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# Physics/Calculating magnetic field strength in realtion to distance

Question
Hello,

I am wondering if you can give me some advice on a formula am am having trouble applying. I am trying to work out how magnetic field strength decreases at distances further away from the magnet. I found this formula on a physics forum (and wikipedia) that said

"In leading order the magnetostatic field far away from the sources goes like 1/r^3, where r is the distance from the source. The field falls off as 1/r^3"

I think I must be doing something wrong in my calculations, because I end up with a result where the field strength further away from the magnet actually increases!
I am wondering if you might be able to tell me what I have done wrong?
The formula said the field strength was in Tesla and the distance in meters so I calculated like this (the magnet is very small) -
7.29 tesla divided by 0.000075m cubed equals 7.29 divided by 4.21875e-13
7.29 tesla divided by 4.21875e-13 equals 1.728e+13

I am thinking that moving 0.000075 meters away from a magnetic maybe, possibly does not increase the magnetic field strength to 1.7 trillion tesla.

I have obviously gone horribly wrong. Can you tell me how to apply this formula in the case of a point in space 0.000075 meters away from a 7.29 tesla magnetic field?
Please bear in mind I have no Physics training/education at all (you may have deduced that already!)

Any advice you can give me would be greatly appreciated.

Thank you and best regards,
Eddie

PS The links to the pages where I found this formula are

and
http://en.wikipedia.org/wiki/Dipole

Thanks again

Eddie,

The formula for the magnetic field which says that it falls off as 1/R3 assumes that the point at which you want to calculate the field is reasonably far away from the origin. Since the distance at which you want to calculate your magnetic field (0.000075m away from the source) is very small this formula doesnt hold. Henc you will get a wrong answer if you try to follow this formula for calculating the magnetic field so close to the source.

Hope this helps.

Palash

Physics

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#### Palash Apte

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