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

Do electrons produce a constant magnetic field?

  1. Mar 6, 2015 #1
    I read somewhere that electrons produce constant magnetic field when moving with constant velocity. But I think it is not true.I thought a reason for it

    Imagine that an electron is stationary and I am moving with a constant velocity.
    In my respect electron is moving, so, it should produce magnetic field. But in ground frame electron should not.
    This is a contradiction as both frames of references are inertial.

    Hence electron should not produce magnetic field.


    Am I wrong or electron actually doesnt produce constant magnetic field?
     
    Last edited: Mar 6, 2015
  2. jcsd
  3. Mar 6, 2015 #2
    I saw somewhere that a moving (or was it accelerating?) electron emits light!
     
  4. Mar 6, 2015 #3

    Dale

    Staff: Mentor

    You are half right. The electron does indeed produce a magnetic field in your frame and does not produce a magnetic field in the ground frame. However, it is not a contradiction but simply a proof that the magnetic field is frame variant. No observed results will actually be contradictory.
     
  5. Mar 7, 2015 #4
    This seems very odd to me .
    This happens only with magnetic field?
     
  6. Mar 7, 2015 #5

    Drakkith

    User Avatar

    Staff: Mentor

    No, as velocity increases, the electric field is seen more and more as a magnetic field. At very high velocities nearly all of the interaction between the moving particle and an observer will be seen as a magnetic field, with little contribution from the electric field.
     
  7. Mar 7, 2015 #6

    Dale

    Staff: Mentor

    It is very odd, but it is the way the universe works. This is due to Relativity. And no, it is not only with the magnetic field. Time and space are relative. Energy and momentum are relative. Electric and magnetic fields are relative. Charge density and current density are relative. Frequency and wavenumbers are relative. And so forth.
     
  8. Mar 7, 2015 #7

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Isn't this the very question that led Einstein to the Special Theory of Relativity? And the question that the opening paragraph of the 1905 paper addresses?
     
  9. Mar 7, 2015 #8
    but i thought that two frames with different velocities are both inertial and are same. Something moving with 0 acceleration and not moving almost have no difference. So i got a little confusion
     
  10. Mar 7, 2015 #9

    Dale

    Staff: Mentor

    I don't know what you mean by "have a little difference" and "almost have no difference".
     
  11. Mar 7, 2015 #10

    Drakkith

    User Avatar

    Staff: Mentor

    That's the crux of the issue, the very point that we're trying to make. Motion is relative and the field as seen by the electron is purely electric, while the field seen by the observer is part electric and part magnetic because there is relative motion between the observer and the electron. It doesn't make sense because you haven't studied relativity.
     
  12. Mar 9, 2015 #11

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    A current-carrying wire produces a magnetic field because of the moving electrons. I understand that they do not move very fast (mm or cm/second). If I placed a compass next to a wire in a dc circiut to detect the magnetic field, could I move the compass until it registered no field in order to determine the speed of these electrons? I guess I could try it, but it's easier to ask!

    Edit: thinking a little deeper, would I now be registering the field of the moving protons?
     
  13. Mar 9, 2015 #12

    Drakkith

    User Avatar

    Staff: Mentor

    No, because drift speed isn't a measure of the actual electron speed. Electrons in a conductor normally move around very quickly and in random directions. If you add up the directions and speeds of all of these electrons you find that they average out to a net zero. When a voltage is applied the velocities no longer add up to zero. Instead, you find that there is a net difference between the charges moving in one direction and the opposite. That's the direction of current flow.

    The protons in a conductor aren't moving. They are locked into the lattice structure of the metal.
     
  14. Mar 9, 2015 #13

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    I know the protons are stationary with respect to the wire, but relative to an observer on my moving compass, which is moving at drift speed, the average electron speed is now 0 and the protons (in this frame of reference) are moving backwards. Hence, no magnetic field from the electrons, and an equal magnetic field from the protons which are opposite in charge and opposite in velocity (from the moving electrons in the wire's frame of reference) in this frame of reference.

    So I'm guessing I can't measure drift speed of a current with a compass.
     
  15. Mar 9, 2015 #14

    Drakkith

    User Avatar

    Staff: Mentor

    Hmm, now you've got me thinking. I'm not sure you would ever register zero magnetic field while current is flowing in a circuit. I just don't know enough to know for sure or explain it.
     
  16. Mar 9, 2015 #15

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    I don't think I could either. If I moved a compass along a wire that is NOT carrying current, the electrons will produce a magnetic field, and the protons will produce a magnetic field that will cancel it out. For example, the electrons produce a field of +1000 T and the protons -1000 T. Add some current, and the electrons Produce +2000 T and the protons -1000 T for a net of +1000 T. To a stationary observer, this would be electrons +1000, protons 0 = net of +1000. So my original thought experiment was to measure the drift speed with a compass, but now I realize a moving compass won't change anything, regardless of the speed. Too bad! I thought I just came up with a drift speed lab for my high school students.
     
  17. Mar 10, 2015 #16
    I think this video actually goes into your question halfway decently...

     
  18. Mar 10, 2015 #17

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    Thanks for the vid. I never knew that.
     
  19. Mar 10, 2015 #18

    Dale

    Staff: Mentor

    Your instinct is correct. B^2-E^2 is an invariant. So, since there is a frame where B is non zero and E is zero this invariant is positive in all frames. The only way for it to be positive is for B to be non zero. So B is non zero in all frames.
     
  20. Mar 13, 2015 #19
    Interesting video, with nice explanation.
    I don't see however how that explains why the magnetic field circulates around the current carrying wire. I thought if you placed a charge particle near a current carrying conductor, the magnetic field would just move it in a circle around the wire at a fixed distance.
    The way this video explains the magnet field, it would seem that the magnetic field would point out radially from the wire.
     
  21. Mar 13, 2015 #20

    Dale

    Staff: Mentor

    You are mixing up the magnetic force and the magnetic field. The two are perpendicular to each other.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Do electrons produce a constant magnetic field?
Loading...