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Lenz's law

  1. Sep 20, 2008 #1

    Refer to example 6.2 shown in the picture, the magnetic flux = -1.2t and the deriative time of magnetic flux should be -1.2, thus, the flux is decreasing and the direction of the induced current should be reverse. but why it say the flux is increasing?
  2. jcsd
  3. Sep 20, 2008 #2


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    It only says that the magnitude of the flux is increasing.
  4. Sep 20, 2008 #3
    the direction flow of the current indicates that the induces B field oppose the increasing flux.... but the d (magnetic flux linkage) / dt should be less than 0, so the induces B field must point into the plane and the direction of the current should be flow from left to right, right?
  5. Sep 20, 2008 #4


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    Yes, correct.

    It is greater than zero because Lenz law is -d(phi)/dt and not just d(phi)/dt.

    No, as you said above, the current inducing induced must itself generate an opposing B-field to counteract increasing flux. Since flux is increasing, this means the resultant B-field must point upwards. And if it point upwards, it means the current must be anti-clockwise, not clockwise.
  6. Sep 20, 2008 #5

    I m really confuse. what about for below example?


    if you say that Lenz law is -d(phi)/dt, then for this case, -d(phi)/dt should be less than zero, the flux is decreasing and the current should flow couterclockwise.
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