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How does current flow?

  1. Apr 5, 2013 #1
    In a simple circuit where i have a bulb connected to a wire that's connected back to the bulb, how will the electrons flow? I'm thinking it would look like this http://postimg.org/image/6jd2dg2md/full/ where as one electron comes out of the negative end, another electron goes in the positive end. Is this the correct way to think of it?

    What about flowing through insulators? Would it be the same thing just that the current is a lot smaller because of the large resistance?

    Thanks for the help :smile:
     
    Last edited: Apr 5, 2013
  2. jcsd
  3. Apr 5, 2013 #2
    Electron always flows from lower potential to the higher potential. A source, a battery, has two terminal. one has lower potential and the another has higher potential thus giving the total potential difference written on the battery.
    potential difference V=V2-V1
    where V2 is heigher potential
    and V2 is lower potential
    This causes the electron to flow from the terminal of lower potential to the terminal of higher potential. The flow of electron turns out to be flow of current but in the opposite direction as electron has negative charge. Instead, if we have a system in which proton is free to move, then we would find the direction of the flow of current would be same as the direction of the flow of proton.
    Here one thing must be remembered that the flow of proton is from hiegher potential to the lower potential.
    From all above you can find that current always flow from the higher potential to the lower potental whether the cause of current is flow of electron or proton.
    It is like you have a flow of water in a pipe and the direction of the flow of water current would always be in the direction of higher presser to lower pressure.
     
  4. Apr 5, 2013 #3
    Hi thanks for the reply but I'm not asking about why current flow but how does it flow. Could you check the first post and see if I'm right?

    Thanks :smile:
     
  5. Apr 5, 2013 #4
    hmm... it is very clever way of asking... :) anyway...then the answer is very simple... current is nothing more than the flow of charges. if one chrge is flowing in any medium, the current is flowing as well. If you can imagine the flow of a charge, then you have your answer how the current is flowing. More the speed of the charges more the current ... for your visulisation you may play this simulation.. it is a java running program..
    http://phet.colorado.edu/en/simulation/battery-resistor-circuit
    I think it may work.. :)
     
    Last edited: Apr 5, 2013
  6. Apr 5, 2013 #5

    BobG

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    Yes, the electrons go from the negative terminal to the positive terminal.

    However, when you're measuring current, you're interested in the flow of energy and the energy flows from the positive terminal to the negative terminal.

    It works kind of like a bunch of students sitting in a row of seats. Kid #1 leaves seat #1, the energy has moved to seat #1. Kid #2 moves from seat #2 to seat #1, the energy moves from seat #1 to seat #2. And so on. And at the end of this, each kid (each electron) has moved one slot forward, while the empty seat (the energy) has moved all the way from seat #1 to the last seat.

    As a result, electricity (the energy) moves very fast, even though the actual electrons are moving very slow. (The actual speed of electricity depends on the components in the circuit, etc, but it's much, much slower than the speed of light, just to dispel another myth - which makes sense, since the speed of the energy still depends on the speed of the electrons to a certain extent, even though you're tracking the holes left by the electrons rather than the motion of the electrons themselves.)
     
  7. Apr 5, 2013 #6
    I think sgstudent is asking how the electrons move inside a conductor, not asking about conventional current flow vs electron flow, or energy flow.

    Conductors, like everything else, are made of atoms. atoms have a positively charged nucleus surrounded by negatively charged electrons. In a conductor the outer shell of electrons surrounding the nucleus can pass from one atom to another. These electrons are called valence electrons, the more valence electrons a certain material has, the better it is at conducting electricity. Remember that, a conductor is already full of electrons, the battery isn't spitting electrons out into it and then sucking them back up. The battery "pumps" the electrons in the conductor around the circuit.

    These electrons do move very slowly, this speed is called drift velocity. The energy however moves much quicker. There are lots of analogy's to explain this, a classic one is marbles in a tube: imagine marbles lined up next to each other in a tube, push the first one and the last one seems to instantly move. The marbles may be moving slow but the energy has traveled fast. (not instantly though. It may appear that the last marble moved instantly but the energy actually traveled at the speed of sound for this system).
    I'm not sure how valid this analogy actually is though. I don't think the energy through a circuit travels at the speed of sound. Here's how I think of it:
    When an electron moves, the electric field it creates moves, causing there to be a net force on the otherwise equilibrium system of electrons. This causes these electrons to move and therefore their electric fields move. The electric field redistribution can travel very quickly around a circuit.
    The electric field at some point in the circuit, multiplied by the charge at that point in the circuit, multiplied by the distance moved by that charge due to the electric field, equals the work done by the electric field on that charge. You can see if no charges are moving, no work is being done. You can also see that the electric field redistribution causing the charges to be moved could cause work to be done far away from the original electric field redistribution very quickly. This is why energy flows around a circuit faster than the electrons do.

    But to simply answer your question "how does current flow?": charges (electrons) inside conductors are pumped around the circuit because of a potential difference. the electrons can and do pass through the battery(in the form of anions).
     
    Last edited: Apr 5, 2013
  8. Apr 5, 2013 #7
    Thanks for describing this beautiful way of demonstration of the flow of electron and the energy as well... :)
     
  9. Apr 5, 2013 #8
    This is so important. The drift speed of the electrons is very slow. Like Bobg said, it depends on the size of the wire and other factors. The energy is transferred at the speed of light. Its like a pulley system, the instant you turn one end, so does the other end. (well, at the speed of light, not instantaneously)
     
  10. Apr 5, 2013 #9
    its also like opening a tap... the water comes out immediately (speed of light???)
    because the water is already in the pipes, it has not been delivered instantaneously from some reservoir.
     
  11. Apr 5, 2013 #10

    BobG

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    I guess adding a statement to dispel the myth that electricity travels at the speed of light was in vain. :frown:

    Or at least consider that the speed of light depends on the medium it's traveling through and that different frequencies of light travel at different speeds through the same medium (hence light being refracted into a rainbow). The speed of electricity will always be less than the speed of light through a vacuum (what we normally use for the speed of light).
     
    Last edited: Apr 5, 2013
  12. Apr 5, 2013 #11
    Well yes but the speed of electricity is still almoust c ofcouse depending on the circumstances , I think the fact why we can't use marbles and water in pipes is that electricity is actually carried by the em force which is mediated by photons which travel at c and so when you apply a charge at one end of a wire the other one is at the same potential almoust instantly becauce the first excited electron made a electric field which made a magnetic field and both of them together make the em field which makes photons so a photon now goes to excite another electron before the actual drift velocity has ever got near that electron so now you have electrons in a wire that move very very slow and yet electricity at the other end which is there almoust instantly.

    I mean we don't wait minutes after we turn lights on for them to actually start to "shine"
    unless the bulb is not burnt out and the being that turned the lights on is a blonde ...
     
  13. Apr 5, 2013 #12
    The battery creates an electric field inside the wire. That field pushes/pulls all the electrons simultanously. So the battery is really transferring energy to all electrons at the same time. The electrons don't have to push each other.

    That simulation has an error in it. It suggests that the drift velocity of electrons is higher at the positive terminal and lower at the negative one. As long as the wire has the same diameter, is made from the same material and the current is the same, the drift velocity of the electrons will also be the same.

    You can not assign a direction to the flow of energy here. All you can say is that energy moves from the battery to the resistor but not which way around it flows through the wire.

    Wait - are you suggesting here that electrical energy is transferred much slower than the speed of light? The speed at which the energy is being transferred depends on the wave propagation speed
    http://en.wikipedia.org/wiki/Wave_propagation_speed
    That is, it depends on the speed with which a change in voltage moves through a wire. And that can be more than 99% of c.
     
  14. Apr 5, 2013 #13
    "sense, since the speed of the energy still depends on the speed of the electrons to a certain extent, even though you're tracking the holes left by the electrons rather than the motion of the electrons themselves.)"

    What do you mean by 'the holes left by electrons'.....I do not think this is part of the explanation of conduction in metals.
    I know that 'holes' are essential to explaining conduction in semiconductors and I know that +ve ions are part of the explanation of conduction in liquids, but 'holes' in metals?......please enlighten me.
     
    Last edited: Apr 5, 2013
  15. Apr 5, 2013 #14
    DrZoidberg this is something I have wondered about for a while now. If the drift velocity doesn't change, how can there be a potential difference across a resistor?

    If (in series) the current is the same on both sides of the resistor, and the drift velocity is the same, then how can there be a different electric field at each side of the resistor?

    There is definitely a different electric field at each side of a resistor, because how else will there be a different electric potential at each side of the resistor. And if the charge is exactly the same on both sides what causes this change in electric field.

    Also, surely the electrons inside the resistor are being slowed down due to heating. (there kinectic energy's being transferred to the resistor (ohmic heating), so the drift velocity will slow down.

    I think this may be something to do with waves. The wave front of the displacement wave being reflected at the impedance mis-match, and causing slowing on both sides of the resistor. Am I correct?
     
  16. Apr 5, 2013 #15
    The field inside the resistor is different from the one in the wire, but the field inside the wire is the same on both sides of the resistor. Potential difference is not a measure of the strength of the electric field. It is the integral of the electric field over a distance. Inside the resisitor the field is stronger because the battery is not the only field source. There is also static charge that accumulated on the surface of the wire that produces a field of it's own. At the ends of the resistor you have a "pile up" of charge. The field created by that charge strengthens the field inside the resistor but weakens it in the wire. The integral of the e. field over the entire circuit stays the same of course and is equal to the P.D. of the battery.
    Inside the resistor the drift velocity may be different but after the electrons leave the resistor their velocity goes back to their previous value. The e. field is constantly transferring energy to the electrons and the electrons are constantly tranferring energy to the atoms around them. However their kinetic energy at any point in time is virtually zero. Because they move very slowly and are very light. It's like pulling a heavy object with a rope. The rope never has much kinetic energy.
    Waves do not occur in a static situation i.e. when the current is constant. So no waves are being reflected.
     
    Last edited: Apr 5, 2013
  17. Apr 5, 2013 #16

    BobG

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    Figuratively speaking. When the conductor loses an electron, it leaves a 'hole' that needs to be filled by stealing an electron from its neighbor. Probably the more accurate description is it becomes positively charged until it steals an electron from its neighbor, leaving its neighbor positively charged, etc.
     
  18. Apr 5, 2013 #17
    Take a look at Ohm's law.
    http://en.wikipedia.org/wiki/Ohm's_law
    The generalized version of Ohm's law is J=σE
    This means the current density inside of metal is ALWAYS equal to the field strength times the conductivity.
    The other way around that means if the current density and the conductivity are both the same, the field strength MUST be the same. So, what I said was absolutely correct. The field in the wire is the same on both sides of the resistor. The resistor leads to a reduction of the field inside the wire because of the "pile up" of charges. So it is not useless.

    I'm hoping you are talking about virtual photons here. But even then your theory sounds rather weird.
     
    Last edited: Apr 5, 2013
  19. Apr 5, 2013 #18

    nsaspook

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    'Energy' flows from both terminals to the load. The direction of charge movement (current) can be either way or both ways at the same time (battery).
     
  20. Apr 5, 2013 #19
    Hi sorry for the late reply, I went to bed :)

    The marbles analogy is great but I would like to add on on this analogy. If there were 3 marbles touching each other and I push one, all 3 would move the same distance right? So is this the same for the battery and wire circuit whereby the electrons are the marbles and the push is the potential difference? So as one charge exits the negative side of the battery at the same time another charge enters the positive side instantaneously? If so then current is Q/t so how should be visualize this? Would it be like fixing a point and counting the number of marbles that pass that point per unit time?

    So increasing resistance would only decrease the current. So can I think of it like this: 2 toy tunnel filled with marbles (like in the analogy) one has a larger diameter than the other. As a for the smaller tunnel less marbles can be lined up, so less charge can flow so the current is smaller?

    Thanks so much for the help :)
     
  21. Apr 6, 2013 #20
    Figuratively speaking. When the conductor loses an electron, it leaves a 'hole' that needs to be filled by stealing an electron from its neighbor. Probably the more accurate description is it becomes positively charged until it steals an electron from its neighbor, leaving its neighbor positively charged, etc.

    Physically speaking, this is completely wrong. This is not the accepted explanation for current flow through a metal.
    I suggest you check the text book explanation.
     
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