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Bullet + wire

  1. May 3, 2006 #1
    question on electromagnetism, moving electrons with respect to other electrons causes a field to be felt right?

    maybe if i skip to my example itl make sence.....
    lets say you fire a bullet parallel to a wire. will the bullet feel a magnetic field, and if so, will it change its tragectory.

    what if the wire is solenoid-like? will that change anything?
     
  2. jcsd
  3. May 3, 2006 #2

    Danger

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    I'm not qualified to answer your question exactly, but I'd think that it would have to be one incredibly strong field to affect the trajectory of a bullet. In addition, most bullets are made of non-magnetic lead, with or without a copper jacket.
     
  4. May 3, 2006 #3

    "question on electromagnetism, moving electrons with respect to other electrons causes a field to be felt right?"

    No, not in the context of your question.
    For example, if your statement were true, than I could rotate an electrically neutral copper disk, say, clockwise, and rotate another electrically neutral copper disk counterclockwise, and the 2 disks spinning close to each other would generate a magnetic field.
    This does not happen.
     
  5. May 3, 2006 #4

    berkeman

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    A net movement of charge will create a magnetic field. A changing magnetic field with respect to time will generate an electric field (which can induce a net voltage in a conductor which loops around the changing magnetic flux).

    If you move electrons that are bound to atoms (like in the bullet example), you are not moving net charge.

    If you have a magnetic field like from a solenoid, and you fire a bullet that flies perpendicular to that magnetic field, then the eddy currents generated in the conductive bullet mass interacting with the magnetic field will have a net effect on the bullet's trajectory.
     
  6. May 4, 2006 #5

    Mk

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    If the wire had current running through it, that would generate an electric field, and the bullet would feel it, but it would not change its trajectory.
     
  7. May 5, 2006 #6

    Danger

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    My bad. I assumed that Skywolf meant an electrically energized wire such as a transmission line.
     
  8. May 6, 2006 #7
    so there is a difference between electrons moving because of a field, and electrons moving with respect to somewhere else,
    i mean, thats kinda where my question came from, if moving electrons caused a field, then why wouldnt moving past electrons do the same thing
     
  9. May 14, 2006 #8
    If you ignore Special Relativity for the moment, there is no difference between electrons moving past an object (whether due to a field or not, although you would have to add the effects of the field as well) and the object moving past the electrons. The problem is that's not the only thing happening in the bulet-wire scenario.

    Remember that the wire (and the bullet) are electrically neutral (at least the example doesn't tell us anything different). That means that, while the bullet is moving past the electrons in the wire (and thus would experience a magnetic field in the "up" direction if I'm doing my cross-products correctly) it is also moving past the protons in the wire at the same speed (and thus would experience a magnetic field in the "down" direction of the same magnitude). These two fields cancel out so the bullet experience no net field. Put another way, what matters is the velocity relative to a net charge. If there is no current in the wire, there is no net movement of charge relative to the bullet.

    Now, if you start running a current through the wire, the electrons begin moving relative to the protons. Therefore the two sets of charged particles have different relative velocities relative to the bullet. Therefore the opposing fields experienced by the bullet are no longer of equal magnitude. The fields therefore do not "cancel out" and the bullet experiences a net field (the direction and magnitude depending on the direction and magnitude of the current).
     
    Last edited: May 14, 2006
  10. May 14, 2006 #9

    rcgldr

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    What if the bullet had a static charge on it?
     
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