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Direction of Induced I in a loop of conductor?

Discussion in 'Electronic Design' started by [email protected], May 26, 2007.

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  1. Guest

    Suppose a loop of conductor is intitially carrying no current and is
    placed in a changing magnetic field for time T, and the induced
    current is allowed to go back to zero. If this is repeated with the
    magnetic field changing in the same way, will the induced I circulate
    clockwise 50% of the time, anti-clockwise 50%?
     
  2. The current will not go back to zero unless the magnetic field
    enclosed by the loop stops changing. The current will always be
    proportional to the rate of change of the enclosed magnetic flux. A
    change in flux is implied by moving the loop into or out of a static
    magnetic field, or by changing the area of the loop, or by the
    intensity of the magnetic field changing. An increase in N-S magnetic
    flux is equivalent to a decrease in S-N magnetic flux, etc. The
    current's direction will be such that the current's induced magnetic
    field will tend to maintain the total flux through the loop constant.
    So, the direct answer to your question is, the magnetic field must
    stop changing for the current to fall to zero. If the field then
    starts increasing from its steady level in the same direction, the
    current will resume in the same direction.
    If the magnetic field was increasing, and then was allowed to decrease
    to zero, the current would reverse, then fall to zero. Then,
    increasing the magnetic field in the original direction would cause
    the current to rise in the original direction.
     
  3. Guest

    No. The direction of the current flow is totally predictable.

    http://en.wikipedia.org/wiki/Right_hand_grip_rule
     
  4. Guest

    Thankyou for taking the time to post me a link, even if it isn't
    relevant ;) The direction is given by Lenz's Law which I completely
    forgot about. Doh!
     
  5. John Larkin

    John Larkin Guest

    It depends on whether the loop has resistance or if it's a
    superconductor. If it has resistance and the loop's L/R time constant
    is fast compared to the rate of change of the external field, the loop
    current will be proportional to the rate of change of the field.

    If there's no resistance in the loop, the current will be proportional
    to the instantaneous value of the appplied field, and follow its sign.

    In between, it's in between.

    John
     
  6. Fred Bloggs

    Fred Bloggs Guest

  7. Guest

    The right hand grip rule explicitly gives the the direction of the
    current induced in a loop, or a stack of loops aka a solenoid. Lenz's
    Law is much less immediately comprehensible.

    http://en.wikipedia.org/wiki/Lenz's_law

    But I suppose relevance is in the eye of the beholder.
     
  8. To be precise, a changing magnetic field induces an emf in a conductor.
    The resulting current flow depends on that plus the impedance in the
    circuit.
     
  9. Jasen

    Jasen Guest

    probably not.

    Bye.
    Jasen
     
  10. John Larkin

    John Larkin Guest

    In the case of a ring exposed to an external magnetic field, the
    current in the ring generates a local field that fights the external
    field, so the emf is a complex function of the result. In the case of
    a superconductive loop, the induced emf is obviously zero, the current
    is finite, and the field created by the ring exactly cancels the
    applied field. And "changing" no longer applies: the current is
    determined by the static applied field, with some constant of
    integration lurking maybe. In other words, the L/R time constant of
    the ring is infinite.

    John
     
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