Monopole

[The Apprentice]

if someone were to melt down the ore thatis used to make magnets into a sphere, is it possible to make all the negative poles pointed toward the center and create a monopole on the outside? ( I know that the negative poles would repel each other, but if some thing had a strong enough pull, could it result in a monopole?)

[insane_alien]

no, that wouldn't result in a monopole at all.

[Bob_for_short]

if someone were to melt down the ore that is used to make magnets into a sphere, is it possible to make all the negative poles pointed toward the center and create a monopole on the outside? ( I know that the negative poles would repel each other, but if some thing had a strong enough pull, could it result in a monopole?)

 

No, the magnetic field in such a configuration will be confined within the body. It is just like two equal electrical charges, one at the center, the other one is on the sphere. No field outside the body. However the magnetic field inside will be radial. I think it is well possible to make.

Edited September 16, 2010 by Bob_for_short

[Mr Skeptic]

The magnetic field lines will always form closed loops.

[Bob_for_short]

The magnetic field lines will always form closed loops.

 

Magnetic field lines are the same as electric dipole lines: they start at one pole and finish at another one.

[Mr Skeptic]

Magnetic field lines are the same as electric dipole lines: they start at one pole and finish at another one.

 

Magnets come in dipoles, there is no magnetic pole for the field line to end at. In one side out the other.

 

Electric dipoles on the other hand have two poles and so their field looks kind of like that of a fundamental dipole except for in the space between the two poles where it is the opposite direction.

[Bob_for_short]

Magnets come in dipoles, there is no magnetic pole for the field line to end at. In one side out the other.

 

Electric dipoles on the other hand have two poles and so their field looks kind of like that of a fundamental dipole except for in the space between the two poles where it is the opposite direction.

 

So a "fundamental" dipole is different from an electric dipole? An electric dipole can also be made with continuous charge distribution, without a gap.

 

OK, Is it possible to construct a ball like the OP asked?

 

I think we can take a ball, cut it in 8 equal parts with three mutually perpendicular planes, magnetize each piece along its axis of symmetry (from the center to the surface) and make them stick together with a glue.

Edited September 17, 2010 by Bob_for_short

[Mr Skeptic]

So a "fundamental" dipole is different from an electric dipole? An electric dipole can also be made with continuous charge distribution, without a gap.

 

Could you give an example? All the cases I can think of of electric dipoles are made of two or more electric monopoles and would have an inverted field between the poles as compared to the outside field.

 

OK, Is it possible to construct a ball like the OP asked?

 

I think we can take a ball, cut it in 8 equal parts with three mutually perpendicular planes, magnetize each piece along its axis of symmetry (from the center to the surface) and make them stick together with a glue.

 

I think the ball could be made but wouldn't be a monopole (nor magnetic if made perfectly). If made of parts it is likely some field will leak between the cracks.

[Bob_for_short]

Could you give an example? All the cases I can think of of electric dipoles are made of two or more electric monopoles and would have an inverted field between the poles as compared to the outside field.

 

A dielectric bar, like a magnet bar but charged with rho(x) = k*(|x|-L)*sgn(x), -L < x < L.

Edited September 17, 2010 by Bob_for_short

[granpa]

some problems can be solved by treating the far field as though it were the result of magnetic monopoles but

it wont work for the internal field within the magnet itself

[Bob_for_short]

Being a student, I heard of Maxwell multi-poles. For example, you take two opposite-sign charges Q at some distance R. Their dipole moment is Q*R. Then you decrease the distance R and increase the charge values so that the dipole moment does not change. In the limit R = 0 you have an electric dipole with practically closed lines.

Edited September 19, 2010 by Bob_for_short

[Mr Skeptic]

A dielectric bar, like a magnet bar but charged with rho(x) = k*(|x|-L)*sgn(x), -L < x < L.

 

Hm, that's an odd arrangement of charge. Getting the charge to switch from -kL to +kL at x=0 would be hard to do though. In any case, it is a mathematical construct and in reality your charges will be point sources rather than smooth.

 

Being a student, I heard of Maxwell multi-poles. For example, you take two opposite-sign charges Q at some distance R. Their dipole moment is Q*R. Then you decrease the distance R and increase the charge values so that the dipole moment does not change. In the limit R = 0 you have an electric dipole with practically closed lines.

 

True enough, but arbitrarily large charges arbitrarily close together is more mathematical than reality.

[Bob_for_short]

Hm, that's an odd arrangement of charge. Getting the charge to switch from -kL to +kL at x=0 would be hard to do though. In any case, it is a mathematical construct and in reality your charges will be point sources rather than smooth....

True enough, but arbitrarily large charges arbitrarily close together is more mathematical than reality.

 

You are right, of course.

 

Let us return to the spherical magnet configuration. The radial magnetic field looks weird to me. Maybe one cannot get a complete cancellation of azimuthal fields inside and it will be something complex, with closed lines.

Edited September 20, 2010 by Bob_for_short

[Moontanman]

if someone were to melt down the ore thatis used to make magnets into a sphere, is it possible to make all the negative poles pointed toward the center and create a monopole on the outside? ( I know that the negative poles would repel each other, but if some thing had a strong enough pull, could it result in a monopole?)

 

 

The first thing to remember is that melting down a magnet destroys the magnetic effect, there is no special ore that is magnetic. A magnetic sphere has north and south poles just like a bar magnet. If you were to forge a bunch of parts that when fitted together would make a sphere, then magnetize them and so that the part that points inward is always one charge and the part that points outward was the opposite charge the pieces would all repel each other if you tried to form them into a sphere, I'm not sure what the field lines would look like.

[granpa]

Being a student, I heard of Maxwell multi-poles. For example, you take two opposite-sign charges Q at some distance R. Their dipole moment is Q*R. Then you decrease the distance R and increase the charge values so that the dipole moment does not change. In the limit R = 0 you have an electric dipole with practically closed lines.

 

the distance between the infinite charges may be infinitesimal but its still nonzero.

A continuous distribution of such dipoles will still result in a finite separation of the resulting (infinite) monopoles.

 

the monopoles actually used to solve these types of problems are(continuous distributions of) finite monopoles separated by infinitesimal distances.