Magnetic Moment and permeablity

[tim9000]

Hi,
I understand that magnetic flux is like electric current and will take the path of least resistance, but I really don't like the current analogy and here's one question where it doesn't help:

Say you've got a piece of Ferro Iron where the magnetic moments are not lined up, you put a magnetic dipole on the ferro Iron and the flux will choose to flow through that on it's path more than it will the air next to the ferro iron. But if the Iron is magnetised to that the flux will still want to flow through the iron (the poles aren't opposing, its like ones north and ones south and they're attracted) will more flux flow through the Iron than before? Or would it be the same amount of flux as before, because while the iron's under the influence of the magnet the dipoles all just flip temporarily to go with it?
I would imagine that if you flipped the magnetised Iron over (so it IS opposing), it would be less flux through it, as some of the dipoles are opposing, and the flux wouldn't really like that as much, but I may be wrong. Any thoughts?

 

[Jeff Rosenbury]

Hi,
I understand that magnetic flux is like electric current and will take the path of least resistance, but I really don't like the current analogy and here's one question where it doesn't help:

Say you've got a piece of Ferro Iron where the magnetic moments are not lined up, you put a magnetic dipole on the ferro Iron and the flux will choose to flow through that on it's path more than it will the air next to the ferro iron. But if the Iron is magnetised to that the flux will still want to flow through the iron (the poles aren't opposing, its like ones north and ones south and they're attracted) will more flux flow through the Iron than before? Or would it be the same amount of flux as before, because while the iron's under the influence of the magnet the dipoles all just flip temporarily to go with it?
I would imagine that if you flipped the magnetised Iron over (so it IS opposing), it would be less flux through it, as some of the dipoles are opposing, and the flux wouldn't really like that as much, but I may be wrong. Any thoughts?

I've never thought that flux was like current. Current is moving charges, but there aren't any magnetic charges.

Flux is more like another flavor of voltage. Iron is like another flavor of dielectric. Just as a dielectric can magnify an electric field, iron (or other high permeability materials) magnifies the magnetic field.

Now consider a dielectric with embedded charges. This is sort of like an electric field magnet. The main difference would be that charges are unipoles while the magnetic bits are dipoles. Perhaps a dielectric with zillions of tiny batteries?

There is also a matter of scale. Iron "conducts" flux a few hundred times better than air. Dielectrics are usually only a few times better than air (though there are some much better dielectrics, we rarely see them.) Conductors OTOH are zillions of times more conductive than insulators. So all current can be thought of as going through a wire while some flux will always go the wrong way.

BTW, pure water is a good dielectric. (impurities make it conductive.) It is made of zillions of tiny dipoles (water molecules are dipoles). The film on the top of water (meniscus) is a place where the dipoles line up and bind together like tiny magnets only with the electric field instead of the magnetic field. Similarly the walls of cells are made of fatty acids that are dipoles. Each wall has two layers which are held together by the dipole action in a manner similar to magnets. So these effects are real, but tend to be on a smaller scale than we're used to seeing with magnets.

 

[tim9000]

I've never thought that flux was like current. Current is moving charges, but there aren't any magnetic charges.

Mmm, I kind of disagree; it's not so much that the 'current' is a result of the charges drifting down the wire, but that the charges are free to move and align themselves in such a was as to channel a small electric field down the wire (to flow). Keep in mind that in AC the charges don't end up moving at all.
Anyway the point is that people frequently use the analogy of electricity as MMF relates to voltage and Reluctance to resistance.

Just as a dielectric can magnify an electric field

I'm not sure what you mean, I know you can collect a large charge on parallel plates, but I don't think you strictly need a dielectric for capacitance.

 

[Jeff Rosenbury]

Mmm, I kind of disagree; it's not so much that the 'current' is a result of the charges drifting down the wire, but that the charges are free to move and align themselves in such a was as to channel a small electric field down the wire (to flow). Keep in mind that in AC the charges don't end up moving at all.
Anyway the point is that people frequently use the analogy of electricity as MMF relates to voltage and Reluctance to resistance.I'm not sure what you mean, I know you can collect a large charge on parallel plates, but I don't think you strictly need a dielectric for capacitance.

These were similes, not the same thing.

A magnetic field is a magnetic field. It is not a current. it is not an electric field. We can think of them one way or another if it helps us understand, but in the end they follow their own rules and don't care a whit what we think.

BTW, current is charge drifting down the wire (at least in a wire, there are other examples of moving currents than wires). AC current just means the charges drift one way first, then back again. Electric fields do not move down a wire (in general anyway). Electric fields are zero inside a conductor (PEC), as are magnetic fields. The fields contain the energy and stay outside the wire (at least ideally).

Study the Hall Effect. It demonstrates this.

The dielectric is the stuff between the plates. Air can be used as a dielectric, but other dielectrics can store more energy (and allow for more charge on the plates) than air (or a vacuum). While you don't need a special dielectric, you do need some sort of insulator.

 

[tim9000]

These were similes, not the same thing.
A magnetic field is a magnetic field. It is not a current. it is not an electric field. We can think of them one way or another if it helps us understand, but in the end they follow their own rules and don't care a whit what we think.

I did say analogy, infact you'll recall I even said 'I didnt' really like it'.
Actually I think you'll find that a magnetic field is actually part of the electric field. Permanent Magnetism is a quantum effect of the electric field and electromagnetism is a consequence of special relativity on charge.

BTW, current is charge drifting down the wire (at least in a wire, there are other examples of moving currents than wires). AC current just means the charges drift one way first, then back again. Electric fields do not move down a wire (in general anyway). Electric fields are zero inside a conductor (PEC), as are magnetic fields. The fields contain the energy and stay outside the wire (at least ideally).

Study the Hall Effect. It demonstrates this.

I've heard differently, if not, how do you explain the extremely fast speed electricity propagates if not via the electric field? it surtainly isn't via electron drift velocity.
I vaguely remember doing a Hall effect sensor lab, however I am curious as the the specific section of the study you are refering. I would be very interested to read it contradict what I said, can you link the section? In fact I think I recall hearing that it had been observed electric 'fringing' around a wire bent at an extremely acute angle.

The dielectric is the stuff between the plates. Air can be used as a dielectric, but other dielectrics can store more energy (and allow for more charge on the plates) than air (or a vacuum). While you don't need a special dielectric, you do need some sort of insulator.

Ah I see what you were saying, I never concidered the use of a dielectric type to increase the charge on a plate, I suppose it is possible, I always just expected it was a mater of manufacturing practicality. Although using most solid dielectrics instead of air would decrease the life of the cap due to partial discharge etc. But I suppose that's the trade off.