# Magnetic Force Inverse Cubed Law?

by Da Apprentice
Tags: cubed, force, inverse, magnetic
 P: 59 I was reading 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? Thanks, Z.C
 P: 155 The inverse cube law is complete nonsense. http://en.wikipedia.org/wiki/Inverse-square_law http://en.wikipedia.org/wiki/Electromagnetism
P: 1,333
 Quote by Dr_Morbius The inverse cube law is complete nonsense. http://en.wikipedia.org/wiki/Inverse-square_law http://en.wikipedia.org/wiki/Electromagnetism
There would be an inverse square law for magnetic monopoles, but these don't exist as far as we know. A magnetic dipole produces a field that follows an inverse cube law.

see:

http://en.wikipedia.org/wiki/Force_between_magnets

P: 35

## Magnetic Force Inverse Cubed Law?

Think,
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."
P: 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/Doc...ation%20of.pdf

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.

 Quote by willem2 There would be an inverse square law for magnetic monopoles, but these don't exist as far as we know. A magnetic dipole produces a field that follows an inverse cube law. see: http://en.wikipedia.org/wiki/Force_between_magnets

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