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Electromagnetic poles- repelling

  1. Mar 3, 2012 #1
    Hi I'm working on a science experiment and I need two electromagnets to push away from each other, but the two small electromagnets i've made (using two bolts wrapped in magnet wire and a D battery) just attract each other no matter which ends I put together. Am I doing something wrong? Do I need more power? Please help. Thank you!
  2. jcsd
  3. Mar 5, 2012 #2


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    This is interesting... Have you tried out all the possibilities? Let's say magnet A has the ends A1 & A2 and magnet 2 has the ends B1 & B2. Did you try out ALL the following possibilities:"A1 to B1", "A1 to B2", "A2 to B1" and "A2 to B2" WITHOUT disconnecting the battery when turning the magnet? If yes and if the magnets attract each other in ALL configurations, could you upload photos of the magnets (with their batteries) sticking to each other in the "A1 to B1" and the "A1 to B2" configuration?
    Last edited: Mar 5, 2012
  4. Mar 5, 2012 #3

    Philip Wood

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    Gold Member

    Forgive me if this sounds patronising, but...
    (1) did you use at least 50 turns of wire on each?
    (2) was the wire insulated? [It's usual to use enameled wire – with the enamel scraped off at the ends, to make the connections.]
    (3) was the current flowing in the same sense through each turn (e.g. clockwise when you looked from one end of the bolt?
  5. Mar 5, 2012 #4
    Remove the bolts and try again. If you have enough turns, even an AA battery can get two coils to repel each other without a core to concentrate field lines. This way you can see if the bolts are the problem. They may already be magnetized permanently.
  6. Mar 5, 2012 #5


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    Another idea: did you check that both batteries are working correctly? If one of them didn't work, you would only have one electromagnet and one simple bolt, and they would behave exactly as you described: attract each other in each configuration. (They same applies if the wire is contacted badly to the battery.) This may sound strange but (in my experience) when in a physics lab complicated lab equipment just doesn't want to work, chances are high that one hasn't plugged the electricity plug into the socket or that one hasn't switched the equipment on. :smile: One often tends to think of complicated problems and forgets the easy ones...

    An easy test you could perform is if you can lift a small steel object with your electromagnet and if it falls down when you disconnect the battery. If this works with both electromagnets, you could rule the above possibility out. Then, photos of your electromagnets would be helpful for further advice.
  7. Mar 14, 2012 #6


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    Dear Rozy94, did you find out why your two electromagnets were always attracting each other? If yes, please tell us, it's really interesting! Thanks.
  8. Mar 14, 2012 #7
    This is interesting and I had never thought about it.

    It could be that one of them has a much stronger field than the other. In this case it reverses the magnetization of the weaker one when placed close near one another and an then absorbs it like a piece of iron.
  9. Mar 20, 2012 #8
    Ok so here are the pictures of the two, it doesn't attract very strongly in the second picture where the two bolt heads are touching so I might just have a battery problem, but if someone sees another problem from the picture please point it out! And the wire is enameled with the ends sandpapered for the connection. Thanks again!!

    Attached Files:

  10. Mar 20, 2012 #9
    @ chrisbaird
    So would the bolts being permanatly magnetized mess this up? Even if it was just slightly? And if so do you know where I could find a regular bolt or how to demagnetize this one, if that's even possible>
  11. Mar 20, 2012 #10
    First of all, this is not the right way to feed a coil. You have no control over the current except for the resistance of the wires. One wire seems to have smaller diameter than the other, and the lengths may not be equal.

    Second, the bolts have different cross-sectional areas. With the same ampere-turns, the flux density in thinner bolt is stronger.

    My suggestion: Take two similar bolts( the nail one is better), and make two coils of equal turns around the bolts. Connect them in series in a way that you get the same poles on the bolt heads. Now you have equal currents in the coils. Measure the resistance of the two coils in series, if it is large enough to limit the current bellow the nominal current of the battery, then no current controlling device is required. If the resistance is small, add a proper resistor in series with the coils. Now the bolt heads are expected to repel one another.
  12. Mar 20, 2012 #11
    Yes, the magnetic field of permanently magnetized bolts could overpower the magnetic field of the coils. The easiest way to check this is to just disconnect the battery so there is no current through the coils and see if the bolts still attract each other.
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