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Iron vs. Opposite Polarity

  1. Aug 26, 2009 #1
    Lets say I take the North pole of a magnet and place it 1 inch from a mass of iron with the same density and shape of the magnet, and somehow measure the force of attraction, I'll label that value "i". And then I take that same magnet and place it the same distance from another identical magnet with the South pole facing the first magnet's North pole, and measure the force of attraction, I'll label that value "p".

    Does i = p or is i < p ? Perhaps p = 2i?
  2. jcsd
  3. Aug 26, 2009 #2


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    I'm certainly no expert in this stuff, but I have an intuitive answer that might or might not be correct. It seems to me that the attraction of the original magnet doesn't change at all. It will, however, combine with the attraction of the second magnet to multiply the overall attraction.
  4. Aug 26, 2009 #3
    Yes, I think the attraction will be 2i, because I know that if two identical magnets are placed N to S and allowed to connect that the resulting field is the sum of the two fields, or at least, not much less. Kind of like two batteries.
  5. Aug 27, 2009 #4


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    There is an easy way to prove what happens.

    If you have access to a digital balance (scales) put the original magnet on a small cardboard or wooden box on the scales. The magnet should weigh the same with either pole pointed down. If it doesn't, the magnet is affecting the scale operation and the box needs to be bigger. Polystyrene or other plastic is OK. Anything light and nonmagnetic.
    The magnet should now have one pole pointed up.

    Zero (tare) the scales.

    Then put the piece of iron in a clamp so that it is held at a fixed distance and bring it over the magnet.

    The magnet will be attracted to the piece of iron and its weight will decrease. Get a distance where it decreases by about 30 %. Measure this distance.

    Now substitute the other magnet with the opposite pole pointed down. This will be attracted to the first magnet and lift it so that it weighs less. At the same distance, check the lifting effect on the scales.

    This is a very sensitive test to try out ideas. What happens if you put another magnet on top of the magnet in the clamp?

    What if you substitute a solenoid with an iron core for the top magnet? How does the current in the solenoid affect its magnetism?

  6. Aug 27, 2009 #5
    my guess is that for a very soft iron and small distances the forces would not differ much as the magnet would magnetize the iron and effectively the iron would behave as yet another magnet. but if you increase the distance or replace with a steel piece the magnet force wud be grater than the iron-magnet force.

    anyway since you have thought about this idea, please do the measurements .. that is the way of science. idle theorizing blunts the mind and sharpens stupidity. please tell all of us about the results.
  7. Aug 27, 2009 #6
    Well, vk6kro's idea sounds good, but I don't have any scales or identical magnets. But I think I could come up with something.
  8. Aug 28, 2009 #7


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    You might find a store that sells them cheaply, like this one in Hong Kong:

    They also sell magnets. Those prices are in US$ and include shipping. I'd get a 0.01 gram one.

    Alternatively, phone your local high school and talk to their Science people. They would probably let you use a digital balance for a while.

    A good way of comparing magnet strengths is to clamp one end of a hacksaw blade in a vice and try to pull the blade into a semicircle from the other end using the magnet. Very strong magnets will be able to pull a blade 90 degrees. If they let go almost immediately, they need remagnetizing.
    Last edited by a moderator: Apr 24, 2017
  9. Aug 28, 2009 #8
    Hey, that's a cool place...
  10. Aug 30, 2009 #9
  11. Aug 30, 2009 #10


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  12. Aug 31, 2009 #11
    That (according to OP) i = p.
  13. Aug 31, 2009 #12


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    Look closely at the curves as seperation distance increases. It looks to me like p=i at small seperations and p>i at larger seperation distances
  14. Aug 31, 2009 #13


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    The original proposal was to place the magnet near iron and near another magnet. Not touching.
    On that web site, I set the material to N52, the dimensions all to 1 inch and the spacing to 0.25 inches. That is pretty close.

    I get 16.29 lb for magnet to steel and 30.69 lb for magnet to magnet.

    So, for what it is worth, the two 1 inch cube magnets attract each other more than one magnet attracts steel and by about double.

    However, if you make the dimensions all 2 inches, the situation reverses and you get 159 lb and 142 lb. Yuk!
    They did use steel plate, though, and that was bigger than the magnets so maybe that was making the difference.
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