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Magnetic Force Inverse Cubed Law?

  1. Mar 15, 2012 #1
    I was reading <crackpot link removed> and was wondering if the inverse cube law for magnetic force still applied for situations where the object being attracted isn't another magnet itself? E.g. if there is an electromagnet attracting an iron nut is the rule still inverse cube and not inverse square?

    Last edited by a moderator: Mar 15, 2012
  2. jcsd
  3. Mar 15, 2012 #2
  4. Mar 15, 2012 #3
  5. Mar 15, 2012 #4
    A "monopole" field goes like 1/r2. Magnetic monopoles don't actually exist as far as we know to date, but some situations can produce a field which is approximately a monopole field over a limited region. For example, if you have a long bar magnet and you stay close to one pole.
    A "dipole" field goes like 1/r3. This is what you get from a current loop or a bar magnet, when you get far enough away that it appears "small."
  6. Dec 31, 2012 #5
    Actually Dazza95 is more correct.

    The inverse square law applies even in real world applications where the magnet is sufficiently asymmetrical to represent a "virtual" monopole.

    Here is a link to an experiment which proves this assertion. The "bar magnet" in this experiment had an aspect ratio of over 100:1

    http://www.u-picardie.fr/~dellis/Documents/PhysicsEducation/general%20rule%20for%20the%20variation%20of.pdf [Broken]

    I would surmise that in dipole magnets that are more symmetrical, the opposite pole is close enough to have a substantial influence on the overall net readings such as to reduce the field strength much more radically as the distance increases than with longer more asymmetric magnets where the opposite pole is at rather great distance . . .

    Unfortunately most physics sources (wrongly) simply throw out the dogma that magnetic field with increasing distance is the inverse cube - - when it is not when extreme aspect ratios are encountered.

    This is an important distinction.

    Last edited by a moderator: May 6, 2017
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