# Calculation of a magnet

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1. Sep 24, 2015

### Ankhen

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. Sep 24, 2015

### Hesch

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.

3. Sep 24, 2015

### Ankhen

Hi Hesch and thanks
Can you explaine it a little more, perhapse with some calculations so I can understand it?

4. Sep 24, 2015

### Hesch

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 ?

5. Sep 24, 2015

### Ankhen

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

6. Sep 24, 2015

### Hesch

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.

7. Sep 24, 2015

### Hesch

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