Thanks again. If there is a wire connecting the source of electricity to the bulb, and I'm trying to describe what happens between (a) the switch first being switched on, and (b) the first moment at which the bulb lights, then whether I'm in an A/C or a D/C situation, is it right to say that '(Negative) charge first flows from the socket down the wire' or '(Negative) charge first propogates from the socket down the wire'?

    Thanks!
    Howard
     
  8. Jul 29, 2012 #7

    Dale

    Staff: Mentor

    Neither way is correct. There are always two wires, one between one side of the bulb and the positive (hot) terminal and the other between the other side of the bulb and the negative (neutral) terminal. When you close the switch current begins flowing through both wires and the bulb in a loop which is called a circuit.

    Also, for all practical purposes the current begins flowing everywhere in the circuit instantaneously, but if you absolutely must speak of something propagating then what propagates is an electromagnetic wave and it propagates from the switch along both wires.
     
  9. Jul 29, 2012 #8
    I see. That's extremely helpful. Would a correct description be 'turning on the switch causes current to flow BETWEEEN the source of electricity and lightbulb?'

    Is the following possible? There is a switch that closes a circuit. The closing of that circuit then has two effects: it closes a second circuit, which lights one light bulb, and it closes a third circuit, which lights another lightbulb.

    Thanks again!
    Howard
     
  10. Jul 29, 2012 #9
    Closing one switch may allow two relay switches to flip, causing the effect you just described.
     
  11. Jul 30, 2012 #10

    krd

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    This is something that has confused me for years - the conventional explanation is that electrons flow.......But do the electrons ever actually travel anywhere?


    My current understanding for say a light bulb with AC. The tungsten is like a football stadium, and the electrons are like the people in the crowd, when the voltage is going one way, the crowd of electrons stand up and make a Mexican wave, when the voltage reverses the crowd make a Mexican wave in the opposite direction - this makes the stadium very hot, and light and heat is emitted.


    I think the "flow" idea is to give electricians the idea they're installing some kind of plumbing.

    It's my opinion, that if the current was strong enough to cause an actual flow of electrons, the conductor would liquefy.
     
  12. Jul 30, 2012 #11

    sophiecentaur

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    Don't forget that there have to be two wires involved. The total current flowing in one wire must be the same as the total current flowing in the other - but in the other direction. So the NET Charge flow from battery to bulb is Zero in the case of a simple resistive circuit.
    You can't ever have a steady current flowing onto or from an object. That would involve the object acquiring an ever increasing charge and, hence, an ever increasing potential. In practice, you can put a small excess positive or negative onto an object but, compared with the values of current that flow in normal circuits, it is tiny: for instance, the Electrical Capacitance of a human is around 100picoFarads (100pF). If 1A flowed for just 1s (just one Coulomb) onto you , you would be charged to a voltage of 10GV.
    Capacitors are specifically designed to have a high capacity between two plates and when you charge them, the same current flows into one terminal as flows out of the other - so there is still no NET charge - just a big difference in charge on each plate.

    The issue of charge flow in metals consisting of negatively charged electrons in the opposite direction is non-negotiable and it is also of no consequence. It affects nothing and you just have to 'get over it'. Faraday (or whoever), didn't get anything 'wrong'; he just assigned an arbitrary sign to his charges. If that were the worst intellectual hoop that electrical theory involved jumping through then life would, indeed, be very easy.
     
  13. Jul 30, 2012 #12

    CWatters

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    It's better to think of electricity flowing from the source THROUGH the lightbulb and back to the source. However people sometines use "between" or "to" the light bulb to mean the same thing.

    Yes. Almost anything is possible with electronics. Years ago you would do this with Relays.

    However I suspect your question has another question hidden in it.... There must be some way for energy/power to get to the second and third circuits. You can't make a circuit with just a switch and a bulb! However a switch can route power from the first circuit to the second etc.
     
  14. Jul 30, 2012 #13
    Electrons do flow, but very slowly. Only the most easily excitable electrons get knocked out and flow. However, in AC, since the direction is changing so quickly, the electrons won't get very far.
     
  15. Jul 30, 2012 #14

    Dale

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    Yes, that is correct. Just remember that there are two wires running between the source and the bulb. Current flows between the source and the bulb on both wires in opposite directions, forming a closed circuit.

    EDIT: I see sophiecentaur and CWatters double-scooped me :redface:. Oh well, at least you are getting consistent feedback.

    Certainly.

    EDIT: And Jakeus314 scooped me on this one too!
     
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