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Cancel magnetic field in one current carring wire

Discussion in 'Electronic Basics' started by Thinking123, Apr 28, 2005.

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  1. Thinking123

    Thinking123 Guest

    Can anyone suggest the best way to shape a single wire carring a
    current so that it's magnetic field cancels itself out? Completely if
    possible. I'm trying to optimize the idea for light weight and varying
    current values. All I can think of would be to bunch the wire into a
    tight zig-zag plane. (From:---------- To:||||||||) Would that work? Or
    would it produce the same field...and just be heavy? Any help would be
    great, thanks.
  2. The cancellation will be nearly complete if half of the
    wire runs back along the same path as the other half.
    Twist the halves together and it will be better yet. Put
    a hole down the middle of one half, enlarge it, and run
    the other half back thru the hole, (coaxially), and the
    cancellation is as perfect as the coaxiality.

    If the single wire has to carry current from one point
    to another in a different place, then you are chasing
    a rainbow. There will be a magnetic field created
    by that current, no matter what path it takes between
    the two separated points.
    It would produce very nearly the same field, and just
    dissipate more heat to carry the same current.
    You could get better help by stating more of your
    real problem and constraints.
  3. Thinking123

    Thinking123 Guest

    You're right, I meant to mention that it had to be between two points.
    Chasing a rainbow then. Are you sure? What if I wrapped it in a coil
    for half the distance, then wrapped it in the opposite direction for
    the other half? Like magnet/solinoid wraps. I just thought of it, but
    that would change the direction of the field and cancel out right? Or
    would that be the same as the zigzag? (This is for a math model that I
    gave up on a while ago when I realized that I would have to create and
    destroy electrons to have a single current between 2 points in a closed
    system. Now this is another idea how to make it realistic...unless it's
    impossible. Just trying to think and get knowledgable feedback. )
  4. The only way to cancel the magnetic field of a current is to have that
    current return by the same path. The best you can do is to use a
    coaxial wire, with the current going out in one conductor and coming
    back through the other. A twisted pair is pretty good. A twisted
    quad (two wires at the corners of the cross section carrying one way,
    the wires at the other two corners of the cross section carrying the
    other way), is better, etc.
  5. Well, I am sure that *if* you have a current not following
    its own return path back coaxially, *then* there will be a
    magetic field produced by that current.
    Almost, but not quite. And the not quite is equivalent
    to what you get if the current does no spiraling around
    and just travels in a straight path along the axis of what
    was the spiral.
    The same in the sense of no net improvement.
    Your zigzag is probably easiest to think about.
    Consider that each zig or zag can be broken
    into two vector components. One is in line
    with the net direction of the zig-zag, the other
    is transverse to it. The transverse components
    of the zigs cancel the transverse components of
    the zags, but the in line components all add in
    the same direction, as much as would the effect
    from a plain straight current flow.
    Now you're on to the practical issue here. In the
    world we inhabit, there is no such thing as a current
    not flowing in a loop in appreciable amounts. You
    can imagine single or small numbers of charges being
    made to movie that way, but either they have to get
    back to their source, (an ever mounting positive or
    negative) region, or you have to have an unreal device
    to keep moving them indefinitely.
  6. Thinking123

    Thinking123 Guest

    Alright, if you're still following this, maybe you can think of a way
    to make this work: Just think of an x-y plane where the magnetic field
    is in the z direction. There are two battteries, DC generators,
    whatever at two points. Say (0,0) and (10,0). If two wires are
    connected between the two sources, there would be a current in both
    wires. Both would generate a magnetic field and cancel out. Can you
    think of a way to shape one of the wires so that it would not effect
    the forces on the whole system? So that it would not produce a net
    force in any direction and only the one wire (along the x-axis) would
    be in the math model? (eg. I think a tall triangle would make the
    effects in the y direction negligable, but then it would produce a
    large effect in the +- x direction...) Any ideas? Impossible?
  7. They would cancel out, if the current flows were
    coaxial, at points outside the outer conductor.
    If the currents are not coaxial, there will be a
    net magnetic field at most points. There could
    be some cancellation, along some surfaces or
    lines, depending on the flow geometry.
    Your question is too vague for me to understand.
    Only moving charges are subject to a force due
    to a DC magnetic field. And the closed system
    does not exert net forces upon itself, (at least not
    in the DC case. If it emits photons, it could.)
    Sorry, I don't know at this point.
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