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Is the force between a magnet and a magnetic material always attractive?  


Mitko Gorgiev
Member Admin
Joined: 3 years ago
Posts: 36
18/08/2020 9:58 am  

It is not true that the force between a magnet and a body of ferromagnetic material is always attractive.

Consider the following experiment: let us score a groove in a piece of styrofoam into which we lay a cylindrical piece of soft iron. Then we place the styrofoam in a plastic vessel of water and bring a strong magnet from outside to its wall. Our floating piece of iron will be attracted to the wall, but since the iron is fixed into the styrofoam, the magnet and the iron will not join, that is, there will be an air gap between them. If we now pull the magnet quickly back, our "boat" will make a movement in the opposite direction.

In both cases the soft iron is moving contrary to the movement of the magnet. This shows that as the magnet is moving towards the iron, magnetic forces of attraction appear, but magnetic forces of repulsion upon moving away.

Let us make another experiment. In a flat piece of styrofoam we thrust a copper plate so that it can stand vertically. We place the styrofoam in a bowl of water and slowly near a strong neodymium magnet close to the copper plate. The "yawl" starts slowly drifting away from the magnet. If we then slowly move the magnet away from the plate, it starts to follow the magnet. When the magnet rests, the plate rests beside it, too.

So, we see that we have here the reverse: repulsion at 'moving towards', but attraction at 'moving away'. Due to this, we may call the ferromagnetism and the diamagnetism diametrically opposite properties. Ferromagnetism is a plus-property, while diamagnetism is a minus-property.

Let’s imagine a hypothetical experiment. A non-magnetized piece of soft iron teleports itself in the close vicinity of a magnet. The magnet will not attract it. Why? Because the "element" of "moving towards" is lacking. No magnetic forces will be evoked by this teleportation.

We usually overlook the "element" of motion, which is essential.

P.S. Soft iron demagnetizes itself outside a magnetic field, that is, it doesn’t "remember" the magnetization.

See also: What is the working principle of an induction motor?