# Null point on a bar magnet

Discussion in 'Physics & Math' started by Quantum Quack, Jan 7, 2009.

1. ### Quantum QuackLife's a tease...Valued Senior Member

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Say we look at a conventional ferr. bar magnet.

We have a south pole and we have a north pole.
Somewhere in the middle th0ugh we have a NULL point where the poles invert.

The question are:
1. What is the best way to describe this null point betweeen the two poles of a bar magnet?
2. How big is it?
3. Can it be defined with the use of normal terms and definitions?
care to discuss?

3. ### James RJust this guy, you know?Staff Member

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There is no null point.

The magnetic field lies around a bar magnet form closed loops. Outside the magnet, the field lines go from the north pole to the south. Inside the magnet, they go from the south pole to the north.

Cut a bar magnet in half and you don't get an isolated south pole and an isolated north pole - you get two bar magnets, each with both a north and a south pole.

5. ### Quantum QuackLife's a tease...Valued Senior Member

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at what point does the north pole become the north pole as distinct from the south pole?

and
how short can a bar magnet theoretically get and still maintain two poles?
I am curious about what conventional thought is on the subject...

7. ### James RJust this guy, you know?Staff Member

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At the point where the physical material of the bar magnet stops. "north pole" and "south pole" are just convenient words to distinguish the two ends of the magnet. Field lines come out of a north pole and go into a south pole.

Practically as short as you want. A single particle like a proton or neutron has a north and south pole.

8. ### James RJust this guy, you know?Staff Member

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I should perhaps add that if you want to be strict about "bar magnet", having it made of iron etc, then the smallest you'll get is a single iron atom.

9. ### Quantum QuackLife's a tease...Valued Senior Member

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I guess it would be safe to say that we coud go almost down to zero distance between south and north poles?

maybe infinitesimal would be better? maybe?

10. ### Quantum QuackLife's a tease...Valued Senior Member

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OK would this be correct hypothetically speaking:

if I produce a slice of magnetic material that is infinitesimally thick would we still have two poles, one on either side of this slice?
I would assume yes ...
btw merry Xmas and happy new year James!

11. ### Quantum QuackLife's a tease...Valued Senior Member

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18,298
if so then there has to be a point where the poles switch, and that is what I am referring to as a NULL point.
as the pole goes from North to South...???

12. ### Quantum QuackLife's a tease...Valued Senior Member

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Similar could be asked for gravity..
At what point is gravity at it's strongest and how small is that point?

13. ### iceauraValued Senior Member

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26,896
No,there is no such point. The poles never switch.

At every point there are field lines traversing it, oriented.

You might as well ask as about the point at which the left and right ends switch.

14. ### Quantum QuackLife's a tease...Valued Senior Member

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18,298
are the field lines static [ stasis ] or are they moving?

15. ### EnmosRegistered Senior Member

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Think "atom".

16. ### BenTheManDr. of Physics, Prof. of LoveValued Senior Member

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What do you mean moving?

You can think of field lines as a sort of "wind". The wind blows a test particle in a specific direction, depending on where the test particle is placed. At each point in the background of a bar magnet, a magnetically charged test particle will feel a force whose direction is dictated by the direction of the field line.

If you think about things in this manner, there is no way the field lines can "move".

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2,745
why? are you suggesting that because of the atoms construct, magnetism exists?

that 'angular momentum' is what causes magnetism and the dipole moment?

Wow

Look up http://hyperphysics.phy-astr.gsu.edu/Hbase/spin.html#c4

but the best experiments to observe are in superconducting material (ultra cold)

then you can see the atoms construct is not as important as the 'state' of the mass, for the magnetism of mass

last thing to 'think' is ........... atom

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2,745
for you,

look up the london dispersion force (van der waal).... and observe the induced dipole attraction

good luck

19. ### John ConnellanValued Senior Member

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QQ: poles are simply convenient terms to describe where the magnetic force field lines enter the bar. No matter how small or big the bar magnet, these force field lines will be there, oriented in the same way all the time. The bar is just a collection of atoms each with a force field with the same orientation.

20. ### Uno HooRegistered Senior Member

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That doesn't sound right to me.

I'm going to dig up my Magnetic Field Handbook and look into it.

21. ### Guest254Valued Senior Member

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I think you might find it beneficial to look at the magnetic field produced by a solenoid. It might help you see what a test charge would do if it could "pass through" the bar magnet.

22. ### FacialValued Senior Member

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2,219
The reason something starts to repel at a different end of the magnet is because the field lines are repelling the object's own field, induced or native, in the same orientation.

From a microscopic view, one could argue that the sum of the magnetic domains, each with their own fields, finally exerts a net repulsion on the ferromagnetic object. For ferromagnetic materials with low hysteresis, ie a nail, the domains are continuously aligned and so the material is constantly attracted. The bar magnet, with high hysteresis, has its domains aligned in one direction so that the point between physical attraction and repulsion occurs roughly halfway through the length of the bar, for a highly hysteretic material like magnets themselves.

23. ### James RJust this guy, you know?Staff Member

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Great! Let me know what it says.