# Thread: How to see the magnetism force?

1. Dear friends:
I alway wondering how the magnetism force (or the gravitation force) was transmitted and how to see them by some instruments. through what particles?
could you please gives me some ideas?
Regards

2.

3. I alway wondering how the magnetism force (or the gravitation force) was transmitted
The carrier particles of electromagnatism ( not just magnetism ) are (virtual) photons. Gravity as currently understood is a geometric property of space-time itself, and doesn't require a carrier particle.

how to see them by some instruments
Magnetism is measured by an instrument called a magnetometer. Gravity can be measured in a variety of ways, for example through accelerometers.

4. May I ask a related question:

Does a magnetic substance have a "half life"? In other words, does magnetism ever 'wear out'? Since it's force is transmitted by particles, I would assume it does?

I know that a magnetic piece of iron can be de-magnetised by hitting it or vibrating it so its magnetic dipoles assume random directions, but do the dipoles ever actually lose their magnetic strength?

OB

5. OK, I know about magnetic force in a particular direction being lost due to mis-alignment of the dipoles, (as I said myself), but;

As I understand it a metal spring will never lose it's force as long as it's not over compressed or over heated?, but this is due to the properties of the physical material of the spring. However, in the case of a magnet; the magnetic force is transmitted by particles? To maintain the force, the flow of particles must presumably be maintained, which must use up energy or mass, so why doesn't it ever wear out?

By the way, I now know what the Curie point is - thanks.

ztdep: Have you ever sprinkled iron filings onto a sheet of paper held over a magnet to illustrate the magnetic field lines? Is this what you mean?

6. Originally Posted by One beer
To maintain the force, the flow of particles must presumably be maintained, which must use up energy or mass, so why doesn't it ever wear out?
Eh there is no flow of particles that transmit magnetism. I see why would you think so but the concept of force carriers from quantum field theory is very misleading and one cannot take it literaly. The magnetic properties of material are given by distribution and aligment of internal magnetic dipoles and there is no need for quantum field theory as long as one is not interested in dynamics during phase transitions.

Let me illustrate on Mangan. Mangan has 25 electrons. The only electron orbital that is not filled is 3d orbital. This orbilal can contain up to 10 electrons, 5 with spin (intristic angular momentum) 1/2 and 5 with spin -1/2. All electrons in this orbital would have same energy. Now there is just 5 electrons to occupy d state. At zero temperature all these electrons will have same spin either 1/2 or -1/2. Why? Because of effect called exchange interaction.
Imagine you would put two electrons with opposite spin to d state. Since the have opposite spin they can be in the same state occupying same space. This however greatly increases their energy because of Coulomb repulsive interaction. If you put two electrons of same spin to d state Pauli exclusion principle forbidds them existence in same state therefore overall energy of the system will be lower. This is principle of exchange interaction. It works between atoms as well.
Now you have atom with 5 electrons whose magnetic moments are not compensated by other electrons. Therefore this atom will serve as magnetic dipole with magnetic moment

where is Lande g factor, is Bohr magneton and is total angular momentum of atom. Multiplying this by density of atoms will give you theoretical maximal magnetisation of material. However in nonzero temperature those electrons do not need to be in state with minimal energy therefore a lot of this potential magnetism will be lost either by electrons in d states spontaneously switching to opposite spin state or by missaligment of atoms themselves. This is qualitatevly mechanism how magnetism is lost by increasing temperature. Quantitatevly this can be computed by this formula:

where M is magnetisation, N density of dipoles, g gfactor, J angular momentum number, Bohr magneton and Brillouin function and x is some temperature dependent function. At high temperatures this leads to Curies law (Curie law is approximation of this).

So yes. You can lose magnetisation by decreasing density of dipoles and/or by misaligment of existing dipoles but you wont lose magnetisation simply by existing over period of time. For example at zero temperature the magnetisation would be practicaly eternal.

7. Hello gere,

Thanks for your explanation, unfortunately it is way above my head - I do not have any knowledge about quantum mechanics. (By Mangan, do you mean Manganese?)

But if I understand the basic idea; magnetism is a feature of quantum mechanics? So somehow a 'field' or a 'force' can exist with no carrier particles?

If so; that's weird, because if I make an electromagnet with a coil of wire and a core piece, then to create a magnetic field I have to send current through the coil. This current has to be generated or be derived from a chemical reaction, both of which use energy. When this energy runs out, the magnetic field disappears, yet with a permanent magnet it seems the 'field' exists for ever - allbeit diminishing in strength owing to gradual mis-alignment of the dipoles.

Do I have that right?

Another thought, leading on from that then: Is the energy one uses to create a permanent magnet (by aligning the dipoles), the same as the magnetic energy one 'gets out' from the magnet in its 'lifetime'? In other words, are permanent magnets like a sort of re-chargeable battery that one can charge up, but there is no extra net power given out beyond what one puts in to align the dipoles in the first place?

OB

8. Hi OB,
yeah I meant Manganese. Magnetism isnt a feature of quantum mechanics. It is purely classical concept rooted within Maxwell equations and corresponding constitutive relations and thermodynamics. These will give you magnetisation vector of ensemble of classical magnetic dipoles. What QM do is it will tell you that these elemental magnetic dipoles are in fact atoms and it will also tell you that projection of their magnetic moment into arbitrary direction must be quantised. Nothing else.

Classicaly fields exists without any carrier particles. Carriers are just concept of perturbative treatment of quantised field theories.

Originally Posted by One beer
If so; that's weird, because if I make an electromagnet with a coil of wire and a core piece, then to create a magnetic field I have to send current through the coil. This current has to be generated or be derived from a chemical reaction, both of which use energy. When this energy runs out, the magnetic field disappears, yet with a permanent magnet it seems the 'field' exists for ever - allbeit diminishing in strength owing to gradual mis-alignment of the dipoles.

Do I have that right?
Yes, but gravity is also a field and it does not diminish in time. Also in superconductors you can have basicaly eternal current which also generates magnetic field.

Originally Posted by One beer
Another thought, leading on from that then: Is the energy one uses to create a permanent magnet (by aligning the dipoles), the same as the magnetic energy one 'gets out' from the magnet in its 'lifetime'? In other words, are permanent magnets like a sort of re-chargeable battery that one can charge up, but there is no extra net power given out beyond what one puts in to align the dipoles in the first place?

Yes of course. Thermodynamics laws hold. Thing is you won`t get energy from magnet just lying around.

9. By the way, I now know what the Curie point is - thanks.
Me too! "Hey Ma!!!! Me and one beer done learn'd us some of that fancy science stuff!!"

10. OK, thanks. That's all a bit mind blowing to me - I think of a magnet "levitating" above another and the magnetic force is counteracting gravity - and this magnetic force could last forever?!

But, of course as you say:

Originally Posted by One beer
If so; that's weird, because if I make an electromagnet with a coil of wire and a core piece, then to create a magnetic field I have to send current through the coil. This current has to be generated or be derived from a chemical reaction, both of which use energy. When this energy runs out, the magnetic field disappears, yet with a permanent magnet it seems the 'field' exists for ever - albeit diminishing in strength owing to gradual mis-alignment of the dipoles.

Do I have that right?

Yes, but gravity is also a field and it does not diminish in time.

It's a shame that magnetic "energy" cannot be used to do useful work - and I can sort of understand why some folk get very excited about machines incorporating magnets that would seem to promise free power.

11. Originally Posted by One beer
OK, thanks. That's all a bit mind blowing to me - I think of a magnet "levitating" above another and the magnetic force is counteracting gravity - and this magnetic force could last forever?!

But, of course as you say:

Originally Posted by One beer
If so; that's weird, because if I make an electromagnet with a coil of wire and a core piece, then to create a magnetic field I have to send current through the coil. This current has to be generated or be derived from a chemical reaction, both of which use energy. When this energy runs out, the magnetic field disappears, yet with a permanent magnet it seems the 'field' exists for ever - albeit diminishing in strength owing to gradual mis-alignment of the dipoles.

Do I have that right?

Yes, but gravity is also a field and it does not diminish in time.

It's a shame that magnetic "energy" cannot be used to do useful work - and I can sort of understand why some folk get very excited about machines incorporating magnets that would seem to promise free power.
JUST. SAY. NO!

.....to the ghastly temptation to think you can make a perpetual motion machine from effing magnets!

The thing is, a table permanently stops a jar of jam sitting on it from falling to the ground. So can a pair of suitably sized permanent magnets. Maintaining something in a motionless condition is not something one should think of as requiring doing continuous work to accomplish it. It happens that most things suspended in mid-air depend on moving the air (e.g. a helicopter blade), which indeed requires constant expenditure of energy. But a pair of repelling magnets is a bit like a table leg - or a steel spring if you prefer. It can achieve the same thing by passive means.

12. Originally Posted by exchemist
But a pair of repelling magnets is a bit like a table ...
A lot like, in fact. They both depend on the electromagnetic interaction for their effect. What makes magnets "mysterious" also makes mundane objects solid.

13. JUST. SAY. NO!

.....to the ghastly temptation to think you can make a perpetual motion machine from effing magnets!

The thing is, a table permanently stops a jar of jam sitting on it from falling to the ground. So can a pair of suitably sized permanent magnets. Maintaining something in a motionless condition is not something one should think of as requiring doing continuous work to accomplish it. It happens that most things suspended in mid-air depend on moving the air (e.g. a helicopter blade), which indeed requires constant expenditure of energy. But a pair of repelling magnets is a bit like a table leg - or a steel spring if you prefer. It can achieve the same thing by passive means.

Oh yes, absolutely. On the first point; I couldn't agree more - no more than you could using gravity.

It's just weird though that two magnets can behave as a spring, but with no physical 'anything' between them - this is what I am struggling to understand.

A spring works because of the properties of the physical metal it is made of, and that is easy to understand. Two magnets, however, can do the same thing but without any apparent physical substance between them to provide the force.

OB

14. Originally Posted by One beer
JUST. SAY. NO!

.....to the ghastly temptation to think you can make a perpetual motion machine from effing magnets!

The thing is, a table permanently stops a jar of jam sitting on it from falling to the ground. So can a pair of suitably sized permanent magnets. Maintaining something in a motionless condition is not something one should think of as requiring doing continuous work to accomplish it. It happens that most things suspended in mid-air depend on moving the air (e.g. a helicopter blade), which indeed requires constant expenditure of energy. But a pair of repelling magnets is a bit like a table leg - or a steel spring if you prefer. It can achieve the same thing by passive means.

Oh yes, absolutely. On the first point; I couldn't agree more - no more than you could using gravity.

It's just weird though that two magnets can behave as a spring, but with no physical 'anything' between them - this is what I am struggling to understand.

A spring works because of the properties of the physical metal it is made of, and that is easy to understand. Two magnets, however, can do the same thing but without any apparent physical substance between them to provide the force.

OB
Well, if you have two objects connected by a stretched spring or rubber band they feel an attractive force, like gravity. We don't find gravity strange.

But for some reason we find a repelling force peculiar. But you get the same between two positive electric charges. So the phenomenon is by no means unique to magnetism.

15. Originally Posted by One beer
Oh yes, absolutely. On the first point; I couldn't agree more - no more than you could using gravity.It's just weird though that two magnets can behave as a spring, but with no physical 'anything' between them - this is what I am struggling to understand.A spring works because of the properties of the physical metal it is made of, and that is easy to understand. Two magnets, however, can do the same thing but without any apparent physical substance between them to provide the force.OB
Magnets are not unique, but because we have access to macroscopic ones, the mystery emerges first there for most people. But the reason that a table is effective in supporting a jar of Marmite (or Vegemite, if you prefer) is also due to a repulsion "without any apparent physical substance between them." Indeed, the reason a spring works is due to the same insubstantial force. So there's a bit of an irony (and not a little bit of circularity) in invoking a spring to explain forces.

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