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View Full Version : making a magnetic wire (where's N and S)?


Sigene
12-20-2011, 04:06 PM
I thought I solve some office boredom and magnetize a piece of a paperclip to float in water and make a compass.

I've just left the wire on top of a disk magnet for a while. Where are the North and South poles on the wire?

Let me explain further. Lets say, I've placed the wire on the North face of the disk magnet. I assume while the wire is in contact, the wire is effectively part of the North end and is North.

When I pull the (now magnetized) wire away, it has to develop a North and South end.

Question 1: Is it absolutely true that the wire is all North when in contact with the North end of the magnet?

Question B: Does the now magnetized magnet have a North and South end at either terminus of the wire (instead of a North and South side) when removed from the disc magnet?

Question III: what determines which end will become North and which South as you pull the wire away?


Bonus question: Could you realistically pack the front and back of a boat with many aligned magnets to where it will be noticeable attracted to the North pole? Would it be better to have them all aligned N/S or would it be better to put them all South facing out so to be simultaneously attracted to North and repelled by South?

naita
12-20-2011, 04:13 PM
Question 1: Is it absolutely true that the wire is all North when in contact with the North end of the magnet?

No, it's absolutely false.

There isn't a North end to a magnet that's full of magnetic northness. There's a field with a direction.

Now for a regular magnet this field is caused by the fields of small bits of the magnet called domains all being lined up, and the magnetisation of your wire is caused by a sufficient number of domains in the wire being forced into line by the stronger magnetic field of the magnet you're using.

naita
12-20-2011, 04:15 PM
Bonus question: Could you realistically pack the front and back of a boat with many aligned magnets to where it will be noticeable attracted to the North pole? Would it be better to have them all aligned N/S or would it be better to put them all South facing out so to be simultaneously attracted to North and repelled by South?

If every side of the boat is attracted to north and repelled by south, then which side of the boat do you think should end up pointing north.

Sigene
12-20-2011, 05:06 PM
No, it's absolutely false.

There isn't a North end to a magnet that's full of magnetic northness. There's a field with a direction.

Now for a regular magnet this field is caused by the fields of small bits of the magnet called domains all being lined up, and the magnetisation of your wire is caused by a sufficient number of domains in the wire being forced into line by the stronger magnetic field of the magnet you're using.

I don't think I said the wire had a north 'end' when it was attached to the magnet. I think I implied that it was all north (i.e. full of magnetic northness).

So can you clarify? Is it absolutely true that the wire is all north (full of Northness) or is it absolutely false like you state? Can we state that the direction of the field lines coming out of the wire are all going the same direction (which to me is nearly the same as saying its all north)?

TriPolar
12-20-2011, 05:08 PM
WAG that your wire shows no significant change in magnetic properties after being removed from the magnet.

MikeS
12-20-2011, 11:13 PM
Question 1: Is it absolutely true that the wire is all North when in contact with the North end of the magnet?No. This is one of the fundamental laws of electromagnetism: magnetic monopoles (http://en.wikipedia.org/wiki/Magnetic_monopole) don't exist.1 Stated differently, you can't have a North pole without a compensating South pole, or a South pole without a compensating North. Rather, the side of the wire that's in contact with the North pole of the disc magnet is a South "side", and the opposite side of the wire is a North "side".

Question B: Does the now magnetized magnet have a North and South end at either terminus of the wire (instead of a North and South side) when removed from the disc magnet?Probably not. Most likely it will retain a north and south side for a while, but depending on the type of metal, the magnetic domains mentioned by naita will become randomized and the magnetization will be lost. It is pretty unlikely that the domains will switch from "all aligned side to side" to "all aligned end to end". In fact, I'd lay good odds on it never happening if you spent the rest of your days performing this experiment. The third question is then moot.
Bonus question: Could you realistically pack the front and back of a boat with many aligned magnets to where it will be noticeable attracted to the North pole? Would it be better to have them all aligned N/S or would it be better to put them all South facing out so to be simultaneously attracted to North and repelled by South?
Technically, yes, but it'd be a miniscule effect. As noted above, the south pole of your hypothetical magnet would be repelled from the Magnetic North Pole, while the north pole of your hypothetical magnet would be attracted. If the Earth's magnetic field was perfectly uniform, these two effects would cancel out and the boat wouldn't move. However, since your north end of the boat is ever so slightly closer to the North Magnetic Pole, it would experience ever so slightly more force. The net effect is that your boat would be attracted to the North Magnetic Pole, but the effect would be so small as to be negligible.

1Except maybe once, in 1982 in Stanford. But that's as may be.

Sigene
12-21-2011, 08:10 AM
Hi Mike


THanks for your reply, but I don't think I've explained it well enough, and I'm not really getting it.

The wire is laying flat on my magnet, in contact with the magnet. THe magnet is on the middle of the wire. So if it is in contact with the magnet, I believe the field lines are all coming out of the wire in the same direction.

Or are you saying, that while it is contact with the magnet, the middle part (the part touching the magnet) is one pole.....south? and the ends are both north? This I can understand, the wire isn't all one orientation, but the middle is one orientation and the ends are the other (Is that correct?)


The first question deals with the wire in contact with the magnet, the others deal with the wire, after it has been removed from the magnet.
I expect the wire to lose magnetism over time, but right now when I pull it away it is still magnetized. and one end points North the other South when floated in water.
So, your statement that it probably won't develop North South ends doesn't seem to hold up empirically (maybe I should take up your odds:p.) I don't need to spend 'the rest of my days' performing the experiment....its happening right now. the floating wire is lined up North and South, indicating there are North and South ends (not sides, nor north ends and south middle)
So at some point, when in contact with the magnet (in the middle), the wire doesn't have a North and South end (maybe a South middle and North ends). Both ends have the same orientation. But then when you pull the wire off, there is a North/South orientation.

My third question still seems valid...what determines which end becomes North and which becomes South?

naita
12-21-2011, 08:55 AM
Hi Mike


THanks for your reply, but I don't think I've explained it well enough, and I'm not really getting it.

The wire is laying flat on my magnet, in contact with the magnet. THe magnet is on the middle of the wire. So if it is in contact with the magnet, I believe the field lines are all coming out of the wire in the same direction.

Or are you saying, that while it is contact with the magnet, the middle part (the part touching the magnet) is one pole.....south? and the ends are both north? This I can understand, the wire isn't all one orientation, but the middle is one orientation and the ends are the other (Is that correct?)


The first question deals with the wire in contact with the magnet, the others deal with the wire, after it has been removed from the magnet.
I expect the wire to lose magnetism over time, but right now when I pull it away it is still magnetized. and one end points North the other South when floated in water.
So, your statement that it probably won't develop North South ends doesn't seem to hold up empirically (maybe I should take up your odds:p.) I don't need to spend 'the rest of my days' performing the experiment....its happening right now. the floating wire is lined up North and South, indicating there are North and South ends (not sides, nor north ends and south middle)
So at some point, when in contact with the magnet (in the middle), the wire doesn't have a North and South end (maybe a South middle and North ends). Both ends have the same orientation. But then when you pull the wire off, there is a North/South orientation.

My third question still seems valid...what determines which end becomes North and which becomes South?

The original magnet, even one clearly painted red and white and labelled with a north and south pole, has the same magnetic properties throughout. If you chop it in four you get four magnets each with the same north-south directions, even if one or more of those magnets consist entirely of material from the "north pole".

Assuming your wire is ferromagnetic and the complexities of magnetism haven't confused me, your wire is made up of similar little magnetic areas as the magnet, but with random directions. Touching the magnet, or just being in sufficiently close proximity to get a strong enough magnetic field, makes those magnetic areas in the wire line up parallel to the magnet's field but opposite making it stick to the magnet. It doesn't matter if it only touches the magnet's "north" end, what matters is the north-south direction of the field. If you put the wire at a right angle to the field you'd get a very short, very wide magnet.

Still assuming ferromagnetism and me not being confused, when you remove the wire at least some of the magnetic areas will stay aligned, giving you a magnet with the opposite north-south direction of the original.

It's possible I've got it backwards though, or that you don't have a ferrromagnetic material, but just trying to figure that out confuses me even more. :)

Why don't you put your original magnet on a floating platform and see which way it points?

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