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Lenz's Law and Electromagnetic Induction

  1. Mar 18, 2007 #1
    For Lenz’s Law, most examples involve the north pole of a bar magnet passing through first in a wire coil, or solenoid.

    If the bar magnet’s north pole moves toward the loop or solenoid, the magnet’s magnetic field lines move from the south pole to north pole? The current in solenoid is then directed to the right, or the induced current moves from left to right in the coil. When the magnet moves away from the solenoid, the induced current will then be directed to the left or move from right to left.

    However, if the magnet’s south pole passes through the solenoid first (in which the magnetic field lines go from the south to north pole), will the induced current’s direction be the same? When the magnet’s south pole is directed toward the solenoid, the induced current will move to the right, or from left to right? When the magnet’s south pole moves away from the solenoid, the induced current will move to the left or from right to left in the coil?



    Thanks.
     
  2. jcsd
  3. Mar 18, 2007 #2

    Doc Al

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    Staff: Mentor

    First things first: The magnetic field "lines" of a bar magnetic extend out from the north pole and into the south pole.

    Second: Lenz's law states that the induced current in the loop is such that the induced magnetic field opposes the change in the flux.

    So imagine a loop in the plane of the paper. You push the north pole into the loop (into the page), thus increasing the flux into the page. The induced magnetic field will oppose that increase and thus point out of the page. (What direction of induced current would create a magnetic field pointing out of the page?)

    If you do the same thing with the south pole of the magnet, the opposite will occur--since now you are increasing the flux out of the page.
     
  4. Mar 18, 2007 #3
    I understand that the magnetic flux increases in both cases when the north or south pole is directed in the loop's direction, but I do not understand the flux's direction in the south pole first case. Why does the flux increase out of the page when the south pole is placed in the loop? (Are there any diagrams that might help me understand this point clearly?)

    Thanks again.
     
  5. Mar 18, 2007 #4

    Doc Al

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    Because the magnetic field itself points out of the page. The magnetic field of a bar magnet points towards the south pole, which means out of the page.
     
  6. Mar 19, 2007 #5
    In the north pole first case, the induced current (from the right hand rule) will move in the direction to the right if the magnetic field points out of the page?

    For the south pole first case, the induced current will generate a magnetic field pointing into the page, so the induced current's direction will be to the left from the right hand rule??
     
  7. Mar 19, 2007 #6

    Doc Al

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    Not sure what you mean by "to the right". Current direction in a loop would be clockwise or counterclockwise viewed from above the page.
     
  8. Mar 19, 2007 #7
    In the north pole first case, I think I mean clockwise by left if the bar magnet is approaching the solenoid's right end?
     
  9. Mar 19, 2007 #8

    Doc Al

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    North pole into loop

    Let's take some care to specify directions clearly. Go back to my example of a single loop in the plane of the paper. Call that plane the x-y plane--the center of the loop is at the origin: 0,0,0. The +z axis points out of the page; -z, into the page.

    Given that, we can now talk about pushing the north pole of a bar magnet into the loop. We start with the magnet along the +z axis, with the north pole closer to the origin. We move the north pole of the magnet towards the loop (in the -z direction, towards the origin).
    • Which way is the bar magnet's magnetic field pointing? In the -z direction (into the page).
    • Which direction is the flux through the loop pointing? In the -z direction (into the page).
    • Which direction is the flux changing? Since the magnetic field is increasing in the -z direction, the flux is increasing in the -z direction.
    • Which way must the induced magnetic field point in order to oppose that change in flux per Lenz's law? Since the flux is increasing in the -z direction, the induced magnetic field must point in the +z direction.
    • Looking at the loop from the vantage point of the +z axis, which way does the induced current flow? Using the right-hand rule for current loops, the current must flow counter-clockwise through the loop to produce a field in the +z direction.
    Make sense? Things will be exactly reversed if you turn the bar magnet around and move the south pole into the loop.
     
  10. Apr 11, 2007 #9
    So as you move the magnet away from the origin - pulling it back (with the north pole of the magnet still facing the origin), the polarity of the origin becomes south pole, and it attracts the magnet again. Does that mean that at that point the current changes in the opposite direction (into the page)?

    Also when the bar magnets south pole is being directed towards the origin, does the origin become sout pole, in order to repel the magnet? is it just the same with the north pole situation, except now with south poles?
     
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