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Homework Help: With two provided metals, find which one is the magnet.

  1. Apr 29, 2010 #1
    1. The problem statement, all variables and given/known data
    I'm given two identical pieces of metal (in size, shape, color, density, etc.) and no other materials to work with besides my hands.

    Using only those two pieces of metal and my fingers, I am tasked with determining which of the two pieces is a permanent magnet (the other is simply a hunk of metal).

    Both objects are solid iron, but only one is a magnet.

    2. Relevant equations

    3. The attempt at a solution
    I originally thought of rubbing the two together to align the domains within the non-magnet, producing a temporary magnet. Then I'd flip one piece of metal around and see if they repelled (from both of the same poles facing each other).

    However, I don't see how that would tell me which one is the magnet.

    At this point, I'm stuck. I have no clue how to proceed.

    Any help would be greatly appreciated!

  2. jcsd
  3. Apr 29, 2010 #2


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    If you rotate a piece of iron with respect to a magnet, what will happen?
  4. Apr 29, 2010 #3


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    Hello BraedenP,

    Welcome to Physics Forums!

    Perhaps you're on to something with your original idea. I speculate that perhaps the answer might have something to do with the way in which the rubbing occurs. I mean, you wouldn't want to just rub the two pieces together all willy-nilly would you?
  5. Apr 29, 2010 #4
    Thanks! I'd have given a proper introduction, but I wasn't sure how far I could deviate from the provided template.

    As for the question, I'd probably just rub one side of each together, so as to align the domains properly. However, if I flipped one piece after the magnetization step, and they did deflect, I still can't wrap my head around how I'd figure out which one was the magnet. Regardless of which one I flip, they'd repel.
  6. Apr 29, 2010 #5


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    I think you need to break the "symmetry", so to speak, regarding the method of which you use to rub them together. In other words, rub them together in such a way that piece A is rubbed by piece B quite differently than piece B is rubbed by piece A.

    To understand what I'm talking about, you might want to research the "rubbing" method to magnetize a piece of metal with an existing magnet. It doesn't really involve rubbing their sides together.
  7. Apr 30, 2010 #6


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    Oh wait! :blushing: turbo-1 had the best hint yet, now that I think about it. Shesh, turbo-1, could you have been more subtle? http://www.websmileys.com/sm/violent/sterb104.gif

    Seriously, ignore what I said about magnetizing a piece of metal from an existing magnet. 'Nothing technically wrong with the idea, but the first thing you would need to do is find one of the magnet's poles. First things first.

    Without giving away an answer, here is a hint along the lines of turbo-1's hint. There's no such thing as a magnetic monopole (as far as we know anyway -- at the very least not something that you could hold in your hand). Magnetic attraction of a magnet is not omnidirectional.

    So if you wanted to find one of the poles of a magnet, even if you didn't care if a particular pole was north or south, how might you go about doing it? http://www.websmileys.com/sm/happy/058.gif
    Last edited: Apr 30, 2010
  8. Apr 30, 2010 #7
    I would balance them both on my finger and see which one turns like a compass needle. :wink:
    Last edited: Apr 30, 2010
  9. Apr 30, 2010 #8


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    Well, that might be tough. The Earth's magnetic field is pretty weak. Yes, that might work, but you'd have to balance the pieces really well. And more importantly, it assumes that you already know the orientation of the poles of the perspective magnet, such that the poles are not facing up and down.

    But there is an easier way. How would you go about finding the poles of a potential magnet (you would need to do this as a first step anyway, if you were to magnetize one from the other, or use the magnet as a compass)? (Another hint: Magnets always have poles. Which means that part of the magnet necessarily lies in between the poles.) What are the magnetically attractive parts of a permanent magnet? What are the attractive characteristics in-between these parts?

    [Edit: it might be useful to imagine that the pieces are both shaped as perfect spheres. Even if a magnet is sphere shaped, it still has poles!]
    Last edited: Apr 30, 2010
  10. Apr 30, 2010 #9
    I know this is not practical. I just submitted this idea as a cute answer. :rolleyes:The interesting thing is that this answer satisfies the phrasing of the question, in principle.

    "Using only those two pieces of metal and my fingers, I am tasked with determining which of the two pieces is a permanent magnet "

    A magnet balanced on the fingers will experience a force, of course. For certain shapes, balancing is very easy, but the problem becomes reducing friction enough to allow actual rotation. However, one could argue that the magnet need not actually turn. Perhaps the finger-tip (which is very sensitive by the way) can feel the small torsional force. Again this is not really a practical idea, but works as a thought-experiment.

    I actually do know how to do this in the practical way, but you are already leading people down that path, so I don't want to spoil that.
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