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Author Topic: A question of attraction...  (Read 1637 times)

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Offline feebix

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A question of attraction...
« on: 14:15:03, 09 June, 2011 »
I wasn't sure where to put this so I've rather cheekily made it fit this section :) (Maybe a general physics section might be good?) Actually, is it even classed as physics? Anyhoo, here's the question. I was fixing a stupidly expensive (and crap) bin this afternoon using my super handy rare earth magnets and yes, I was "In space" when I fixed it lol  When I was considering how thick a metal plate to use to meet the magnet I wondered the following....
When a magnet is attracted to a bit of lets say iron. If you have a 1mm thick one inch by one inch plate and then a quarter inch thick one inch by one inch plate, is there any more attractive force involved BECAUSE the sheet of metal is thicker? This is with the same sized magnet of course. Can't quite get my head around that one. Anyone?   :nerd:
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Offline Nomis Elfactem

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Re: A question of attraction...
« Reply #1 on: 14:38:22, 09 June, 2011 »
The short answer I guess is No, because the initial source of the magnetic force is the magnet and as that doesn't change size the force remains the same.

The only caveats with that is that a) neither of the sheets of metal have been "attached" to a magnet for a long period of time prior to you running the test as they them selves can become magnetised over time and that b) you don't put an electric current through either sheet as that can induce a magnetic forces too (hence the term "electro-magnetic").

I could of course be talking complete tosh....

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Offline feebix

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Re: A question of attraction...
« Reply #2 on: 16:08:55, 09 June, 2011 »
I suppose it might also depend on how one measured the "force" I'm asking about. Putting Gausse aside and measuring the speed at which the magnet pulls the metal toward it, could that speed not increase as the metal is made thicker and (possibly??) more "receptive" by a factor of X?  Therefore increasing magnetic action on said metal? I know the Gausse wouldn't change but would the speed of attraction change? Or the force necessary to disengage metal and magnet?

 I too could be talking utter shash. It has been known to happen of rare occasions :)
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Offline Ian Straton

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Re: A question of attraction...
« Reply #3 on: 16:32:59, 09 June, 2011 »
Simon is right the attractive force is related to the flux of the magnetic field which is generated by your magnet, any ferrous material will be attracted to your magnet with the same force regardless of size.   You can prove this quite easily, what would happen if you walked past a ship with a magnet in your pocket?

if the attractive force was related to the mass of metal being attracted you would instantly be pinned immovably against the ship, in reality you just walk on by thus proving that the force of attraction is defined by your magnet not the metal being attracted.

as for the speed of attraction, you need to measure the total speed (or more properly the accelerations) of the whole system: metal and unrestrained magnet.  You will find that the two speeds will always add up to the same total speed regardless of the mass of the attracted metal (where the metal is more massive than the magnet the magnet will move more quickly and vice versa).

Offline feebix

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Re: A question of attraction...
« Reply #4 on: 18:55:59, 09 June, 2011 »
But the guy on the Churchill advert just points his magnet at that pile of metal and it all flies at him!? lol I obviously appreciated the fact you wouldn't b pinned to the ship. My question was more addressing tiny variences that may or may not have been present depending on conditions.... "May not" being the answer then :) So even two identically sized bits of metal, one being 100% pure and one 50% pure (with non ferrous contaminants) would presumably have the same results......?
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Offline Ian Straton

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Re: A question of attraction...
« Reply #5 on: 22:35:51, 09 June, 2011 »
The 50% alloy would react as if the field was half as strong since only half its mass would be accelerated by the field the, 50% non ferrous mass would add inertia without being accelerated by the magnetic field.

Offline Marmalade

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Re: A question of attraction...
« Reply #6 on: 15:12:12, 18 August, 2011 »
What happens when you get to a certain mi nimum size, then? Intuitively (and no necessarily correctly!), one Fe atom is going to be subject to a much lesser force - this suggests that there is a minimum size above which the force is more or less constant and below which it drops to near zero?

Also, to keep my head functioning, can you please top mixing measurement units?! A one sq. in. plate 1mm think compared with one an inch thick - what's all that about then?
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Offline Ian Straton

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Re: A question of attraction...
« Reply #7 on: 11:07:17, 31 August, 2011 »
What happens when you get to a certain mi nimum size, then? Intuitively (and no necessarily correctly!), one Fe atom is going to be subject to a much lesser force - this suggests that there is a minimum size above which the force is more or less constant and below which it drops to near zero?

Also, to keep my head functioning, can you please top mixing measurement units?! A one sq. in. plate 1mm think compared with one an inch thick - what's all that about then?

No, there is no minimum size*, a single atom of Fe will respond to a magnetic field in exactly the same way as any other sized piece of Iron, ie the total acceleration of the magnet/iron system will still be the same, its just that the single atom of Fe will move *really* fast and the magnet not much at all.  After all a block of Iron consists of many single atoms, if a single atoms were not affected by the magnetic field then neither would the whole block.

(* in fact the minimum size is an electron since the electromagnetic force is transmitted by exchange electron/photon pairs however since this discussion is based around identifiable metals it makes no sense to discuss individual sub atomic particles.)

As to the measurements issue, Lol!  Feebix is a master craftsman, as such he chops and changes between units according to whichever best describes the thing he is currently talking about! You get used to it..

 

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