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B Bar magnet determining north pole

  1. Apr 21, 2016 #1
    I came across this problem:

    Explain how you would determine which was the north pole of a bar magnet, without using any other magnet.

    I was thinking of using a solenoid, and dropping the magnet in. Then you can determine whether it was a north or south pole by whether the current flows clockwise or anticlockwise. I was thinking that one of these would produce a negative current, but I am not sure which. Is there any way to do this?
     
  2. jcsd
  3. Apr 21, 2016 #2
    Wouldn't it just be a matter of hanging the magnet from a string?
     
  4. Apr 21, 2016 #3
    That won't work if the magnet isn't strong enough.
     
  5. Apr 21, 2016 #4
    Well if there are no restrictions on the magnet strength then it is a solution!
     
  6. Apr 21, 2016 #5

    berkeman

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  7. Apr 22, 2016 #6

    CWatters

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    Does the earth count as another magnet?
     
  8. Apr 22, 2016 #7
    What if it is a horse-shoe magnet?

    No
     
  9. Apr 22, 2016 #8
    In the end all you are trying to do is create a moment to rotate the magnet, so again, if you have both poles facing the ground there will still be a torque trying to rotate the magnet...from a string!
     
  10. Apr 22, 2016 #9

    berkeman

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    You will still get a small torque that will align the magnet. There is an easy trick to make the torque for a horseshoe magnet much larger in the Earth's magnetic field. Can you think what that trick might be? :smile:
     
  11. Apr 23, 2016 #10
    Vacuum?

    What if you were on a planet with no magnetic field?
     
  12. Apr 23, 2016 #11

    berkeman

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    Water is pretty frictionless for slow movements. No, I had a different improvement in mind... :smile:
    Then you would need to wave one pole of the magnet past a coil of wire and measure the polarity of the voltage induced in the coil. Have you read about magnetic induction yet in your studies? :smile:
     
  13. Apr 24, 2016 #12

    cnh1995

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    The solenoid will have induced current and hence, it will momentarily act as another magnet (electromagnet). What if you passed current through the solenoid using a battery and brought the magnet near its axis?
     
    Last edited: Apr 24, 2016
  14. Apr 24, 2016 #13

    tech99

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    Suspend the magnet horizontally using a thread. Connect a metre or so of wire from the + to - of a 1.5 volt battery. Hold the magnet closely over the wire. It will point according to the direction of the current using the Cork Screw Rule.
     
  15. Apr 25, 2016 #14
    We're doing that. Would the north pole make a positive current or a negative current first?
     
  16. Apr 25, 2016 #15
    I don't know

    thanks :smile:
     
  17. Apr 25, 2016 #16

    cnh1995

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    The solenoid will become an electromagnet. Using the direction of current, you can determine the poles of the electromagnet. Then take the magnet near its axis. You will feel either attraction or repulsion, from which you could tell the polarity of the magnet.
     
  18. Apr 25, 2016 #17
    That's what I was thinking, but I was wondering which direction the current would flow.
     
  19. Apr 25, 2016 #18

    cnh1995

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    Current direction will be simply from +ve terminal to the -ve terminal of the battery(conventional current,of course).
     
  20. Apr 25, 2016 #19
    Thanks :woot:
     
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