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Question: Why do magnets attract and repel?

Asked by lucasjacobs to Meeks, Pete, Stephen, Steve, Tom on 21 Jun 2010 in Categories: .

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  • Photo: Steve RoserSteve 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.

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  • Photo: Stephen CurryStephen 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.

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  • Photo: Tom HartleyTom 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.

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  • Photo: Marieke NavinMarieke 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….

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  • Photo: Pete EdwardsPete Edwards answered on 21 Jun 2010:

    Hi Lucas
    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.

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Comments

  • Photo: StephenStephen commented on 15 Jun 2010:

    Lucas – your question has stimulated quite a bit of chatter among the scientists on the back-channels (e.g. twitter).

    I’ve been digging a bit and – to my relief – nothing that I said at first is wrong – phew! Magnetic forces arise due to the relative movement of charged particles (e.g. electrons, protons). If you will accept that like charges repel and opposite charges attract (due to the electrostatic force which we don’t really understand – we just know that it’s there) we can start from there.

    When charges are moving relative to one another then Einstein’s theory of relativity comes into play – this tells us that when we (or an electron) observe a moving object it actually appears slightly shorter that it does when it’s not moving (weird but true!). The changes are tiny unless the speeds involved are very large but they are significant. This means that the charge density (charge per unit length) in the moving object increases slightly and that gives rise to an extra bit of electrostatic force. We we would see this for example in an electron that was moving parallel to a wire in which there was an electric current – it would be attracted to the wire by what we would call a magnetic force (if the electron was not moving or there was no current in the wire, there would be no force between them).

    (So electrostatic and magnetic forces are, in a way, two sides of the same coin. That’s why we often talk about electromagnetism – the two phenomena are not easily separated!)

    Now – to jump to magnetic materials. The structure of many atoms results in orbitals containing unpaired electrons that (to a first approximation) are whizzing around the nucleus, giving rise to a tiny current. So once again you have moving charges. These will interact with other moving charges (in other magnets) in the same way as my electron plus wire example above.

    OK all that may seem very complicated – you can probably see that I still don’t fully understand it myself but then not many people do. But your question was still a great one and has reminded me of why I loved physics so much – it really tries to dig into the heart of things.

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  • Photo: StephenStephen commented on 15 Jun 2010:

    Tom is spot on about the idea that electomagnetic forces are thought to arise from particles swapping photons back and forth. But of course that just raises other questions: why do they swap photons? how do they know which direction to throw them in?

    It’s a pretty mysterious place this universe, innit?

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  • Photo: lucasjacobslucasjacobs commented on 18 Jun 2010:

    Ye i sure is thank you for your answers! =D

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  • Photo: MeeksMeeks commented on 21 Jun 2010:

    ooh loads of answers -love it!

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