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Well I need to create a reasonably powerful electromagnet and I am in

  1. Mar 24, 2012 #1
    Well I need to create a reasonably powerful electromagnet and I am in Gr. 12. I havn't been able to find a proper guide/book to create a electromagnet.

    I plan on using a ferrous core and 38 gauge wire. I am going to wind the wire around a spool.

    My questions:

    What should be the diameter and length of the spool?
    How many turns should I wind up?
    What should I use as my power source?

    Thank you for reading this, your help is appreciated :)
    Last edited: Mar 24, 2012
  2. jcsd
  3. Mar 24, 2012 #2


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    Gold Member

    Re: Electromagnet

    Roughly speaking, the more ampere turns you have, the more powerful your magnet will be. So, more turns and more current are the name of the game.
  4. Mar 24, 2012 #3
    Re: Electromagnet

    Electromagnets made in the solenoid geometry are relatively weak compared to magnets in the "C" geometry or "H" geometry, because the field lines in air are relatively long. Electromagnets using ferrite rather than soft iron are relatively weak because of the relatively low permeability of ferrite. Ferrite is used at high frequencies where a laminated iron design is not suitable. For dc magnets, solid iron is fine.

    This shows a C geometry electromagnet (but not an efficient design). The two grey boxes represent the coil cross section.


    The magnetic field is proportional to the number of amps I and the number of turns N, or NI (amp-turns). Available power supplies are usually in the 1 amp to 5 amp range, with a wide range of voltages. A good conductor for a small magnet is probably 18 Ga enamel or formvar coated, because its current rating is about 2.3 amps. Larger magnets should use a lower gauge (larger diameter). 38 Ga. is a very poor choice (22 milliamps rating). See Table at


    In the end, the current limit will be lower than in the Table, because the real problem will be getting the heat from resistance (I2R) losses out.

    In the image of the C magnet above, if the height of the gap is g, then the magnetic field in the gap is (from Ampere's Law)
    [tex] B=\frac{\mu_{o}NI}{g} \space \space \space Tesla [/tex]
    where μo = 1.26 x 10-6 in mks units.
    How big a volume of magnetic field do you need?
  5. Mar 24, 2012 #4
    Re: Electromagnet

    I need to a pull a magnetic ball 15cm away; ball is 1 cm in radius.

    What type of power should I use? 9V battery would be sufficient? what else can we use?
  6. Mar 24, 2012 #5
    Re: Electromagnet

    In your case, a solenoildal geometry is best. See magnetic field formulas at
    Us a 1/2-inch diameter steel rod or bolt, about 6-inches long.
    I would use 20 Ga. wire (10 ohms per 1000 feet), which is good for 1 amp. I would use 2 "D" cell batteries in series, and a 300-foot long wire (1 pound), available on eBay for $20.00. If you are really lucky, you could buy a 1-pound coil of 20 Ga. "magnet wire" on a plastic spool like this one,
    http://www.ebay.com/itm/Magnet-Wire...tDomain_0&hash=item2a1a96cc0b#ht_778wt_931and find both ends of the wire accessible. In this case just stick the steel rod inside.

    [added] Ths formula is more accurate
    Last edited: Mar 24, 2012
  7. Mar 24, 2012 #6
    Re: Electromagnet

    umm the one you listed as a spool how would i be able to find both ends? and how many 2d batteries should i put in series? i can buy one off local store which is 0.683433013 in a spool would it be sufficient?
  8. Mar 24, 2012 #7
  9. Mar 25, 2012 #8
    Re: Electromagnet

    Thank you Bob. I was wondering how long would 2 D cells last?
  10. Mar 25, 2012 #9
  11. Mar 25, 2012 #10
    Re: Electromagnet

    This is really easy to do.

    diameter- atleast .5 in.
    length- 3-4 in.
    power source- atleast 9V

    The main thing is to wrap the wire as tightly as possible and to get as many turns around a single point of the base of the spool. The more you have the stronger it will be. Also, make the base of the spool hollow and fill it with either nails or a bolt, which will enhance the magnetic field strength.
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