Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Determine the north pole?

  1. Jan 30, 2007 #1
    1. The problem statement, all variables and given/known data
    Explain how you would determine which was the north pole of a bar magnet, without using any other magnet.

    2. Relevant equations

    3. The attempt at a solution
    If it's small enough you might be able to make it into a compass like object but if its heavy than the magnetic field of the earth is not strong enough to deflect it. So this might not be 100% correct answer. The answer should encompass all sizes of magnets.
  2. jcsd
  3. Jan 30, 2007 #2
    Is it allowed to use current carrying wires?
  4. Jan 30, 2007 #3
    I think the best way is to use a string/thread and tie it at the middle of the bar magnet. Then just hang it out by holding the string/thread and let it move by itself without applying any force from you hand/fingers as much as possible.
  5. Jan 31, 2007 #4
    if that doesn't work, are you allowed to magnetize something much smaller and lighter with the original magnet?
  6. Feb 1, 2007 #5
    As long as that something is not another magnet. But you still couldn't tell which was the north and south pole because if it was steel and you lined the south pole with it, the steel will induce a north side facing the magnet. And vice versa.
  7. Feb 1, 2007 #6
    But you still woudn't know which was the N and S pole without another compass , which is a magnet hence disallowed.
  8. Feb 1, 2007 #7
    I think so since the question only said you can't use another magnet. So you are suggesting loop a wire with current around a magnet and notice the direction of the current. This should induce a magnetic field along with the intrinsic magnetic field in the electromagnet (with a magnet core). If the resultant magnetic effect is greater than the place where the current is travelling towards is the N pole and vice versa.
    Last edited: Feb 1, 2007
  9. Feb 1, 2007 #8

    Now i know which way north is w/o a compass--are you sure the problem is this constrained? If nothing else you could use the night sky.
  10. Feb 3, 2007 #9
    I said that another compass was needed because you actually don't know which is N and S on the original magnet (the S will point towards the S and N towards the N but you don't know which is which), which you have made it into some sort of a compass if folllowing Ruben's advice. So you suggest using to the night sky as a guide but you still would need to know which was N and S before hand or find the information in a book. But the original people who found out had to use a proper compass hence another magnet. In this way I think the way suggeted by P3X-018 and elaborated by myself is a better answer to the question. The question clearly stated that another magnet cannot be used.
    Last edited: Feb 3, 2007
  11. Feb 3, 2007 #10
    I guess I'm properly confused, owing much to my igorance, no doubt. My thinking was as follows, A big bar magnet suspended on however frictionless a bearing will be under the most minute torque. I have no idea,but guessing it could hang for days or weeks, months centuries even, So assuming its a compass its in its own right, we need to either make a Focault like pendulum and wit saa long time or diminish the inertia-thatis sympathetically magnetize a needle, paying attention to orientation, and stick the now low mass N/S indicator into a cork, whatever,

    Basically build a scale mod of the bigem, If it points toward Polaris, bingo
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook