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Magnetic field of permanent magnet

  1. Mar 9, 2012 #1
    Is it possible to estimate the magnetic field strength of a permanent magnet from the heaviest weight that the magnet can lift? The heaviest weight that can be lifted can be found by trial and error.

    Any comments or suggestions appreciated.

    Best regards,
  2. jcsd
  3. Mar 9, 2012 #2
    I don't know much about electromagnetism, so I'm not sure if there is another equation unknown to me that makes this easier, but you the acceleration due to gravity is appox. 9.8 m/s^2 at ground level. For an object to be lifted straight up by a magnet the acceleration due to the magnet has to be > than 9.8 m/s^2.

    Using Newton's second law of motion and the known masses of objects, you can guess at the force that is being applied, though if you had a laser accelerometer (which aren't cheap, but many high schools might have them) you could know far more accurately.
  4. Mar 10, 2012 #3
    Hi inkpot,

    As far as i know, there is'nt any formula you can apply to find magnetic field, unless there is an electric charge.

    If there is an electric charge, then

    Field strength = Magnetic Force/(charge * velocity)

    Hence I am assuming, its not possible to accurately determine the Magnetic field strength, of a magnet, based on the heaviest load it can carry, as there are other factors that affect magnetic field, and only by using this formula will you be able to determine the field strength.

  5. Mar 10, 2012 #4
    As the magnet in question is a permanent one why is it necessary for an electric charge to be present in the equation relating field strength and force?

    Is there a good source on the fundamental nature of magnetism that anybody could suggest?

    There is a very interesting discussion at https://www.physicsforums.com/showthread.php?t=3467

    Best regards,
  6. Mar 19, 2012 #5
    An idea could be to use TWO permanent magnets aligned so that one repel the other.
    When second magnet is "floating" over the first one with no motion the force on the first magnet is the wheight of the second !
    This gives NOT info on the force of the magnet: we have to read also the distance at which the equilibrium is obtaied: the distance will be higher for a stronger magnet...
    Unfortunately this distance depends also on the strenght of the second magnet.
    If someone make experiments and find a formula... i will be hasppy to use it !

    I have build a nice non-contact scale using two repelling magnets with a load cell below the first :-)

  7. Mar 19, 2012 #6


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    It depends on what you are lifting. You might be able to get a rough estimate by treating the bar magnet as a pair of magnetic monopoles. The magnet will induce a magnetic field in the material you are lifting, depending on the magnetic susceptibility of the material. A table for many material is found at http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_6.html
    I think it's possible, but it gets a little messy so it's not typically covered in the classroom.
  8. Mar 19, 2012 #7


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    One method could be to move a search coil or wire through the field at a known speed and find the induced emf in the wire.
    Another would be to use a Hall Effect transducer.

    Methods that involve picking up heavy weights are not very satisfactory because they are only measuring the field when the magnetic circuit is completed (the equivalent of a short circuit for susceptance when the weight is actually in contact) and does not tell you the field in the absence of the 'iron' weight. It would be affected greatly by the 'fit' of the test weight to the magnet.
  9. Mar 23, 2012 #8
    Thank you all for your responses.

    My reason for making the enquiry was part of an attempt to use magnetism, either with permanent magnets or electromagnetism, as a means of damping the motion of an object floating in water.

    However, subsequent investigations seem to show that this may not be a practical option, mainly due to the way that the magnetic force declines very quickly with increased distance from the magnet. A supplier of rare earth magnetics has a very good calculator on this subject, see http://www.kjmagnetics.com/calculator.asp .

    A similar site can be found at http://magda.ucc.ie/

    Best regards.
  10. Mar 23, 2012 #9
    Thank you inkspot !
    I have found the site http://magda.ucc.ie/ really useful...
    Now I am looking for formulas used in this site to generate the graph :-)
  11. Mar 24, 2012 #10
    I am glad that you found the magda site of use.

    Best regards,
  12. May 31, 2012 #11
    Force to an iron between 2 magnets
    Two magnets (A and B)has the same pulling energy, say 50 lbs. Put face by face on the same north poles (in repelling mode) in 10 mm distance. If a 3 mm bar iron strip/steel trip put in the middle of these 2 magnets, What will the forcstrip irone exerted on the strip iron/steel ? Howdo you calculate ?
  13. Jun 1, 2012 #12


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    A magnetic field will not provide any damping unless you use eddy currents, induced in a metal plate / disc. On its own, it will just produce an offset force without taking any energy out.
    If you want to damp oscillations of a floating object then the thing to do is to use water damping with discs, drogues, seives or paddles under the water.
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