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Physics/The behavior of C core electromagnet in relation to a permanent magnet

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C core electromagnet and permanent magnets
C core electromagnet a  
Hello Steve,

Thank you again for all your answers - they have helped my understanding a lot.
I was wondering if I could ask you another question today about the behavior of magnets in relation to each other.
I have been using a program called FEMM to run a simulation of an arrangement of magnets that has given me some unexpected results. My knowledge of this program is pretty basic, so I am not sure if the results are due to my lack of proficiency with the program, or are based on a deeper misunderstanding of the magnet's behavior. I have posted a question on a FEMM forum regarding the use of the program, but I was wondering if you might be able to give your opinion on the physics of this particular magnet set up, so I can see if I am on the wrong track in the first place.

As you can see in the attached images, I have an electromagnet with a kind of C shaped core. In the air gap is an object containing two axially magnetized permanent magnets, both stuck to a piece of plastic between them, fixing them in place in relation to each other. This magnet-plastic-magnet object is contained in a tube which allows it to move up and down (but not flip around).
The idea is, when the electromagnet is switched on in the way shown in Pic 1, the lower pole of the electromagnet will repel the bottom permanent magnet while the upper pole of the electromagnet will attract the top permanent magnet, resulting in the object moving up. When the current and field is reversed in the electromagnet, as in Pic 2, the upper pole of the electromagnet repels the top permanent magnet and the lower pole of the electromagnet attracts the bottom permanent magnet, moving the object down.

Is this what would actually happen do you think?
I have based my reasoning on the fact that opposite poles attract, and like poles repel. Have I applied this correctly here? Would the magnets behave as I have imagined?
In the program I am using I am unable to specify which parts are free to move and which are fixed in place, so the two permanent magnets effect each other and make it harder to clearly determine what would happen.
I would really appreciate your opinion!
Best regards,
Eddie

Answer
Hello Eddie,

An ideal C shaped electromagnet would generate a uniform magnetic field. A single bar magnet inserted into a uniform magnetic field would have a torque on it to align it like a compass needle (assuming it was not initially aligned). But it would not have a translational force on it. The 2 poles of the bar magnet would have forces on them of equal strength and opposite direction. So translationally, the forces would cancel.

Reference:
http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1164&context=physicska
Go to section 29-4.

I'm picturing the interactions of your 2 bar magnets with the electromagnet's field individually and I come to the conclusion that neither would have a net upward or downward force on them. No real electromagnet has ideal characteristics, but I expect the force on your object would be at best minimal.

Sorry for the delay. I've been having some health issues.

I hope this helps,
Steve

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Steve Johnson

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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.

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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

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