Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electron Drift Velocity

  1. Feb 6, 2009 #1
    The electron drift velocity in a wire is about 0.1 mm/s.
    http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/DC-Current/Current.html [Broken]

    If a coil were constructed from a helical nonconducting tube and filled with mercury such that if a current was passed through the mercury, a magnetic field would be detected. If then the mercury were caused to flow through the tube, would there be a measurable change in the magnetic field?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 8, 2009 #2


    User Avatar

    Is this a correct restatement of your question?

    "Can I apply a velocity vector to a material that cancels out the average velocity of charge in that material thus reducing the current (change in charge vs time) past a point to zero?"

    If true, a consequence of no current would be no magnetic field due Ampere's law.
  4. Feb 8, 2009 #3
    Yes, that's a fair restatement.

    If the speed and direction of mercury flow were equal and opposite to electron drift, wouldn't the resistance of the mercury increase dramatically?

    Actually, I was thinking more of causing the mercury to flow in the same direction as electron drift, for instance, would a flow at 1 cm/s cause a 100 fold increase in the magnetic field?

    After further thought, it seems that the electrons would still enter and leave the mercury at the same rate so ultimately the magnetic field would be the same. There may be a short time during which the field may increase due to electrons still in transit from the cathode to the anode.

    Does this make any sense?
  5. Feb 8, 2009 #4


    User Avatar
    Science Advisor

    Isn't it current and not electron flow that causes a magnetic field?
  6. Feb 8, 2009 #5


    User Avatar

    Staff: Mentor

    But electrical current is the net flow of electrons, usually in the conduction band of a metal or other conductor. That's what makes skeptic's question so vexing.....
  7. Feb 8, 2009 #6
    lemme see if i understand this. is the implication that i can build a coil out of a length of hose, then pour mercury into a funnel at one end. and then that a wire coil next to this hose coil would register an EMF as gravity pulled the mercury through the hose? wouldn't water also work? seems like the answer is no, otherwise this would be a common practice by now since it would be a cheap generator with no moving parts wherever one had a water source at an elevation.

    i guess either you get a back emf through the fluid with no voltage to oppose it, or there's some electron spin or other physics i don't understand.
  8. Feb 8, 2009 #7
    Proton soup don't forget the protons!) It doesn't work because the current is the net flow of charge and the mercury is made of roughly equal number of protons and electrons. For an electrically neutral liquid, only when an electric field is applied is there a net increase in the amount of charge moved through a cross section. There is probably some second order affect that might be measured such as an increase in resistance due to higher temperature from the movement of the liquid.

    If the liquid is ionic you could get a current flow from the mechanical flow but the charge density would be lower than the charge density in a metal so you would need to move the fluid pretty fast to get a large current.
  9. Feb 9, 2009 #8
    yeah, that makes sense. :redface: i don't i've ever seen a problem that considered induced currents from proton flow, tho.
  10. Feb 9, 2009 #9
    I've never used a proton current in a circuit either. Of course we all use ion flows, with charge due to protons, in batteries and gas tubes.

    I don't know much about them but proton exchange membrane fuel cells separate the proton current and electron current. Although the electron current is the one passed through the external circuit it wouldn't work without the proton current in the fuel cell.
  11. Feb 9, 2009 #10
    I think I understand this better now. As we cause the mercury to flow, not only is the rate of flow of the electrons is increased but also the rate of flow of the positive ions which cancels out any increase in magnetic field.
  12. Feb 9, 2009 #11


    User Avatar
    Science Advisor

    OK, now I'm confused. I thought the net flow of electrons was electron drift (very slow, like the second hand on a clock), and was a seperate thing from current, which is the net flow of electron charges, and always travels at the speed of light for a given medium.
  13. Feb 9, 2009 #12
    Yes it is confusing for several reasons. The first is that conventional current flow is from positive to negative even though the electrons which make up the current, flow from negative to positive. This is due to an error made by Ben Franklin. It's never been corrected because it really doesn't make any difference.

    The second reason is that even though the electrons are traveling slowly, the signal travels at nearly the speed of light. Think of it as a tube filled with marbles. You push a marble in on one end and another one pops out at the other almost instantaneously.
  14. Feb 10, 2009 #13


    User Avatar

    Darn. I typed a long reply and got logged out. Basically I think it will boil down to a question of inertial reference frames.

    Here is the reader's digest version. Think of the wire in the link you provided. Imagine a guy walking along the wire with velocity exactly equal to the drift velocity. He sees no net change in charge past him thus no current thus no magnetic field.

    So if you could move the structure such that it moved in such a way to exactly cancel the net change in charge relative to you, ya you would see no magnetic field. But someone else, if they moved (or stayed put) in a different way would still see one.

    I think one could probably demonstrate this with a hall effect probe. There should be a constant voltage observed when it surrounds a wire with a constant current. Now move the probe back and forth along the wire. A change in voltage should be observed as the change in charge relative to the probe changed. Anyway, I think it should work like that. I might even try it tomorrow.
  15. Dec 3, 2009 #14
    I think you all are making this more difficult than it has to be. Whenever an electrically charged particle moves, it creates a magnetic field. When electrons move through a wire (mercury in this case) from the negative end to the positive end, it creates a magnetic field. If you took that mercury with electrons moving through it and moved it in the direction of electron flow, the velocity of the electrons in space would increase as would the strength of the magnetic field produced by them. However, the protons in the mercury nuclei would also now be moving in the same direction at a speed equal to the speed increase of the electrons. Since the protons' charge is opposite that of the electrons, this would induce a magnetic field in the opposite direction of the field induced by the electrons and equal in strength to the increase in strength of the electron's field. The net magnetic field would not become stronger or weaker. Also, you should consider the electrons that previously weren't moving because they weren't free or valence electrons. If you consider those, the net field still would not change because the number of protons is equal to the total number of electrons in a pure substance like mercury.

    I hope this answers your question clearly.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook