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Magnet Behavior in Solenoid

  1. Feb 22, 2015 #1
    The field lines inside of a solenoid all go in one direction, right? So why doesn't a magnet that's placed in the solenoid simply get forced one way or another. I tried this at home by placing a magnet in the solenoid(near the outside of the coil so that it stuck out a bit), and when I turned on the current, the magnet slid either out of the solenoid or into the center of it(where it just stopped). I'm assuming that it slides out of the solenoid or gets pulled in depending on the polarity of the magnet/current. After it stopped in the middle, I pushed it further to the other end so that it was on the opposite end of the solenoid from where it started. When I turned on the current, it slid back into the center. Thus, it didn't simply get pushed one way or the other. Can anybody explain why this happens?
     
  2. jcsd
  3. Feb 22, 2015 #2
    I am not quite sure but I believe its due to electromagnetic induction in the coil according to Lenz' Law if at all what I think is what you asked.
    When the magnet is being brought near the coil, a current is induced in it such that it opposes the cause of change in magnetic flux in the solenoid.
    For instance if the north pole of the magnet is being brought near the solenoid in right-left direction, a current will be induced in the coil in the anti-clockwise direction making it a kind of like a north pole which opposes the magnet coming near.
    Similarly if the magnet in the similar orientation was moved away from the solenoid, the induced current in the solenoid coil will be such that it attracts the magnet, hence clockwise direction.
    So when a magnet is being brought close to solenoid, it tries to repel it and when inside a little, for some coils of the solenoid the magnet is being brought close while for others it is being moved away.
    Hope it helped!
     
  4. Feb 22, 2015 #3

    davenn

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    hi there
    welcome to PF

    Consider the polarity and position of the magnetic poles of the solenoid and how they interact with the poles of the permanent magnet
    Have a think and see what answers you can come back with :smile:

    Dave
     
  5. Feb 22, 2015 #4
    Thank you Faris and Dave. I thought about the poles of the solenoid and I think I understand why it does what it does.
     
  6. Feb 22, 2015 #5

    davenn

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    good ... then have a go at writing down what you think is happening and we can see if you are on the right track :smile:

    s couple of drawings showing the N and S poles of the solenoid and the magnet would be a great start :wink:
     
  7. Feb 23, 2015 #6
    This is my idea: The current in the coil produces a magnetic field with a North and South pole. In the case that the South of the permanent magnet is facing the North of the solenoid, the North and South will attract and the magnet will be pulled into the solenoid. It will keep going further until it reaches the point when the South pole of the solenoid gets closer to the magnet and pushes the magnet with enough force to keep it from going further. In the case that the North and North poles are nearest each other, they will simply repel.
     
  8. Feb 24, 2015 #7

    davenn

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    yup as will the 2 south poles, so the magnet will sit in the middle of the coil at a point where there is a balance between the 2 similar poles repelling

    a cool experiment to play with
     
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