Tagged: Magnet

Magnetic fields, again


English: The magnetic field of a bar magnet re...

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I asked a couple of questions about magnetism before and I have to say I was not fully convinced by the answers – so here is another way of stating what puzzles me.

Imagine a static magnet. Now magnetism cannot propagate instantaneous as if it did the magnet would surely immediately cease to be magnetic – even though we know, via Oblers’s Paradox if nothing else, that the universe is finite, we must surely also assume it is very large and contains a very large number of magnetic objects.

So magnetism is propagated at some finite speed. Naturally we will assume that speed is c , the speed of light and the propagation is via photons. But what are these photons?

If they are real, physical particles, then they must carry energy and so the magnet should ‘run down’ – otherwise it would be a perpetual motion machine. But static magnets apparently run down very slowly – so slow I have never been aware of it really happening, though I have no doubt it does.

So what else might they be? Presumably a function of quantum electrodynamics (QED) as formulated by Feynman? In this case then the energy of these “virtual” photons is a function of the uncertainty principle.

This would essentially mean that the strength of these magnetic photons would be limited by \delta E \delta t \leq h where E is the energy of the QED photon, t time and h the familiar Planck constant – a very small number indeed.

Here the particles can come and go in an instance or presumably, live mysteriously for a very long time at very low energies. But no actual energy is expended unless this virtual photon is “observed”.

So, is this right? And if it is, how did anyone explain magnetism before QED? And if it is wrong how do magnetic fields propagate.

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Thanks to @PootBlog but I still don’t get it


Magnets have many uses in toys. M-tic uses mag...

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I have been on holiday with the family in a not exactly sunny, but still perfectly pleasant Bilbao, and so have not replied to this as soon as I probably should have.

But thanks to Damian Counsell (@PootBlog) who responded to my annoying twittering by explaining one of the magnetism issues I had:

I have to say, though, I am less than convinced. Say I create a new permanent magnet. How do other magnets know about it? In other words how does this “field” propagate? As it carries information it must surely propagate at the speed of light – but how? Is it a property of space-time itself?

A second thing I don’t understand about magnetism


Magnet

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Magnets attract (or repel) one another. But how?

How does the electromagnetic force get turned into kinetic energy? Do all the electrons inside a magnet ‘rush’ to one end of the magnet – if so why don’t we see an imbalance of charge (or maybe we do – after all magnets can be used to generate current).

Fear these questions may lead some of you to doubt I have any sort of physics qualification at all – but I do not recall being taught any of this, at ‘O’ level, ‘A’ level or as an undergraduate.

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Things I don’t understand: magnetism


A stack of ferrite magnets

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OK, I have a (astro)physics degree, so maybe I should understand this, but I don’t. Been meaning to ask my brother (PhD in high temperature super conductivity) – but never yet gotten around to it – so here goes.

For magnets to attract (or repel) one another, they must emit some sort of radiation: magnets cannot just “know” they are close by. So, what is this radiation? Presumably it is electromagnetic which presumes magnets are emiting radiation constantly.

Now, I am sure magnets (as in toy magnets of the sort pictured here) decline in strength over time, but they also seem to last many years with no easily noticeable sign of decline. So how can they emit electromagnetic radiation for years in this way?

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