Nuclear Magnetic Resonance (NMR)

[kajunbee]

Yes, a very interesting experiment and a wealth of knowledge to be gained. I've been doing research on nmr so as to better follow what is being discussed. This lead me to some other questions.  So as I understand the precession frequency is related to the strength of the static b field. And the static field will be provided by disk shaped magnets. Itsu stated that the precession frequency is 4.2 MHz. So I found a Larmor frequency calculator online and it gives a field strength of .1 Tesla. So if this is correct how do you determine the strength of the magnets that you will use. Is the magnetic field strength between 2 magnets additive. Meaning if one is 900 gauss and the other is 1000 gauss would you say the field strength is 1900 gauss or half or something in between. Also how does the separation distance effect the strength and uniformity of the field. I ask because according to the calculator a small deviation in field strength greatly influences the frequency. Hopefully this post makes sense to you and I'm not totally misunderstanding.

[Smudge]

Hi kajunbee,

Ferrite magnets have been chosen because they will give a field strength of 0.1 Tesla at the water sample.  To establish the field strength that a given magnet will provide you use the known remanent field characteristic of the magnet material (B_rem) then create a load line on it.  That load line is related to the demagnetizing factor for the magnet shape.  That only gives you the surface field at the magnet poles, so you need more calculations to determine the field at some distance, and since the magnet poles are not point sources you cannot use the often quoted inverse square law relationship with distance.  This whole process is very complicated so the easiest way to do it is by the use of finite element programs that are available.  For the annular disc magnets we should really use a 3D program, but as I don't have one I used a 2D program called FEMM.  Putting in the magnet dimensions and spacing and the ceramic characteristics this yielded the 0.1 Tesla value.  As to your question, is the magnetic field between two magnets additive, the answer is yes but with qualification.  If two magnets close together have opposing fields then there is the possibility that they can partially demagnetize, in which case you can't then simply subtract the original fields.  This doesn't happen with NdFeB magnets because they are difficult to demagnetize, but force a ferrite magnet close to a NdFeB one and almost certainly the ferrite one will demagnetize or even get reverse magnetized.

Smudge

[ramset]

working thru inevitable sorting of unique test protocol to establish procedure [shared with permission of builders]

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