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Force between two bar magnets.

  1. Mar 24, 2014 #1
    1. The problem statement, all variables and given/known data

    You're given two bar magnets as shown below. In the gap between the two bar magnets, there's a bit of copper that's 1mm thick. The area of the bar magnet is 2.5cm2, and the flux density is 0.5T. Compute the force between the two bar magnets.

    attachment.php?attachmentid=67956&stc=1&d=1395674521.png

    2. Relevant equations



    3. The attempt at a solution

    So the force is given by this formula:

    F = B * H * A

    H = [itex]\frac{B}{µ0}[/itex] = [itex]\frac{0.5T}{4∏ * 10^-7}[/itex] = 397887

    F = 0.5T * 397887 * ##2.5cm^2## = 497385.

    The answer I get is about 20 000 times bigger than what I'm expecting.

    Where am I going wrong?

    F is the force.
    B is the flux density in the air gap.
    H is the field strength in the air gap.
    A is the area in the air gap.

    I'm sorry for the use of the wrong symbol, but ∂ was the closest I could get.

    Also, I realized that I have a gap filled with copper instead of air, just as I was about to post this. I have no examples involving gaps with anything else than air as far as I'm aware. I will keep looking for this and update this post if I find anything.

    Any input is appreciated.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Mar 24, 2014 #2

    SteamKing

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    Staff Emeritus
    Science Advisor
    Homework Helper

    Well, for one thing, you are sloppy and inconsistent with the units of your calculation. Is area supposed to be in cm^2 or m^2? It makes a difference.
     
  4. Mar 24, 2014 #3
    I'm sorry for this. 2.5cm2 should be 0.00025m2.

    Also, I've overlooked the fact that my equation should contain 1/2.

    I therefore get 24.87N, and that looks alright.

    Thanks for your input, even if it's just to let me know I'm not doing my work properly.
     
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