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Quick Electromagnet question

  1. Jun 1, 2012 #1
    I have another question, what would happen if i used a cylindrical permanent magnet as the core for an electromagnet. Assuming the poles of the solenoid are in matching direction, will the magnet field be stronger? What happens if the poles of the solenoid are in conflicting directions with the permanent magnet?

    any help would be really appreciated!
  2. jcsd
  3. Jun 1, 2012 #2


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    Assuming that the electromagnet's field is not strong enough to change the permanent magnet, the two fields, at any point, will add vectorially, as fields tend to do. So you can end up with mutual total or partial cancellation of the fields at most locations.
    I remember reading (In New Scientist many years ago?) of a clever variation on the scrapyard electromagnet system which used a strong permanent magnet. An equally powerful electromagnet was wound round it which was pulsed for a short time, to make the permanent magnet 'let go' of its load. It used no current for most of the time (making the wiring and running costs cheaper) and was fail safe. Big permanent magnets, these days, particularly, are not too costly.
  4. Jun 1, 2012 #3
    lets say if the electromagnetic field of the solenoid is twice stronger than that of a permanent magnet. Both poles are still in the same direction, are aligned at their central axis. (the solenoid is wrapped around the magnet, so both have the same length). What would happen?

    Would the total magnet field be the sum of the solenoid field + the permanent field? or is there some kind of special equation to find out?

    Would the fields be not match correctly and ended up reducing each other? if yes, then is there a special equation where i could find out how to match the fields evenly?

    I'm sorry to ask the same question twice. :( b/c i'm still a bit unclear, considering the common notion is that electromagnets works best with a ferrite core, but not much has been said regarding a magnetic core :(

    Thanks in advance!
  5. Jun 1, 2012 #4


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    The material of which a permanent magnet is made will, I believe, not have a high effective permeability at the levels of externally generated field strength that an ordinary coil would produce (or you could demagnetise the permanent magnet). Most electromagnets (used to at least) use soft iron cores with little hysteresis - exactly the opposite to using a permanent magnet core.

    I think the answer to your question would depend where you want to know the field strength. Like I said, I think you would be best to treat the electromagnet coil plus core and the permanent magnet as separate field generators and then add the fields vectorially. The electromagnet would actually need iron for its field to be comparable with that of the permanent magnet, I think. But then the iron core would represent a low reluctance path for the permanent magnet's field - so it clearly isn't that simple. The construction would need to be coaxial and most of the field would be concentrated between the poles of the permanent magnet and the torus of iron around it. No problem if you wanted to lift a car up but no good if you wanted a wide extent of external magnetic field.

    If this is just a thought experiment, then a very weak permanent magnet could be combined with a 'strong' air-cored electromagnet (loads of turns and loads of current) and then the fields would add up as I initially suggested, I am sure, because there would be no hi premeability around to distort the fields.
  6. Jun 1, 2012 #5
    Thanks for the answer! Now I have a much clearer picture in my head :)
  7. Jun 3, 2012 #6
    I've recently used a 3mm x 3mm cylindrical nib magnet as the button to turn the coil on and off. The button is located in the coils center and I've found no effect in regards to increase or decrease in field strength.
  8. Jun 3, 2012 #7


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    How sensitive have the measurements been in your setup? You would need a special arrangement of weak magnet and strong air cored electromagnet, I think. You probably have just the opposite situation.
  9. Jun 3, 2012 #8
    Does anyone know how to calculate the attraction strength of a permanent magnet to soft iron rod?

    This is the permanent magnet.

    The cross sectional diameter of the iron rod is 12mm, separated by a distance at 1/4" and 1/2". Both are restricted on an axis. Will changing the diameter of the iron rod change the attraction strength?

    Is there an equation for this?

    *I attached an image of my current setup.

    Attached Files:

    Last edited: Jun 3, 2012
  10. Jun 3, 2012 #9
    Not very sensitive at all. I determined that the rotor doesn't slow down or speed up with or without the PM, pemanent magnet, trigger/button. No, I have exactly that, an air core, single bifiliar wound, 1/2 sphere 200mm dia upper rim/equatoral and 12mm hole(air core) at the bottom southern most region. I would think that this setup, 3mm PM, isn't strong enough to have a adverse effect with my particular design.
  11. Jun 3, 2012 #10


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    It would be hard to do better than using the calculator on the link. The force will be very dependent on actual spacing.
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