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Lenz's law

  1. Aug 4, 2007 #1
    http://img222.imageshack.us/img222/1203/lenzslawfe6.jpg [Broken]
    Thank you.
     
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Aug 4, 2007 #2

    Doc Al

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    In the diagram: The magnet provides a magnetic field through the coil indicated by the blue arrow marked "B". As the magnet moves, the magnet field changes; the change is marked [itex]\Delta B[/itex]. The induced current creates a magnetic field that opposes that change; the current direction is shown by the thick red arrows; the induced field from that current is shown by an arrow opposed to [itex]\Delta B[/itex] marked [itex]B_{induced}[/itex].

    Does that help?
     
  4. Aug 4, 2007 #3
    Ok, I understand, and how is possible decreasing of the field? I can't understand that.
     
  5. Aug 4, 2007 #4

    Doc Al

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    In the top right and bottom left diagrams, the change in the field is opposite to the field and thus the magnitude of the field is decreasing. Taking the top right as an example: The field from the magnet points to the right (since its a south pole). Since it's also moving to the right, the field is getting weaker--the change in field points to the left. Make sense?
     
  6. Aug 4, 2007 #5
    So you say that the magnet (from the example top right) is "pulling" the magnetic field from the wire (since the wire its it self a magnet)?
     
  7. Aug 4, 2007 #6

    Doc Al

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    In that diagram (top right) the field from the magnet is being pulled to the right, thus reducing the field inside the coil due to the magnet. This action induces a current in the coils which creates a field that opposes this change.
     
  8. Aug 4, 2007 #7
    I still can't see the difference between the decreasing and increasing examples. Just the magnet is moving in different direction. I think that in the both examples the field is increasing.
    http://img247.imageshack.us/img247/3561/74457189pi7.png [Broken]
    http://img247.imageshack.us/img247/8823/48879275gp1.png [Broken]
     
    Last edited by a moderator: May 3, 2017
  9. Aug 4, 2007 #8

    Doc Al

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    Realize that the magnetic field of a bar magnet is strongest near the poles. So if the magnet is moving away from the coil, the field through the coil due to the magnet is decreasing (getting weaker); if it's moving towards the coil, the field is increasing (getting stronger).
     
  10. Aug 4, 2007 #9
    Is it like two permanent magnets? Ex. Let's say that there is one permanent magnet. I put close to it other same permanent magnet so they are attracting each other and there are 2 magnetic fields together. So when I'll pull out the second permanent magnet the magnetic field will be weaker, right?
    http://img216.imageshack.us/img216/9650/90406307hj7.gif [Broken]
    btw- Why it wants to keep it constant?
     
    Last edited by a moderator: May 3, 2017
  11. Aug 4, 2007 #10

    jtbell

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    No, that's not what Doc Al is talking about. Imagine that you start with a coil made out of plastic, not metal, so that it can't carry a current. Start with the magnet close to the coil. The flux through the coil, of the magnetic field produced by the magnet, is relatively large. Now pull the magnet away from the coil. The flux through the coil, of the magnetic field produced by the magnet, decreases, because the magnetic field is weaker far from the magnet than close to it. Of course, this doesn't have any other effect, because the coil is non-conductive, so there is no induced current.

    Now, replace the plastic coil with a metal one, wired into an electric circuit, and perform the same motion with the magnet. The flux through the coil, of the magnetic field produced by the magnet, changes in exactly the same way as before. But now, because the coil is conductive, and it's part of an electric circuit, this changing flux induces a current in the coil. This current produces an induced magnetic field, which is indeed rather like the field produced by a bar magnet (a dipole field). This induced field is in addition to the original field produced by the magnet.

    In this case, the induced field is in the same direction as the field produced by the magnet, so as to try to "reinforce" it and maintain a constant total magnetic flux through the coil.
     
    Last edited: Aug 4, 2007
  12. Aug 4, 2007 #11
    Ok, thank you. But look. Always the electrons and in general the atoms tend to have lower magnetic or electric force. So when you get close the permanent magnet to the conductor, the field increases, but when the field is decreasing (getting weaker) why it wants to get increased again?
     
  13. Aug 5, 2007 #12
    Anybody know?
     
  14. Aug 5, 2007 #13

    Doc Al

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    I think you are asking why there's a negative sign in Faraday's law--why does the induced field oppose the change due to the moving magnet. Think of it as a consequence of the conservation of energy. If, as you went to push the north pole of a magnet towards the coil, the induced current created a field in the other direction then the bar magnet would be sucked into the coil. It would speed up (increasing its kinetic energy) and the current in the coil would increase (increasing its energy as it heats up or drives some other device)--you'd end up getting free energy. The way things actually work--as described by Lenz's law--is that it takes work to push the magnet into the coil (or pull it out): No free energy here. To create that current in the coil you have to exert a force--do work--on the magnet.

    Does that help?
     
  15. Aug 5, 2007 #14
    I just wanna know why the magnetic field of the coil (when it is weaker, like in the top right example), why it wants to get stronger? Isn't the atoms tend to have weaker force (weaker magnetic field)?
     
  16. Aug 5, 2007 #15

    Doc Al

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    All four examples in the diagram operate according to the same principle. What's special about the top right?
     
  17. Aug 5, 2007 #16
    I said the example when the magnetic field is getting weaker no matter the top right or below left. I just want to know why it wants to have stronger field? Isn't weaker field better?
     
  18. Aug 5, 2007 #17

    Doc Al

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    It "wants" to keep the field constant, not stronger or weaker. If the field through the coil (from the moving magnet) is getter stronger, the current acts to make it weaker; if the field is getting weaker, the current acts to make it stronger.
    No.
     
  19. Aug 5, 2007 #18
    And when the magnetic field of the coil gets weaker, is it like when it was in first time, when there is no permanent magnet around the coil?
     
    Last edited: Aug 5, 2007
  20. Aug 5, 2007 #19
    Anybody knows?
     
  21. Aug 5, 2007 #20

    Doc Al

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    Again, it's not clear what you are asking. You must distinguish between (1) the induced magnetic field due to the current in the coil, and (2) the magnetic field in the coil due to the permanent magnet.

    Note that the induced EMF, which creates the current in the coil, is proportional to the rate of change of the magnetic field in the coil due to the moving permanent magnet (what I labeled as (2) above). If you stop the magnet from moving, the induced EMF goes to zero. If you pull the magnet out far enough, eventually the field goes to zero and the rate of change of the field is essentially zero and the induced EMF goes to zero.
     
  22. Aug 5, 2007 #21
    When the permanent magnet gets closer to the coil it lines the domains (the atoms) of the coil and it makes field with opposite poles of the permanent magnet. So when I'll close the permanent magnet there will be increasing of the field and when I will pull back the permanent magnet then there will be not lining up of the two fields.
    http://img216.imageshack.us/img216/9650/90406307hj7.gif [Broken]
    So I can't see any weaknesses of the field of the coil.
    http://img247.imageshack.us/img247/3561/74457189pi7.png [Broken]
    http://img247.imageshack.us/img247/8823/48879275gp1.png [Broken]
     
    Last edited by a moderator: May 3, 2017
  23. Aug 5, 2007 #22

    Doc Al

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    I think I finally understand your question...
    No, this is not the correct way to view things. You are treating the coil as a piece of magnetic material (like iron) that is attracted to the poles of the permanent magnet. But that's not the effect of interest here. (What if the coil were made of non-magnetic, copper wire?)

    The right way to view this is to realize that the permanent magnet is surrounded by an non-uniform magnetic field. When that magnet is moved, the field through the coil changes, which induces an EMF that creates a current in the coil.
    No, the magnetic field created by the current in the coil is not always oriented opposite to the poles of the permanent magnet. It is oriented in such a way as to oppose the change in the field through the coil due to the movement of the magnet.

    This diagram is not true in general--sometimes the poles are opposite; sometimes they are the same.
    In this diagram, since you are pushing the magnet towards the coil, the poles of the magnetic field of the coil will point in the opposite direction: You'll have two north poles facing each other, thus repelling and resisting the push.
    In this diagram the orientation of the poles of the coil are show correctly, but this creates an attractive force that resists the movement of the permanent magnet.
     
    Last edited by a moderator: May 3, 2017
  24. Aug 5, 2007 #23
    And how is possible that one field can be weaker? What should I have to make some magnetic field weaker?
     
  25. Aug 5, 2007 #24

    jtbell

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    The magnetic field of a magnet decreases with increasing distance from the magnet, like the electric field of a charge decreases with increasing distance from the charge. So if you move the magnet further away from some point, you decrease the magnetic field at that point.
     
  26. Aug 5, 2007 #25
    So if I pull back the permanent magnet from the coil (like in the top right example) I will decrease the field right? That means that before the magnetic field was increased with the pulling in the permanent magnet?
     
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