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# Electrical Engineering/Stacking an electromagnet on a permanent magnet

Question

Magnet stack
QUESTION: Hello Cleggsan,

I hope you are genki. I was wondering if I could get your sober, rational comment on my latest wild and fanciful idea. I THINK I have reasoned it out correctly but....
I have a design for an electromagnet that I need to make more powerful. I can't increase the number of turns or increase the level of current, but I was wondering what the effects might be if I stacked the electromagnet on top of a permanent magnet.
"Stacking two identical magnets will behave similar to a solid magnet of the same dimensions as the two stacked magnets." How might this work if the electromagnet is placed on top of the permanent magnet? When I activate the electromagnet, will the magnetic field it produces include (some of) the permanent magnet's field as well? Could this be used to increase the strength of my electromagnet?
I have drawn a bar magnet under an electromagnet, under a piece of soft iron. Each component is separated by a layer of plastic to stop/reduce the field of the permanent magnet from magnetizing the other objects while the electromagnet is inactive. If the current from the electromagnet is weak, will the overall magnetic field be weak at the point of the soft iron object, or will the permanent magnet's comparatively stronger field be used to increase the overall magnetic field produced by these three things? (I find it hard to explain myself clearly when talking about magnets - sorry)

Thanks, as always - Domo

Best regards,
Eddie

ANSWER: Magnetic fields are magnetic fields and are regardless of the source of the field.  And, magnetic fields are additive, that is, if they are phased correctly, as opposed to being out of phase, they will add up linearly minus losses from distances separating them.

In your idea the only thing you must be worried about is when the fields reverse for whatever reason the anti field could cause demagnetization of the permanent magnet.  You must decide if that is going to be a problem in your scheme.

By the way, lots of the questions you are asking are similar in nature to what hard disk drive makers are facing as they design storage drives for computer use.  Have you consulted this field of endeavor?

Mucho Gracious

---------- FOLLOW-UP ----------

Magnet stack
QUESTION: Arigato Cleggsan,

I have been looking at hard disk drives since you suggested that, thank you. I never knew about that at all - I will look into it more.
Reversing the fields is not a part of this design, so I am thinking there should be no danger of demagnetizing the permanent magnet.

Now, I am trying to calculate the magnetic field at the end of the electromagnet, labelled Point B in the attached image, and I was wondering if you could tell me whether I am going about it in the correct way.
I have imagined that when the electromagnet is switched off, the magnetic field at Point B is equal to the magnetic field at Point A minus losses from the distance between point A and B, (i.e. quite weak) would this be correct?
How about when the electromagnet is activated? Here I am not sure whether the field at Point B would be equal to

The field at Point A + the field at Point B minus the loss from the small distance of the plastic in between the top of the permanent magnet and the bottom of the electromagnet

Or

(The field at Point A minus the loss from the distance between Point A and B) + the field at point B

It seems to me that the first option will be a much stronger field than the second option. Have I reasoned this situation correctly, or am I making a mistake/mistakes?
When the electromagnet is generating a magnetic field, will the permanent magnet's magnetic field at Point B experience the same loss from the distance as it does when the electromagnet is inactive? Can the active electromagnet effect/reduce this loss from distance experienced by the permanent magnet?

Thank you again and best regards,
Eddie

iron filings
The strength of a magnetic field reduces at the rate of the square of the distance. In order to get the best efficiency from magnetic circuits is to have the sources and fields as physically close together as possible.

The plastic spacer between the two sources should be as thin as possible.  The coils should be compact in order to keep the fields as tightly coupled as possible.

The high school lab experiment where iron filings are sprinkled on a sheet of paper and a magnet is slowly pulled up from underneath as the filings shape into the magnetic field shape. It is instructive in what happens with the concentration near the center of the field and how rapidly the field diminishes with distance.

Electrical Engineering

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#### cleggsan

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