Hi.

Is there a method of calculation for a magnetic field distant from the magnet itself ?

 

Say:

 

================================================X <--- long iron bar

N

magnet

S

================================================Y <--- long iron bar

 

 

A magnet at left end, of known field strength, intimately attached to polished surfaces, between long iron bars,

produces a magnetic flux between far right points X and Y.

 

How are the bars length a factor in the value of the flux at the far end ? For sure iron purity, spacing, thickness... have an influence too; but let's discuss only the length. Would there be a zero flux at an infinite length ?

For a high permeability material, ideally all of the flux would be in the material and the flux return would be at the end. Which is how magnetic clamps (that can be turned on and off, with permanent magnets) work.

No simple method, because the flux leaks over the whole length or the ferromagnetic bars.

 

You have to evaluate the reluctance of the circuit between the bars, and combine it with the magnet's behaviour, which can be simple (ferrite, Sm, Nd) or not (Alnico). It gets simpler if the bars have a high permeability and don't saturate.

 

If you're lucky you can neglect the flux around the magnet that doesn't pass through the bars, and the flux at the far end of the bars.

 

You've grasped here the difficulty of magnetic circuit design. It doesn't resemble an electric circuit, because no insulator is available here, so fields must be created right where they're needed. It resembles making electric circuits in the Ocean.

 

All designs depend on that. For instance, the coils of a transformer are concentric instead of using one iron leg each. Or the stator winding of a motor or generator is around the rotor winding or very near to the air gap, and not farther away where more room would ease the design. This is what Gramme design improves radically.

 

Type I superconductors would be DC "magnetic insulators" (see Meissner effect, for instance at Wiki) but only for inductions of few mT. In HF, conductors block magnetic fields, but the shallow currents that do it mean losses.

Understood !

Thanks, gentlemen.