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Lorentz force question

  1. Dec 1, 2009 #1
    The lorentz foce has been bothering me. Becouse it seems to sugest an abosolute velocity.


    Say you have tw parrellel wires, with equal current running in the same direction. According to Lorrentz force, they attract.

    Now, the two wires are plasma. The curents are running side by side, at equal velocity (equal current).

    Everthing is the same. The electrons should still attraced.

    A little wierd, but not too bad.

    NOW. Simply remove the plasma. Two streams of electrons, in space. The Lorentz force should still attracked, as the only thing that has changed between these experements is the medium. EXCEPT in space, there is no medium, which means that there is NOTHING to compare the speeds to. THe electrons are still traveling at the same velocity (which no longer makes sence) And they are still atrackting.

    See what I am getting at?

    Anyone know where my thinking is wrong?

    Thanks!
     
  2. jcsd
  3. Dec 1, 2009 #2

    CompuChip

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    You are talking about a force.
    Force induces acceleration of the wires.
    And special relativity doesn't work for accelerated frames.

    If you want, attach an accelerometer to some electrons in both wires, and measure it. However, this will still only give you information about the change in velocity of the wires, starting from some arbitrary reference. In other words, about the relative velocity of the wires.
     
  4. Dec 1, 2009 #3
    The Lorentz force is perpendicular to velocity, so in the case of wires, the lorentz force wont change the velocity that is generating the lorentz force.


    Where did special relitivity come into this?
     
  5. Dec 2, 2009 #4
    I'm not quite sure I fully understand your scenario, but even if we took it at face-value, it does not seem to imply absolute velocity. Under all of those circumstances (electrons in wire or in space), you only compare the relative velocities. You can't say that the electrons have an absolute speed in space any more than you can say they had absolute speeds when you did the experiment with wires on Earth.
     
  6. Dec 2, 2009 #5
    I guess what I am asking is

    in F=QVB. What is the "V" relitive too?

    It can't be the relitive speed between the two electrons. Becouse the lorentz force exists when electrons are traveling in the same direction, with the same velocity.

    It can't be the relitive speed of the medium, becouse the lorentz force works without a medium.

    so what else can V be relitive too?
     
  7. Dec 2, 2009 #6

    berkeman

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    This is actually a pretty interesting question. If an observer moves at the same constant velocity of a charged particle, he must not see (be able to measure) the B-field being generated by that particle? Hmmm...
     
  8. Dec 2, 2009 #7

    Dick

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    Plasmas are overall neutral. The 'current' is a difference in the velocities of the negative and positive components of your plasma. If you remove the positive component the situation is completely different. Now you also have an electrostatic force between the two streams. They will repel. And yes, if are travelling at the same speed as a moving charge you don't see any B field. The mix of E and B fields an observer sees generated by a charge is completely dependent on the motion of the observer.
     
    Last edited: Dec 2, 2009
  9. Dec 2, 2009 #8

    berkeman

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    Awesome. Thanks, Dick.
     
  10. Dec 2, 2009 #9
    How does that work in a wire??

    If you have two parrell wires, with equal current flowing in the same direction

    Do they not repel despite the charges having equivalent velocities?
     
  11. Dec 2, 2009 #10

    Dick

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    Wires are like plasma. You have a population of free electrons, but you have an equally large population of metal atoms that are missing an electron. Wires are overall neutral. The only difference with a plasma is that the atoms aren't mobile. Just the electrons.
     
  12. Dec 2, 2009 #11
    To add to Dick's remarks, there are a few things at work for beams of charged particles moving in parallel. Moving right along with the beams, the electrons will only experience electric repulsion as he says--just Coulomb repulsion.

    Changing to the frame of reference where the charged particles are in motion: There is the introduction of a magnet field, an increase in mass, and time dilation. These are all effects explained by relativity. All contribute to a slower percieved divergence of the constituent particles of the beams
     
  13. Dec 2, 2009 #12
    I believe that if you have two extremely relativistic electron beams side by side, there is no net force, because the attractive Lorentz force is exactly cancelled by the repulsive Coulomb force. If the beams are less than relativistic, the repulsive Coulomb force is dominant. This one of the problems in trying to combine low-velocity heavy-ion beams.
    Bob S
     
  14. Dec 2, 2009 #13

    Dick

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    Really? Parallel beams? Same velocity? In a vacuum? This is a pretty idealized question. What Lorentz force are you talking about? I'm kind of a believer that absolute velocity doesn't matter. I've had arguments with accelerator people about issues like this and they never really made their objections clear. Can you?
     
  15. Dec 2, 2009 #14

    berkeman

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    Actually, this brings up an interesting follow-up question. The electrons in parallel wires carring the same current have the same drift velocity in the same direction. But we still generally in EE problems calculate the B-field from one wire, and apply that B-field to the other wire to calculate the attractive force. But this must be wrong, if the electrons in one wire are moving with the same drift velocity as the other wire, they must not experience any measureable B-field? I can see how we can get a repulsive force for opposing currents in parallel wires, but is the attractive force zero for equal parallel wire currents?

    Dang, time for an experiment!
     
  16. Dec 2, 2009 #15

    Dick

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    Hi berkeman, there is no B field between the electrons in the two wires. There is a B field between the electrons in each wire and the stationary charges in the other wire. Being KI6EGL, I think you know what the result will be. Put away the experimental apparatus. It's RELATIVE velocity of ALL charges. Not just the electrons.
     
  17. Dec 2, 2009 #16
    When you say extremely, this is a little fuzzy. That is, how extreme?

    In the limit as v-->c, mass goes to infinity, so any finite applied force results in an acceleration that goes to zero.

    Other than that, I'm equally currious. In the limit, does the magnetic force equally cancel the electric force? It seems it should, but I couldn't demonstrate it.

    Dick, and Bob. I think were looking for this condition: F = 0 = Bv +E
     
  18. Dec 2, 2009 #17

    berkeman

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    Hah! I'm beginning to see that the simplistic way of understanding (and teaching and tutoring) this case needs a bit of modifying. This is great.

    But, now that I see that the electrons in one parallel wire don't experience any Lorentz force from the electrons in the other wire, it would seem that the force from the passing + atoms in my own wire would be much larger than the passing + atoms in the other wire. The Biot-Savart force from such nearby passing charges would be quite large! Is it not an issue because of perceived local charge neutrality? And if so, how is that different from the passing + charges in the other parallel wire? Thanks.
     
  19. Dec 2, 2009 #18

    Dick

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    Forces between charges in the same wire only create turbulence in wire they are in. Like, uh, eddy currents or something like that. That turbulence is uh, RESISTANCE, yeah, that's it. resistance. The charges are supposed to be confined to each wire. The idealization here is that we have a uniform AVERAGE current flow in both wires. Only the average counts in the long run. Variations are supposed to average out.
     
  20. Dec 2, 2009 #19

    berkeman

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    I was thinking along those lines, but I need to do some calcs to convince myself of this. Thanks, this has been a very enlightening thread for me.
     
  21. Dec 2, 2009 #20

    Dick

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    A privilege, berkeman.
     
  22. Dec 2, 2009 #21
    INTERESTING!!!

    I am having a TAD bit more difficult time understanding this.

    So, the force that causes two wires to attract (with equal currents)

    Is NOT the lorentz between the electrons of one wire on the electons on the other, but is the lorentz force between the electrons in one wire, and the stationary positive charges in the other?


    If I am following everything.

    WHAT if you have a plasma, that is spinning. (with a differnt radius)

    Since the charges are traviling with the same velocity, maby the lorentz force will have no effect?

    BUT since spinning is accelerating, they are not at the same velocity, so what effect does this have??

    Would this produce a magnitic field similar to a solenoid (stationary magnetic field) and the velocity concerning the lorentz force is compared to the center of the mass of all the plasma?
     
  23. Dec 3, 2009 #22

    Born2bwire

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    Close, but that is due to how we choose the reference frame. Note that berkeman has chosen the frame moving with the electrons and I think he has opposing currents. Let's look at it in some frames if the currents are running in the same direction.

    As I understand it, the movement of the wire is from the attraction of the currents in the wire on the positive lattice of the wire's atoms (and vice-versa, yay Newton). A wire is a quasi-neutral material. Well, neutral for most purposes but if you start adding static electric fields you could cause charge separation and stuff. But for the purposes here, the wires are neutral so there is no direct Coulombic force between the wires. However, the moving charges of the currents induce magnetic fields which will have a net Lorentz force on the moving charges in the other wire. This force displaces the currents, the electrons, which then drag the previously stationary ionic lattice of the wire with them. This is looking at the situation from the frame of the atoms in the wire's lattice.

    So if the currents are running in the same direction and we look at it from the frame of the electrons, then only the ions are moving which give rise to magnetic fields that interact. It would be different if we had the currents running in opposite directions. Here, one wire would have stationary electrons and moving ions and the other would have moving electrons and ions. Here it would be more complicated.

    If we were to look at it in the frame of the moving electrons, then the electrons would be stationary and the ions and atoms of the wire would be moving. In this case it would be magnetic fields from the wire's ions interacting with the other wire's ions (and then the wire's ions dragging the "stationary" currents with it). This would be true for current running in the same and opposite directions.

    But, special relativity allows us to look at this problem in a reference frame that is moving along the length of the wire differently too. In this frame there could be no Lorentz force from the magnetic fields, but we now have electric fields which will give rise to the same force that we would find in the other frames. This comes about by the length contraction, the contraction of the currents along a preferential direction makes the wire appear to be electrically charged. That is, if we are moving along the wire, the electrons move in one direction and the positive ions move in another direction (these are the atoms of the wire which now move since we are moving along the wire). Because the velocity addition in the Lorentz transformation is different for velocities moving with the frame compared to those moving against the frame, we have a different contraction for the electrons than the ions. This results in different charge densities over a given length between the electrons and ions and thus a net electrical charge which gives rise to a Lorentz force from an electric field.
     
  24. Dec 3, 2009 #23

    berkeman

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    Why would there be electric field forces? The two wires are electrically neutral in all reference frames.


    EDIT -- besides, the electron drift velocities in wires are sub-walking speed, nowhere near relativistic speeds.
     
  25. Dec 3, 2009 #24

    Born2bwire

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    This is done as an example problem in Griffiths, 12.3.1.

    He sets up the problem as follows, let us say that we are looking at a unit charge q that is moving with velocity u in the direction +x. At some distance away from from the charge is a wire that has electrons running in the -x direction and ions running in the +x direction both with speeds of v. This would correspond to the situation where we have a current running in the +x direction of 2*v*\lambda (\lambda is the charge densities of the ions and electrons) and a current element at some distance away running in the +x direction as well. If we are in the current element's frame of reference, we both see moving electrons and moving ions in the wire but in moving to this frame, we have to do Lorentz transformations. This is done first by adding the speeds that the ions and electrons move in the wire. Since they move in opposite directions, their transformed speeds are different. This means that the electrons will have a different length contraction than the ions, so the electrons per unit length is now unequal to the ions, and thus we now have a net electrical charge. Since the current element is at rest in this reference frame, there is no Lorentz force on it from the magnetic fields from the wire, the Lorentz force is due solely to the electric fields of the wire. This turns out to be the exact same force that we would predict for parallel wires if we extend the analysis by changing the current element to a line current.

    It does not matter whether or not the charges move at relativistic speeds, this result will come out the same. I just wanted to point out that the forces and mechanisms involved in this problem can depend greatly on your reference frame.

    EDIT: Griffiths also notes that a similar treatment can be found in Purcell.
     
  26. Dec 3, 2009 #25
    From Bob S
    I believe that if you have two extremely relativistic electron beams side by side, there is no net force, because the attractive Lorentz force is exactly cancelled by the repulsive Coulomb force. If the beams are less than relativistic, the repulsive Coulomb force is dominant. This one of the problems in trying to combine low-velocity heavy-ion beams.

    I attach a calculation showing that for two parallel equal-charge particle beams, the repulsive electric (Coulomb) force and attractive magnetic (Lorentz) force cancel as the particle velocity v=βc approaches c.
    Bob S
     

    Attached Files:

    Last edited: Dec 3, 2009
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