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# Physics/Ferrous metal attracting a magnet

Question

Magnet movement
Hello Steve,

A while ago you helped me with a question regarding a design I had - Thanks very much for that, it was really useful to me. I have made some chances to the design, and I was just wondering if you could tell me whether my understanding of the principle involved is correct.
My original design was a plastic tube containing an electromagnet at the bottom end. On top of the electromagnet is a permanent magnet. When the electromagnet is switched off, the permanent magnet is attracted to the iron core of the electromagnet and the permanent magnet is held in place. When the electromagnet is switched on it produces a magnetic field which repels the permanent magnet, lifting it up. The structure of the tube prevents the permanent magnet from moving to the side, or from moving more than a set distance away from the electromagnet.
The problem with that design was the power needed to push the permanent magnet up was too much for the coil of the electromagnet, and the current required would have caused to much heat if on for longer period of time. In order to deal with this, I have added a non magnetized ferrous ring at the opposite end of the tube to the electromagnet. The idea is that I can send a briefer pulse of current through the electromagnet, pushing the permanent magnet up to a point in space where the attraction of the permanent magnet to the ferrous ring will cause the permanent magnet to continue moving up, and stay at the top of the tube, attracted to the ferrous ring. In this way, the coil does not need to be charged for very long and the heat may be managed. Every element in the structure is fixed in place except for the permanent magnet.
Have I imagined the behavior of the system correctly here? If the permanent magnet is free to move, and the non magnetized ferrous ring is fixed, when the permanent magnet is brought closer to the ferrous ring, will the attraction between the magnet and the ring cause the magnet to move?

Best regards,
Eddie

Hello Eddie,

That idea looks reasonable. However I would have one suggestion. In your sketches, you indicate turning the electromagnet off when the permanent magnet has risen half of the distance to the ferrous ring. I would worry that intermittently the permanent magnet would fall back down to the electromagnet. There could be several causes. I would recommend extensive testing watching for that to occur. I suggest that you anticipate that you might need to maintain the current in the electromagnet longer, until the permanent magnet would definitely be captured by the ferrous ring. In fact, after you determine a pulse that seems reliable, that you increase it by 30% as a margin for success.

I hope this helps,
Steve
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thanks Steve, that is exactly the type of advice I was looking for. I will follow your suggestion re the 30%. Thanks again!

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