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# Electrical Engineering/Altering the permeability of an electromagnet

Question

Electromagnet Field Di
Hi Cleggsan,

I hope this message finds you genki.
I was wondering if I could ask you about the factors determining the strength of an electromagnet, specifically the permeability value that is put into the equation
Magnetic field = permeability x turn density x current

I am interested in the way the permeability value is arrived at for this calculation. It's my understanding that the permeability of material at the core of the electromagnet is multiplied by the permeability of free space to get the relative permeability that is then put into the equation. eg Iron permeability 200 x permeability of free space = relative permeability = 200 * 1.25663706* (10^(negative 6)) = 0.000251327412 (Am I correct so far?)

"An inductor with a closed-loop core can have a higher magnetic field...than similarly constructed coils with a straight core...because the entire path of the magnetic field lines is within the high permeability core, while in an inductor with a straight core the magnetic field lines emerging from one end of the core have a long air path to enter the other end."

I am interested in this idea of the "long air path", and I am wondering what the possible effects might be of positioning some high permeability metal rods parallel to the electromagnet's core. Would it be possible that the path of the electromagnet's magnetic field would then flow through these rods as the field lines return to the other end of the core? I read on a website about magnetic shielding that "At kHz frequencies or lower, it is generally necessary to use permeable (magnetic) materials (μr>>1) to divert magnetic fields. Since these materials have a reluctance much less than air, magnetic field lines can effectively be rerouted by providing an alternative path through a permeable material such as steel or mu-metal." (http://www.learnemc.com/tutorials/Shielding02/Practical_Shielding.html)

If this applies here, could I divert the electromagnet's magnetic field as it travels through the air through the metal rods on the return journey to the other end of the electromagnet? (I understand that the rods will be attracted to the electromagnet, but I imagined these rods held in place by some kind of plastic housing so that the distance between the rods and the copper coil of the electromagnet remained fixed).

I have never heard of this being done to electromagnets, but I have not been able to figure out why it would not work. Am I missing something here? As always, your comments would be much appreciated.

Best regards,
Eddie

You cannot change the permeability of a magnetic material.  You can alter the magnetic field wherein the magnetic material is part of the circuit.  In magnetic circuits where there is open air around any part of the magnetic material if a material that has conductivity of magnetic fields the circuit will be modified due to the addition of the field disturbing material.  The position and size and permeability of the metal object(s) placed in the circuit can focus, scatter or other cause distortions and shape changes to the field in the air.

In a magnetic circuit employing electric coils the number of turn of wire, material of the form in which the wire coil is wound, the materials or lack thereof, all effect in some way the field shape and geometric properties of the field pattern.

Does this help.

Electrical Engineering

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#### cleggsan

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