Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electromagnet for attracting Non-ferrous metals

  1. Dec 2, 2011 #1
    Good morning:

    I'm looking for information on electromagnets for attracting non-ferrous metals. One of the kids on the block is asking for assistance in research for a science fair project. She has a 1951 pamphlet describing the effect. And I found a construction article, but very little explaining the physics.

    I won't do the project, but I will assist with the research. And it would be nice if I understood the physics. I fully expect to get asked for explanations. This subject didn't come up when I was going to school 40 years ago and it hasn't come up in the last 40 years working in the electrical power field.

    The building of a coil isn't the the main issue. The science fair part is to propose a hypothesis, and then test. And that means understanding the physics. I don't know enough about it to make any suggestions.

    Here is what I think I know so far:
    If one lays a copper washer on the face of an AC powered electromagnet, the magnetic field through the copper washer induces a current going around the washer. This circulating current induces a magnetic field looping through the center of the washer, opposite polarity to the electromagnet. This repels the copper washer from the electromagnet. This should be an application of Lentz's Law.

    Now if one puts two washers on the face of the electromagnet, both are repelled from the electromagnet. However, the two washers are attracted to each other. Their magnetic fields have the same polarity.

    So, if the first washer is mechanically fastened to face of the electromagnetic so it can not move, then the second washer will be attracted to the first.

    What I don't understand yet (of many things)
    How come the electromagnet doesn't push the second washer away?
    Is the attraction between the two washers more than the repulseive effect of the electromagnet on the second washer?

    I don't mind doing the reading. So If anyone has any references - that would be gracious.

    Oh, if my explanation is goofed up, I'd be interested in knowing that as well.


  2. jcsd
  3. Dec 2, 2011 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    I thought it was opposites that attract.

  4. Dec 2, 2011 #3
    If the info you seek is anywhere, it's in this book:

    http://www.lindsaybks.com/bks2/elmag/index.html [Broken]

    I used to have a copy of it years back, but no longer. I don't remember how much it explained about the physics.
    Last edited by a moderator: May 5, 2017
  5. Dec 3, 2011 #4
    Ice: the reasoning sounds reasonable

    Andrew: yes, opposites attract...that's what Ice meant...the fields in the washers are "the same" ...both having, say, the "north" pole towards the electromagnet and so the back of the first washer has its "south" in front of the "north" of the washer behind it....and so both fields get together the same way two magnets do.
  6. Dec 3, 2011 #5

    Oh...and about the second washer being attracted to the first one, instead of being repelled by the electromagnet...I wonder if this is always the case or depends on the magnitude of the field in the electromagnet...

    To start, we should probably consider that the field seen by the second washer is not the same as the first one...the second washer sees the summation of the electromagnet's field minus the field from the first washer...isn't it so? So, there is a weaken field there opposing it versus the attraction to the first washer...

    ...then again, would there be a point where the electromagnet is SO strong that it also repels the second washer? Or the stronger it is so is the field from the first washer? Is there a point where the first washer cannot produce enough current to continue to weaken the main field in the same proportion?

    ...also, the attraction force between magnetic parts gets stronger at reduce distances (gap) and weakens as distance increases...this plays in favor of the washers getting together...doesn't it?
  7. Dec 3, 2011 #6
    That is a page from the 1951 pamphlet. Although the one I read looks like it has been through the Korean War - Oh, maybe it did. It almost explains the physics.

    Here is a link to the construction article we have:

    I'm mostly looking for references on the physics.

    Last edited by a moderator: May 5, 2017
  8. Dec 4, 2011 #7
    I was hoping for a bit more collective knowledge. Thanks anyway.

    This forum is addictive and interesting. I did enjoy hanging out here for a couple of day. Thanks for that as well.

    Addictive or not, I've got to get back to work.

    I'll be back.


  9. Dec 23, 2011 #8
    Hey Ice, I'd be very interested to know more about your theory. Could you provide references for your theory about copper washers attracting each other with an AC electromagnet? I have read up on Lenz's Law, but can not find anything talking about two non-ferrous washers attracting each other with an AC electromagnet.

    Also, could you please provide specifications for the electromagnet? Such as voltage and possibly the easiest and most economical "parts" I could build said electromagnet from? I'm curious what size washers you would use, and if the material could be made from gold instead of copper; if so, do only like materials attract each other?

  10. Jan 25, 2012 #9
    Levi -
    I wasn't ignoring you , I just got back from work a few days ago.

    The references I have are the ones in post 6. I don't know enough about the physics to tell you if gold dust will be attracted. Just a guess, I assume you are interested in picking up the fines coming through a placer mine sluice that pass over the carpet. If so - you and 10,000 other miners. I can garrantee that if I had that figured out I wouldn't be discussing it here - it would be with my patent attorney (I wish :( But I don't.

    As I understand, the magnet shown in the post 6 references will attract any non-ferrous conductors. So yes, it would attract gold. What I don't know:
    What factors govern the size of particle that will be attracted. For the magnet shown in the references, I can see a bit of the physics in the attraction of a 1" diameter piece. I'm not seeing the physics for the attraction to aluminum dust.

    There are commercial non-ferrous seperators on the market. Do a Google search. They may use the same physics as this magnet and attract the non-ferrous metals. Or they could use a conventional electromagnet, which will repel the non-ferrous conductors. I have not done any inquiries, so I don't know what size particles they work on.

    Good Luck

    The Worm
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook