# Magnetic field strength diminishes proportional to inverse cube or inverse square?

1. Nov 22, 2010

### magnetics

I have read a number of journal article that state that magnetic field strength diminishes inversely proportional to the square of the distance. BUT more than one has stated that the field strength is inversely proportional to the cube of the distance from the surface of the magnet.
Which one is correct?

2. Nov 22, 2010

### Staff: Mentor

Re: Magnetic field strength diminishes proportional to inverse cube or inverse square

It depends on the source of the field and how close you are to it.

A "monopole" field goes like $1/r^2$. 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 $1/r^3$. This is what you get from a current loop or a bar magnet, when you get far enough away that it appears "small."

3. Nov 23, 2010

### granpa

4. Nov 24, 2010

### magnetics

Re: Magnetic field strength diminishes proportional to inverse cube or inverse square

Thank you jtbell, that makes a lot of sense now. Could you possibly point me to a reference book/article that defines this?

Also in practical terms for a permanent magnet, say rare earth with Max. Energy Product of 40MGOe how long would it have to be before it's capable of taking on the properties of a monopole at each end?

5. Nov 24, 2010

### granpa

Re: Magnetic field strength diminishes proportional to inverse cube or inverse square

dont forget about the magnetic field of a single electron moving through empty space

6. Nov 24, 2010

### LURCH

Re: Magnetic field strength diminishes proportional to inverse cube or inverse square

That tends to be a matter of practical application. To be treated as a monopole, one need only be so close to one pole that the effects from the opposite pole are "negligable." Where that threshold lies is up to the individual and usually dictated by the purpose for which the magnet is being used or examined.