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Electric charges experiment

  1. Nov 26, 2009 #1


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    I set up a simple experiment where a dc current carrying bare wire conductor runs parallel with a electrically charged plastic ruler. When no current flows in the wire and the ruler is charged with electrons (by rubbing it with a fur) the conductor will be attracted to the ruler. When dc current flows in the wire , no matter in what direction nothing its changing , the wire still remains attracted to the plastic ruler. When a permanent magnet is approach to the conductor while life the conductor will be attracted or repulsed depending on the current direction (obviously). I thought that the electrons on the ruler will be at half of the speed relative to those moving in the wire , so there will be two electron flows in opposite direction which will lead to a repulsive magnetic force between the ruler and the conductor! What do I'm missing (if I'm missing something)?

    (this is a double post because i don't really know where should belong ...sorry)
  2. jcsd
  3. Nov 26, 2009 #2
    The electrons on the ruler are stationary. There is no current in a plastic ruler-it is an insulator.
    Bob S
  4. Nov 26, 2009 #3


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    that's the point! If an electric field in one frame of reference is a magnetic force in another dynamic frame, than the electrons in the wire would be influenced by the magnetic field of the stationary electrons on the surface of the ruler, which are traveling at half of the speed relative to those in the wire...so the stationary electrons are seen as moving by the electrons in the conductor, so were is the repulsive magnetic force? The experiment is simple you can do it in 10 minutes...
  5. Nov 26, 2009 #4


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    Relative to the electrons in the wire, the ruler electrons would be moving at full speed, not half speed. Anyhow, there's no magnetic force because the wire is neutral overall.
  6. Nov 27, 2009 #5
    The attractive force between the ruler and conductor is electrostatic: the negative charge on the ruler induces a positive charge on the side of the conductor closest to it and a negative charge on the side furthest away from it. This happens because of Gauss's law and its application to perfect conductors. This force is not influenced by magnetic fields (since all the charges are stationary) or by currents in the wire (that electric field is perpendicular to the one caused by the static charge).

    Also, if you want to consider the electrons in the ruler moving at half the speed of those in the wire then you also have to work out the relative velocity of the permenant magnet with respect to this frame of reference.
  7. Nov 27, 2009 #6
    In our frame of reference, a current would be influenced by the charge on the ruler by using the first half of the Lorentz force equation:

    F = q(E + v x B) = qE + I x B

    which is just the force on a particle of charge q in an electrostatic field E.

    You can Lorentz-transform this into any other frame, including one where the charges in the wire are static.

    Bob S
  8. Nov 27, 2009 #7


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    The magnet is there just for fun it has noting to do with the experiment, was on my desk and i checked if the current really flows (i have some bad alligator clips that's it...i do have a ammeter...just because)
    The attraction to the charged plastic ruler of course is electrostatic,...
    And yes they move at the same speed relative to those in the copper wire...(my mistake...I'm not perfect...yet :)
    And yes the wire is neutral (over all) but so are two conductors carrying current and they still have a magnetic interaction between them...

    - What would have been your opinion if i would say that when the current flows, the conductor is repealed of the ruler?
    Last edited: Nov 27, 2009
  9. Nov 27, 2009 #8
    If the wire had capacitance C per unit length and voltage V with respect to another conductor, it would have charge Q=CV per unit length. In this case, this wire would either attract or repel the ruler.
    Bob S
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