# Calculating magnetic field strength (complex)

okay guys, i think im out of my depth on this problem.

Im doing a long investigation on eddy currents, and i derived a formula. Im reaching the end of the investigation, and so i wanted to compare my experimental data to expected data. One of the factors i was investigating is the relationship between the terminal velocity of a magnet and its magnetic field strength in the scenario where a magnet is dropped down a copper tube.

However, as the magnet was too strong for my magnetic field measuring probe to measure, i measured it at 1 cm away from the magnet.
Is there any equation that can determine the actual magnetic field strength of a magnet, from the magnetic field strength of a magnet at 1 cm away.

Also, just as a side note, is there any way to determine the height of eddy currents algebraically. I know that the height of the eddy currents is the cross sectional area of the eddy currents divided by the thickness of the conductor. (ie h=A/z), but how do you determine the cross sectional area of the eddy currents.

Thanks so much!!!

Um, guys, i really need your help :)

This is a point most textbooks don't even touch.

I've once read that if you have a small magnetic dipole, the field will follow an inverse-square law. But I'm not sure.

About calculating eddy currents, you would need more details such as how the magnetic field is distributed - and this will probably be complicated. Remember that even for the simplest cases (spire, solenoid...), the equations are only approximations.

I really wish I could help you more, but I'm not an expert in this subject.

Is there any equation that can determine the actual magnetic field strength of a magnet, from the magnetic field strength of a magnet at 1 cm away.
Here is an on-line magnetic field and force calculator that may help. There are also other on-line calculators on the web if this is unsatisfactory.

http://www.kjmagnetics.com/calculator.asp

Bob S

You probably wont be able to do this , but ill tell you so can learn what it is .
you could put your magnet near a gas discharge tube of lets say neon , and the magnetic field will alter the electrons orbital a little bit and cause a shift in the spectral lines
and the energy shift in the light would be the field strength multiplied by the bohr magneton , this is called the zeeman effect . and you would haft to measure the wavelength of the light .
and your bar magnet might not be strong enough to get any size-able shift .

@cragar:

Despite the practical problems (such as cost), this is the first time I've heard Zeeman effect could be used backwards.

Really cool!

I think this is how astronomers measure the B field strength of stars , by looking at the shift in the light .