# Repel forces of magnets

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1. Jul 14, 2015

### Stoney

Hi
Just wanted to know,

If I have two magnets of the same size with a pull force of say 30kg and use them to repel each other, the top magnet at a position close to the surface of the bottom magnet would be able to support around 30kg (I know repel force is not measured in kg). If I then replace the top magnet with a smaller one will it still be able to support around 30kg at the same position as it is still being repelled from the bottom one with around 30kg of force?

Thanks
Stoney

2. Jul 14, 2015

### Hesch

What do you mean by "smaller one"?
The force is proportional to the cross section area of the airgap between the magnets, if the B-field, H-field and distance are kept constant.

3. Jul 14, 2015

### Stoney

Sorry, say the bottom magnet is 50mm x 25mm x 10mm with a force of 30kg and the smaller top magnet is 50mm x 25mm x 3mm with a force of 9kg, would this top magnet still be able to support around 30kg as it is being repelled by 30kg of force from the bottom magnet. I hope I have explained myself properly. Thanks for your help.

4. Jul 14, 2015

### Hesch

The magnet itself has no "force". It's the magnetic energy in the airgap that produces force.
The magnetic energy density in the airgap = ½*B*H [ J/m3 ]. If you close up the magnets this energy density will be increased due to changes in the H-field and the B-field. So somebody must add this energy to the airgap, and this is done by using force to close up the magnets.
Energy = force * distance.

That's why magnets are attracting/repelling each other. It has nothing with "north" and "south-poles" inside the magnets to do. They don't exist.

Remember that two magnets will always attract each other with the same force ( not 9kp vs. 30kp ). Newton states that with his law: action = reaction.

The cross section area of the airgap, using two different magnets, must be regarded as the area of the smallets magnet, because this is the area that "limits" the airgap.

Last edited: Jul 14, 2015
5. Jul 14, 2015

### Stoney

Ok I think I understand now, thanks for your help.