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Lenz's law - some doubts in this

  1. Jul 27, 2011 #1
    Lenz's law -- some doubts in this

    1. The problem statement, all variables and given/known data
    Dear members,
    Please refer my attachment lenz law
    When the bar magnet with its N-pole is moved towards the coil, the induced current produced in coil S opposes the motion of magnet. It will happen so, if the induced current in the coil S produces magnetic field lines from left to right i.e., if the induced current flows through the coil S in clockwise direction (when seen from left).
    Which means the face of the coil facing the moving bar magnet is North pole.


    2. Relevant equations
    Now my question is if the bar magnet with its S pole is moved towards the coil, what is the direction of the induced current in the coil. I have visualised this assuming the face of the coil is south so that there is repulsion between magnet and coil and attached this as lenz1. Members please let me know whether my attempt is correct.


    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Jul 27, 2011 #2

    PeterO

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    Re: Lenz's law -- some doubts in this

    What ever happens when you move the N end of the magnet near the coil, the opposite will happen when you move the S end of the magnet in the same way.

    Peter
     
  4. Jul 31, 2011 #3
    Re: Lenz's law -- some doubts in this

    i dont understand sir
     
  5. Jul 31, 2011 #4

    PeterO

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    Re: Lenz's law -- some doubts in this

    I was trying to boost your confidence so you would see that your answer was correct.

    The supplied diagram shows the direction of induced current when the North pole of the magnet was moved towards the coil.
    I was reminding you that if you instead were to advance a South pole towards the coil, then the current would be induced in the opposite direction.
     
  6. Aug 1, 2011 #5
    Re: Lenz's law -- some doubts in this

    Thanks a lot sir.
     
  7. Aug 1, 2011 #6
    Re: Lenz's law -- some doubts in this

    Sir, another doubt. I move the magnet from right to left in lenz1 attachment and the direction of the induced emf is also from right to left, that is looking near the face of the coil facing the magnet. So, how can we say that the direction of induced emf is opposite to motion of magnet.
     
  8. Aug 1, 2011 #7

    PeterO

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    Re: Lenz's law -- some doubts in this

    I don't say that!

    Magnetic field is directed INTO a South Pole - I think by definition - that is to the right in your diagram.

    As the South Pole is moved closer to the coil, the coil is subjected to an increasing amount of "to the right" directed magnetic field.

    The coil will try to create its own "to the left" directed magnetic field to oppose that increase.

    In order to create that field, the current is induced in the direction you identified initially

    Note that the induced effect opposes the increase in the field, not the field itself. If the magnet were to stop moving so that the field, while in evidence, is not changing, the induced effect ceases also.
     
  9. Aug 2, 2011 #8
    Re: Lenz's law -- some doubts in this

    understood 90 % sir but a little confusion. lenz law states that " the direction of induced emf is to oppose the changing magnetic flux" so i infer that the direction as indicated by arrow in my attachment will oppose the changing magnetic flux. am i correct,sir?
    Thanks again for clarifying patiently sir
     
  10. Aug 2, 2011 #9

    PeterO

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    Re: Lenz's law -- some doubts in this

    Yes you are correct - at least it looked like it.

    The current will be "up the front" of the coil, and "down the back" of the coil.
    This will produce magnetic flux to the left [the right hand grip rule will show this].

    The approaching South pole subjects the coil to an increasing rightward flux, which is why the coil produces a leftward flux to try to cancel out that increase.
    The key in lens law is the word I highlight here:
    " the direction of induced emf is to oppose the changing magnetic flux"

    It doesn't only matter what direction the imposed flux is, but the way it is changing.

    You can have a Pole approaching a coil, or moving away from the coil.
    Moving closer, the change is for the field to get stronger or increase
    Moving away, the change is for the field to get weaker or reduce.

    We have to know both the direction of the imposed field, and whether it is increasing or reducing.

    In this case: right field, increasing which induced leftward field to try to cancel.

    If the magnet was moving away, it would be: right field reducing which would induce rightward field to try to make up for the reduction.

    In the Original drawing, it was North Pole approaching.
    That means: left field increasing which induced rightward field to try to cancel.
    If the North Pole was moving away we would have:
    left Field reducing which indices a leftward field to try to make up for the reduction.

    Peter
     
  11. Aug 3, 2011 #10
    Re: Lenz's law -- some doubts in this

    Sir, in the attachment img1 and img2, i had been asked to find the direction of induced current. for img1 the answer given in the book is
    (a) The direction of the induced current is along qrpq.

    (b) The direction of the induced current is along prqp.

    In case a) south pole is approaching the coil, so south pole will be induced at the face of the coil to create a repulsive force. According to the end rule, current should be flowing towards the south pole and away from the north pole and hence the direction of the induced emf should be along prqp
    Again in the case of b) for the left hand side coil, south pole is approaching so south pole is induced at the face of the coil so the direction of the induced emf should be along qprq
    Please clarify sir. Thanks in advance, sir
     

    Attached Files:

  12. Aug 3, 2011 #11

    PeterO

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    Re: Lenz's law -- some doubts in this

    The pictures are very small and difficult to identify which way the coil is facing but.

    in the first case, you interpretation that a South Pole is induced is correct, though to me an unusual way of putting it. I am not sure what you mean by the end rule? I have never heard any such rule??, Certainly the current will be up the front of the coil and down the back of the coil.
    As best I can make out the direction the coil is drawn, that means from q-r-p-q.

    Looking from the magnet end of the coil it will be a clockwise current. [obviously if you looked from the left end, which physically would be easier as the magnet is really in the way looking from the magnet end, the current is anti-clockwise.

    I will now try to see the second diagram.
     
    Last edited: Aug 3, 2011
  13. Aug 3, 2011 #12

    PeterO

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    Re: Lenz's law -- some doubts in this

    I agree with their answer.
    Again, for the same reasons, the current is up the front o the coil, and down the back of the coil. that means form q to p inside the coil, then from p - r - q in the outside connection, so p -q -r -p.
     
  14. Aug 3, 2011 #13

    PeterO

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    Re: Lenz's law -- some doubts in this


    It seems to me that what you call the end rule is giving you the wrong answers.
     
  15. Aug 4, 2011 #14
    Re: Lenz's law -- some doubts in this

    Clock Rule
    According to the clock rule "When an observer, looking at the face of the coil, finds the current to be flowing in the anti-clockwise direction, then the face of the coil will behave like the North Pole. While if the current is in the clockwise direction, the face of the coil will behave like South Pole.
    http://www.tutorvista.com/content/science/science-ii/magnetic-effects-electric-current/summary.php
    Sir, this rule has been mentioned as End rule in my text book.
    End rule
    When looked
    from one end, if the
    current through the
    solenoid is along
    clockwise direction the nearer end corresponds to south pole
    and the other end is north pole.
    When looked from one end, if the current through the solenoid is
    along anti-clock wise direction, the nearer end corresponds to north
    pole and the other end is south pole
     
  16. Aug 4, 2011 #15
    Re: Lenz's law -- some doubts in this

    Sir,
    please see this attachment. Though this diagram illustrates right hand grip rule, the diagram is similar to one given in my book to illustrate clock or end rule, in which current is flowing towards south pole and away from the north pole.
    So in the previous example, south pole of the magnet approaches the coil so south pole is induced in the coil, so current should flow towards S end and away from N end, which made me to say that the current flows along qprq because at the q end we have S pole and at the p end we have N pole.
    Correct me if i am wrong, sir
     

    Attached Files:

  17. Aug 4, 2011 #16

    PeterO

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    Re: Lenz's law -- some doubts in this

    This diagram is quite clear, since drawing the core makes it VERY easy to identify which way the coil is wound.

    In the previous example, I think the coils were wound in the other direction - though it was hard to be sure with such tiny diagrams.

    For that reason I would never refer to "the current flowing towards the South end" or away from the South end for that matter. I always look to see whether the currecnt would be clockwise or anti-clockwise. I then check which end of the coil it will come out of if flowing clockwise or anti-clockwise, as appropriate. ******* make sure you read the next post, on page 2
     
  18. Aug 4, 2011 #17

    PeterO

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    Re: Lenz's law -- some doubts in this

    Actually the diagram is drawn incorrectly. It shows the "free" wire disappearing up behind the core at both ends!!!!!
    by comparing to the other coils, it is clear the wire at the "S" end , or Right Hand End , should be drawn up in front of the core then curving over the top like every other loop.
     
  19. Aug 5, 2011 #18
    Re: Lenz's law -- some doubts in this

    Thanks a lot for all your explanations, sir
     
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