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# Physics/Magnet powder in an electromagnet

Question
Hello Steve,

How are you? I hope you are doing well and feeling healthy.
A while ago you were kind enough to answer a question I had on using a loose powder of magnetic grains of Neodymium as a core material for an electromagnet. http://www.allexperts.com/user.cgi?m=6&catID=1358&expID=70048&qID=4904706
I have been thinking more on this, and I was wondering if I could ask your opinion on a couple more points.
Supposing I have a plastic cylindrical container filled with a fine grain magnet powder of Neodymium granules. Around the cylinder is a copper coil. On top of the cylinder is a permanent magnet. The permanent magnet attracts the grains of magnet powder (or at least the grains closer to the top I assume) and the permanent magnet is held in place.
I want to power on the coil to produce a magnetic field that aligns the Neodymium grains in a way that would then repel the permanent magnet, and I am interested in the factors affecting this operation. I want to calculate/estimate how much force needs to be generated by the coil and I guess I have two questions.

1. I am thinking about how many turns the coil needs to have, and how much current needs to be put through it etc. Calculating an electromagnet's field is obviously very different for an air core and a ferrous metal core. What about this electromagnet? Can I treat this magnet powder core electromagnet as the same as a standard iron core electromagnet in terms of having a ferrous core that amplifies the magnetic field of the copper coil?

2. Let's suppose the permanent magnet on top of the Neo powder has a surface field of 1000 gauss. Let's also assume that the Neo powder, when the magnetic fields of all granules are aligned, has a magnetic field much stronger than this - strong enough to repel the permanent magnet.
In order to cause all the particles of Neo powder to align in a way that repels the permanent magnet, does the coil need to produce a field greater than 1000 gauss? In the case of a standard soft iron core electromagnet with a permanent magnet attracted to it while the coil is powered off, I am thinking that the electromagnet needs to produce a field greater than 1000 gauss to repel the permanent magnet. However I am wondering in this is necessarily true for the magnet powder core electromagnet. The permanent magnet on top of the powder is keeping at least some of the powder aligned with its 1000 gauss field, but in order to move/flip around the granules of powder, is a 1000 gauss field required from the coil, or can some of that field be supplied by the magnet powder graules?
Supposing the grains at the top of the container near are attracted to and held in place by the permanent magnet, but the grains at the bottom are not. Is it reasonable to assume that activating the coil would flip these previously un-aligned grains more easily, and the resulting alignment of these grains would add to the force of the magnetic field being created by the coil?
The idea is for the grains of permanent magnet material to do more of the work, so the coil does not need to have so many turns and/or use so much current. Is this a flawed reasoning do you feel?

Thanks Steve for all your help in the past. I profusely apologize if my writing is unclear. My thoughts on this topic are a little unclear I guess!

Eddie

Hello Eddie,

1. Can I treat this magnet powder core electromagnet as the same as a standard iron core electromagnet
My Answer: The effectiveness of core materials varies. We don't know that much about the Neodymium powder. Check out the Relative Permeability data in the table in this wikipedia site
https://en.wikipedia.org/wiki/Permeability_(electromagnetism)
Notice that the table's Medium column includes a Neodymium magnet. And the score in the Relative Permeability column is quite similar to that of an air core. But this isn't powder. I expect the powder to be much better than air. Does the literature about the powder include data about permeability?

2. does the coil need to produce a field greater than 1000 gauss?
My Answer: If the permanent magnet and the Neo powder core are close, nearly touching, so the strength of the permanent magnet's field is 1000 gauss at the nearest of the Neo powder, yes. The electromagnet would need to generate a field greater than the 1000 gauss.

can some of that field be supplied by the magnet powder graules?
My Answer: Not if those granules are oriented in the wrong direction. They will continue to point in the wrong direction.

Is it reasonable to assume that activating the coil would flip these previously un-aligned grains more easily, and the resulting alignment of these grains would add to the force of the magnetic field being created by the coil?
My Answer: My expectation for the condition of the grains at the bottom before activating the electromagnet would be similar to the grains at the top. I believe the permanent magnets field would be channeled through the grains of Neo much the same as if you had an iron bar in contact with the permanent magnet. The end of the iron bar is magnetized because the bar channels the field that would otherwise have curved around to the other pole immediately after emerging from the magnet. I expect the cylinder full of Neo powder would do much the same thing so that the entire mass of the powder would be aligned as well as the closest powder.

Remember Eddie that I have warned you that the questions you have presented me with are way outside my expertise.

Yes, I am feeling well now. Again I was slow with this reply, sorry.

I hope this helps,
Steve
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thanks Steve! I am sorry for being late to add this comment. I appreciate the time you have obviously taken in considering my question. Your answers are always a model for the type of reasoned, logical thinking that I try to follow. Many thanks.

Physics

Volunteer

#### Steve Johnson

##### Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

##### Experience

I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

Education/Credentials
BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University