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Basic question about Lenz law

  1. Jul 12, 2008 #1
    Basic question about Faraday's law?

    Hi guys,

    Ok I have a small problem with faraday's law. Taking the example of a north pole of a magnet coming close to a solenoid. My question is when the magnet is inside the coil and is stationary why is the induced current zero. Wouldn't the magnetic field of the magnet and the coil add up and B increase creating and induced current. I know the real answers for these questions is zero but I don't understand why? Any help would be greatl appreciated. Thanks!!

    EDITED
     
    Last edited: Jul 12, 2008
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  3. Jul 12, 2008 #2

    CompuChip

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    If I read your post correctly one of the underlying questions is: why doesn't a stationary magnetic field produce a current? Note that the important principle here is Faraday's law, Lenz's law only tells you the direction of the current, if there is any.
     
  4. Jul 12, 2008 #3
    Yeap you are right:smile:. Sorry. I'll try to change it. Anywhere if you can help me with this question it would be great. Thanks!!
     
  5. Jul 12, 2008 #4

    Doc Al

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    Re: Basic question about Faraday's law?

    What do you mean by "the solenoid has a magnetic field"? A solenoid is just a loop of wire. Are you talking about the magnetic field of the magnet?
     
  6. Jul 12, 2008 #5
    Re: Basic question about Faraday's law?

    Ok well you are right. It is a just a piece of wire that had no current to begin with hence no magnetic field. The second question is what really bothers me. Where lets say that the magnetic field of solenoid was zero .When the magnet is moved inside the coil and is kept stationary shouldn't the new magnetic field be the magnetic field of the magnet and not zero. Hence change of B hence induced current. What I'm basically asking is why should there be a change of flux to get an induced current. Also I think I read somewhere that if an object has a magnetic field lets say 70 T and is kept constant there is no induced current. I thought to have magnetisim there should always be a current (it shouldn't be induced but some sort of current).
     
  7. Jul 12, 2008 #6
    Re: Basic question about Faraday's law?

    To understand this, it would be much better to ignore the solenoid for the moment.

    It is all about maxwell's equations, when you change a magnetic field, it induces an electric field. What really induces the current in the solenoid is this induced electric field. In your case the induced electric field is circular in space because the magnetic field is linear. (look at maxwell equations for these), and this circular electric field will induce an emf in the loops of the solenoid thus the current.

    So to get an induced current (which is actually an induced electric field which manifests itself as a current when there is a physical conducting material is available) you should have a changing magnetic field, not a constant one.

    Thinking in terms of a changing flux just makes the calculations easier and simplifies the notion in a beautiful manner. When you see something that you can apply the amperian loop technique to find the induced current, you say that "great I will just look at the change of flux through the amperian loop and work out the current, and I will not struggle with vector fields and differentiating them. and use the right hand rule or the direction instead of struggling with unit vectors for direction"
     
    Last edited: Jul 12, 2008
  8. Jul 12, 2008 #7
    Re: Basic question about Faraday's law?

    Thank you for the help guguma :smile: It does make sense. But going back to the solenoid question. When the magnet is moved inside wouldn't the magnetic field change from 0 to some number. Isn't this a change of flux. My other question is what about some object that already has a magnetic field. Then does it have a current to begin with (not talking about induced).
     
  9. Jul 12, 2008 #8

    Doc Al

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    Re: Basic question about Faraday's law?

    I assume that you mean: The external magnetic field (from the permanent magnet) through the coil is zero.
    Sure. The magnetic field through the coil starts out at zero and becomes equal to the field of the permanent magnet. The field through the coil is not zero when the magnet is moved close to the coil.
    While the field within the coil (due to the permanent magnet) is changing, there will be an induced current. Once the field stops changing (when you stop moving the magnet), the induced current becomes zero once again.
    Not sure how to answer that without an appeal to Maxwell's equations. (And then you can always ask "why do those equations apply?") A changing magnetic field creates an electric field--that induced electric field drives the induced current.
    I think you're asking about the field from a permanent magnet (say) and whether that field is also associated with a current. I'd say sure. Semi-classically, you can think of the magnetic field as created by the current of electrons "orbiting" the nucleus. (That picture is woefully inadequate, but a good start.)
     
  10. Jul 12, 2008 #9

    Doc Al

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    Re: Basic question about Faraday's law?

    Of course it is. And as long as the flux is changing, there will be an induced current in the solenoid.
    See my last post.
     
  11. Jul 12, 2008 #10
    WoW:smile:. Thanks a lot DocAl. I appreciate your lengthy response :smile:.You actually cleared many of my doubts but I still have a bit of confustion with that moving the magnet question. So if the magnet is kept stationary would the total magnetic field be greater than zero. Oh I got is it that when the magnet is stationary the total magnetic field is greater than zero but constant. Hence no change in magnetic flux. Am I right?
     
  12. Jul 12, 2008 #11

    Doc Al

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    You are correct. What creates an induced current in the coil is a changing magnetic flux.
     
  13. Jul 12, 2008 #12
    Re: Basic question about Faraday's law?

    You are right when you introduce the magnetic field to the solenoid the first time, you will see an induced current and while the magnet is moving AND while the magnetic field is still changing (the flux through the loops) you will see induced current. But when the magnetic field inside the solenoid due to the permanent magnet becomes constant, there will no longer be induced current in the solenoid, because you would not be having an induced electric field, thus emf to move the free charges in the solenoid anymore.

    For your other question see Doc Al's post. I do not know much about permanent magnets, but it has to do with electron motion and electron spin.

    But if you are making the magnetic field in a lab or something, there should be current. Actually any kind of moving charge distribution will manifest a magnetic field whether it be due to single tiny electron motions in the magnet, or motion of an enormous electron army in a giant solenoid.
     
  14. Jul 12, 2008 #13
    Thanks DocAl.You are doing a great job here:cool:. Now I can log off happy. Thanks again. I should thank guguma as well for the help. Bye!!
     
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