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Calculation of a magnet

  1. Sep 24, 2015 #1
    Hi. Can somebody make a calculation for me?
    how strong do a magnet need to be to lift a mass of 75 kg iron 1 meter straigth up in 1 second.
    Best Regards morten
  2. jcsd
  3. Sep 24, 2015 #2


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    It cannot be calculated just like that, because it depends on the shape of the iron and the magnet ( actually the shape of the airgap ).

    So correct answers could be 1T or 100T, depending on the shapes.
  4. Sep 24, 2015 #3

    Hi Hesch and thanks
    Can you explaine it a little more, perhapse with some calculations so I can understand it?
  5. Sep 24, 2015 #4


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    The attraction is not due to some "magnetic poles" inside the magnet, but due to magnetic energy density in the airgap between magnet and iron, which is:

    Edens = ½*B*H [ J/m3 ].

    Thus the energy: E = Edens*(volume of airgap) = Edens*(cross section area)*(width) of airgap = Edens*A*s.

    The force: F = dE/ds = Edens*A.

    Is the width of the airgap 1m in the starting position, or are magnet/iron closed up all the way ?
  6. Sep 24, 2015 #5
    Can you calculate it with both airgap 1m and with the magnet/iron closed up.
    It would really be a big help.
    Do you have a email or facebook I can contact you? It seems like you know you way around this stuff,and I have some psysics-questions i prefer not to ask in a open forum
  7. Sep 24, 2015 #6


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    Say that the total area, A = 0.1m2, then

    Fdens = m*g / A = 7370 N/m2

    Edens = ½*B*H = 7370 [ J/m3 ] →

    B*H = B2 / μ0 = 14730 [ J/m3 ] → B = 0.136 T.

    Now, if s = 1m, try to calculate the H-field needed ( B = μ0 * H ). Then calculate the amount of ampere-windings needed using Amperes law:

    circulationH⋅ds = N * I.

    You should use a -shaped magnet.
  8. Sep 24, 2015 #7


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    PS: You will need about 216000 ampere-windings by a distance = 1m ( × 2 airgaps ).
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