Question: Why do magnets attract and repel?
Steve Roser answered on 14 Jun 2010:
Magnetism is all to do with electrons. Electrons can line up in materials leading to lines of force, which attract and repel other magnetic material. In iron for example the four unpaired electrons help other nearby electrons line up in the same direction and overall the net result is electrons pointing in teh same direction
Convinced? me neither. Its all very subtle actually and to do with quantum mechanical effects.
Stephen Curry answered on 14 Jun 2010:
Hey Lucas – this is one of the best questions yet: simple and yet completely fundamental.
My answer is therefore going to be a grave disappointment! Even though I’ve got a degree in physics, it’s nearly 25 years old and looking a bit rusty! I do remember that magnetism derives from the movement of electrons (I *think* it may even be a relativistic effect of electron motion) but that doesn’t come anywhere near being ‘an explanation’
But even the forces that we do ‘understand’ aren’t understood that well. Why does a positively charged proton attract a negatively charged electron? I’m not sure anyone really knows. We just know that they do because that it one of the fundamental properties of charged particles.
Tom Hartley answered on 15 Jun 2010:
Hi Lucas, I am going to try to answer – but check back to see the comments, because I will probably make some mistakes…
This is a surprisingly difficult question to answer. I work with big magnets (the MRI scanner has an incredibly strong ring-shaped magnet which goes around your body when you are inside it), and although I am not a physicist I would like to know more about how magnets and magnetism work, so I ask my physicist colleagues lots of questions. The short answer is I don’t know, because so far I’ve never heard an explanation which makes complete sense to me (but remember I am a biologist and psychologist by training, so I don’t have all the tools and training needed). I am going to try to tell you what I’ve grasped, but there could be some important mistakes – I expect the physicists will be able to answer you more clearly, and certainly more accurately.
First of all, here is my favourite scientist, Richard Feynman, trying to answer your question. Feynman was a brilliant scientist and an outstanding explainer of science. He helped to devise the theories that explain magnetism. However, he gets a bit stuck here. http://youtu.be/wMFPe-DwULM
The problem seems to be that magnetism is a very basic property of matter (along with charge and mass), and it is not easy to explain in terms of any underlying mechanism. Sometimes they talk about exchanges of photons between particles. If you imagine two people on skateboards* rolling along in parallel, then the photons would be like tennis balls thrown between the people. When one person throws a ball, they get a slight kick which changes the path of the skateboard (pushing them away from the other skater). Equally, when the other person catches the ball they get a slight kick which pushes them away. So that’s how repulsion would work. You can think of attraction as the same thing, backwards (in time – I think). This idea that magnets attract and repel one another by exchanging photons is all very well, but it raises lots of tricky questions in my mind.
Rest assured that physicists do understand magnetism, in the sense that they can describe and predict the magnetic properties of materials under different conditions and the magnetic fields they produce in great detail and with great accuracy. This understanding is at the root of developments such as MRI – so we know it works. However, this knowledge is usually expressed in the form of equations, which explain what happens, but don’t necessarily have any straightforward interpretation in terms of everyday experience.
*I got this idea second-hand from my wife’s former boss at the Institute of Physics, but it may have become a bit garbled in the translation – if so – my bad.
Marieke Navin answered on 21 Jun 2010:
Wow what a difficult questions Lucas!! To be honest I don’t really know, gulp! It is caused by the movement of electrons in the atoms of the material. All of chemistry is in fact caused by the movement of electrons…
I feel like this answer isn’t any good, let’s see what the other’s have said….
Pete Edwards answered on 21 Jun 2010:
Sorry I only just picked up on this but I think you have your answer from the others so I’m not going to muddy the water with yet another explanation.