General > General Technical Chat

Please remind me about magnetic field strengths, Tesla conversion from Newtons

(1/1)

Infraviolet:
I should know this, I did a physics degree many years ago, and it should be easy to find the right equations online, but I can't seem to find the right search term.

I need the equations to convert from magnetic pull strength (usually given in kg, clearly in reality a measure of kg*9.81 to give N) force to get me an understanding of the magnetic flux density (mT). Just like for gravity you have a mass, and for electrostatics a charge, I want to work back from the pull strength to get a magnetic "charge" which I can then drop in to an inverse cube law and get the milliTesla field strengths at various distances along the axis from a disc shaped magnet.

Can anyone remember the equations? Is there an extra variable I'd need to supply from somewhere which means that knowing the pull strength in newtons at the magnet's surface, and the distance of this surface away from an imaginary point at the magnet's centre where one can pretend the dipole resides, won't be enough to let me derive the mT flux density with distance graph?

Thanks

berke:
The magnetic field drops as $$r^{-3$$ only because it's a dipole field, it results from two nearby opposite and equal charges (monopoles) cancelling eachother.  The field for a cylinder magnet is non-trivial but apparently closed forms are known, this paper https://arxiv.org/pdf/0909.3880 gives the equation for the field of a cylindrical magnet in terms of a special function C and provides a numerical procedure for computing C.  At a sufficient distance you should be able to pretend that your magnet produces a field indistinguishable from a dipole field.  In that case you want to know the magnetic moment.  The magnetic field of a dipole is given in eq. (5.56) of Jackson (Classical Electrodynamics) as $$B(x) = \frac{3 n (n \cdot m) - m}{\|x\|^3}$$ where $$n$$ is a unit vector such that $$x = \|x\| n$$ and $$m$$ is the magnetic moment.  Hope this gets you back on the right track.

Navigation

[0] Message Index

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod