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Magnet Question, Please Help

  1. Aug 22, 2008 #1
    Hello, please forgive my ignorance. I need help coming up with a formula I can't seem to find on the internet. Assume I have a magnet that can attract a maximum of 5kg iron sphere against gravity from a distance .2m. My question is how much could that same magnet attract at a distance of .1m.

    Wikipedia says that generally "the strength of the field falls off inversely with the cube of the distance."

    Does that mean you could calculate by Mass * (Change In Distance)^3?

    Please help...
  2. jcsd
  3. Aug 22, 2008 #2


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    Science Advisor

    Your answer depends on the size and shape of the magnet and size of the iron sphere, compared to 100 cm. If it is a bar magnet of length much smaller than 10 cm and a small sphere, the magnetic field would fall off like 1/r^3, but the force on the sphere would fall off like 1/r^4. This is the force law for a dipole in the field of a dipole.
    If the bar magnet is longer than ~10 cm, then force falls off more like 1/r^3.
    Last edited: Aug 22, 2008
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