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Electric Magnet - equations

  1. Apr 14, 2006 #1
    I am interested in building electric magnets. I have tried without success to find equations on what will make a stronger magnet. If I double the number of wraps, will the magnet be twice as stong? If I double the volts, will this make the magnet twice as strong? Does the type/gauge of wire impact the strength of the magnet?

    I have built a few simple magnets to date and they work, but now I am interested in building even stronger magnets but I am not sure what I can do to maximize the strength. Any thoughts or links would be greatly appreciated. Thanks!
  2. jcsd
  3. Apr 14, 2006 #2


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    Staff: Mentor

    The magnetic strength is linearly related to the number of turns and the current. The current comes from the drive voltage and the resistance of the coil, I=V/R. You might try howstuffworks.com to look for info on how electromagnets work:

  4. Apr 14, 2006 #3
    More info.....

    I originally created an electric magnet using a 12-volt drill battery and a coil with 90 wraps. The second time I used the same battery and used a coil with 500 wraps. My second attempt did not seem to be any stronger - I used a hanging paperclip from 1/2 inch away to measure field strength. I expected the second attempt to be considerably stronger (over 5X as strong).

    The only other difference is that the first attempt used 14 gauge copper wire and the second attempt used 20 gauge copper wire. Would the wire gauge be that big of an issue?
  5. Apr 14, 2006 #4


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    Staff: Mentor

    -- Be careful shorting out a 12V battery with a coil of wire. The resistance of the wire is pretty low until you get a lot of turns. Calculate the resistance of your coils and stay within the rated current specs of the battery.

    -- Wrap the coils around an iron rod if you aren't doing that already. That will give you the biggest increase in pulling power.

    -- When doing these experiments, put a current meter in series to measure the coil current. Be sure to use a high-current setting on the current meter, so that not much series resistance is inserted by the meter. (A DVM on the 10mA setting inserts many Ohms of resistance in series with the measurement....)
    Last edited: Apr 14, 2006
  6. Apr 14, 2006 #5


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    Gold Member

    When you changed the wires, you changed the resistance. Since a 20 gauge wire has a smaller diameter, it allows less current to flow through, hence a weaker (magnetic) field. In order to effectively compare the two electromagnets you made, you should have used the same 14 guage or 20 gauge.

    EDIT: berkeman got to it while I was typing
  7. Apr 14, 2006 #6
    Thank you for the posts. I guess I am looking for an equation that will tell me that if I want to double the strength of an electric magnet and I am changing the wire gauge, how many wraps to I need in the coil?

    One other note...I understand that wrapping the wire around an iron rod will maximize the megnetic field. I am wrapping my magnet around a concrete cylindar with a 1 5/8 inch diameter that has a very high salt content. Are there equations that will tell me the impact to the magnetic field depending on the material that the coil is wrapped around?

    Thanks again for the info.....
  8. Apr 14, 2006 #7


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    The magnetic field (for a solenoid) depends on the number of turns, the current through the wire,the permeability of the material that the wire is wrapped around (core), and the length of the wire.

    For a solenoid that has no core(just air), the magnetic field is found by:

    B = (N*μ0*I)/L

    where N = number of turns
    μ0 = permeability of vacuum
    I = current
    L = length of wire

    Since you are using something other that air as the core, the magnetic field is found by:
    B = (N*μ*I)/L

    where u = K*μ0

    K is the relative permeability of the core material.

    Iron core solenoid
    Magnetic Properties of Ferromagnetic Materials
    Last edited: Apr 14, 2006
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