[TommyJ]

‘How to be a magnet’. It depends on the size of ferromagnetic and it’s crystal domain alignment.
In a half-field shell electrons are not paired and their tiny magnets are pointing in the same direction. This is intrinsic magnetism of electrons.
But if an atom is magnetic, it doesn’t need that the material made of lots of these atoms are magnetic.
Crystals. Ferromagnetic - a bunch of atoms are aligned in the same magnetic direction. Aligned domains (bunches of bunches) of atoms. This is a quantum property - aligned to macro size.
Domains of material can point in different directions.
Piece of iron may not have a magnetic field at all, because all domains are pointing to different directions.
However if you apply a strong magnetic field from outside the material, you can make one solid unified piece of magnet.
Magnetism is a quantum property magnified to the size of the object.
These criteria are difficult to fulfill. There are few materials that can do that: Fe, Co, Ni, Gd.

Wikipedia:
The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units A/m. A good bar magnet may have a magnetic moment of magnitude 0.1 A·m2 and a volume of 1 cm3, or 1×10−6 m3, and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter.

[Eternal Student (OP)]

Hi again.

     Thanks for your reply TommyJ.  This was mainly about why a material is magnetic but that's a perfectly sensible place to start.  This section in particular is useful and something like this is usually taught at school level:
 

Domains of material can point in different directions.
Piece of iron may not have a magnetic field at all, because all domains are pointing to different directions.

   
   Ferromagnetic objects are considered as a collection of little magnetic regions called domains.  Every domain is a tiny magnet in its own right.  When these domains are aligned then the overall piece of Iron will behave like a magnet.
   With this in mind it is possible to argue that all the potential energy you require to explain what happens in reply #39 was actually always there.  There was always the potential to recover kinetic energy by allowing other lumps of iron to approach any one of these domains in your magnet.  The total potential energy available is then the sum of the potentials for all the domains.
    To phrase this another way, the little bit of energy we put in to induce permanent magnetism in our Iron bar wasn't creating the massive store of potential energy.  That store of potential energy was always there.  Inducing magnetism into our Iron bar just made it possible for us to access that store.

OK, that's one argument that could be presented and (I think) might be acceptable for school-level.   Anyone else have another argument?

Best Wishes.
 

[yor_on]

Made me smile ES

"   We remember  1.    We forget about 2.   We don't even ask the question why isn't the weather in France related to my choice of breakfast?"

Hmmm, entanglements?

[yor_on]

Well, there is a symmetry to it?
Magnetism I mean.

https://www.nap.edu/read/10118/chapter/7

[alancalverd]

"Mineralogy Crystallography and Metallurgy" was Tuesday and Thursday afternoon, more than 50 years ago, but IIRC Tommy is right - the process is one of forcing the naturally random moments and domains into alignment, so the work done is principally overcoming the "thermal" randomness of the raw lump of material. Once scrunched, whether they remain aligned or not depends on the impurities present or added to the iron, so you can get "soft" or "hard" magnetic materials by judicious alloying and tempering.

I tried using the analogy to teach 10-year-olds the value of tight binding in a rugby scrum but our masters at Twickenham thought I was mad and wouldn't add it to the curriculum. Don't they learn anything at primary school?

I once managed to shunt a car, just enough to magnetise it without rendering it undriveable. Made navigation easy thereafter as whatever road I took, the compass indicated "south". 

 

[Eternal Student (OP)]

Hi.

I once managed to shunt a car, just enough to magnetise it without rendering it undriveable. Made navigation easy thereafter as whatever road I took, the compass indicated "south". 

    I'm sorry to hear this.  Did you try getting the car warm and then driving into something else, or just go around the car hitting it with a small hammer?
    Seriously, how did they de-magnetise the car?

[alancalverd]

I didn't bother - just ignored the compass. Which reminds me of another adventure.

Lost with a rental car but no map, I phoned my client and explained the predicament. He said "This is Ireland. Stop the car for a few minutes. The wettest window is on the west." No problem.

[Eternal Student (OP)]

Hi Yor_on,

Well, there is a symmetry to it?
Magnetism I mean.

     I'm not sure what you meant.   Also, I only find a thing that I have to download on the website you linked to.  It looks like an entire book.   I was a little concerned about downloading anything since I don't recognise the site.   Is it really worth downloading?  Are you strongly recommending it or was it just a few sentences in the book that you could copy-and-paste into the forum directly?

Best Wishes.

[Eternal Student (OP)]

Hi again Alancalverd,
  for the joke.

Lost with a rental car but no map,

   That reminds me, did you ever hear anything from your school Physics teacher friend?   Don't worry too much if you didn't.

[yor_on]

So what are you thinking of ES? One doesn't necessarily need to download the book to read that page about symmetries? I thought you might refer to laws and properties but if it is something else it's time for you to state it, as nothing seems to fit?

[alancalverd]

That reminds me, did you ever hear anything from your school Physics teacher friend? 

Actually my son, who may join the forum but said en passant that it is indeed a bit byzantine at present.

[yor_on]

Okay ES. I'm guessing again but it might have a relevance to 'fields', and this EM 'photonic force' of 'virtual particles'? Is that what you're aiming at?

Can't help linking this one. https://hackaday.com/2016/03/21/just-when-you-thought-magnets-werent-magic-magnets-are-mechanisms/
=

Now if that is the what you're thinking of we need some history to it, well, as I think. Because the modern representation is connected to special relativity.

so first,  one should read this   https://en.wikipedia.org/wiki/History_of_special_relativity

And when that is done, this    https://en.wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity

and then this     https://en.wikipedia.org/wiki/Magnetostatics

not that it make it simpler :)
=

As I dabble in physics again, we should probably add this too.

" Whether a particle is or is not elementary depends on the description level. As our knowledge about microscopic physics increases, objects once considered elementary were found to have a substructure that can be modeled on a more detailed level. Thus atoms, once thought of as elementary are today described as consisting of nuclei and electrons; nuclei are described as consisting of protons and neutrons, and the latter is thought of consisting of quarks. Whether quarks have substructure is presently unknown.

A particle is considered to be elementary (on a given description level) if it is sufficient to describe it by an irreducible unitary representation of the Poincare group or its Lie algebra. This group is dictated in QFT by the symmetries of space-time at experimental microscopic scales. The 10 independent observables of this Lie algebra are the four components of a 4-vector ##p## describing momentum, three components of a 3-vector ##J## describing angular momentum, and three components of another 3-vector ##K## describing infinitesimal boosts.

For massive particles such as the electron, one can construct from these an additional 3-vector ##q## describing the particle position in an observer-dependent frame. For massless particles such as the photon, a sensible position vector does not exist. "

https://www.physicsforums.com/insights/physics-virtual-particles/
=

Just a comment. It makes it time dependent, aka depending on ones 'frame of reference' as I see it. And to understand ones 'local frame of reference' correctly you need to see that your local 'clock' never lies, and that it will be the clock you use to define all other 'clocks' relative.

But it should also be connected to HUP, aka 'virtual particles' spontaneously creating a 'pair production' of 'real particles', under a time and energy constraint. The more 'energy' the more 'virtual particles/energy' creating those 'real particles'.

But still governed by HUP and Planck time. That one might be questionable but that is how I think about it. I can't find a link to the question if a pair production can exist over a Planck time before annihilating itself but I would expect it to be governed by the uncertainty principle.

And that should place it outside our local definitions of what's real. And so outside Plank time, except in very special circumstances as the 'Big Bang' in where it is assumed to create 'real particles'. And to do that you need something more than just a 'spontaneous pair production'. You need 'something' changing it.

So there are two aspects to it. One what I tend to call 'global', frames of reference interacting, the other one strictly local
=

While this may move it into 'New Theories' we could add that if 'time' and 'clocks' is something belonging to SpaceTime, and then add that what was before it involved no clocks, it might be one approach to defining it. You could also differ between the idea of 'time' as a law, versus the idea of 'clocks' as its property proving its existence to us.

The point there being that 'dimensions' as such isn't a preexisting factor defined this way. Instead being a result of the Big Bang and clocks coming into existence. And what it should do to our universe is to define it as a exception, a 'singularity'.

[Eternal Student (OP)]

Late editing:  Skip to the summary at the end of this post if you get bored while reading it.

Hi again,   
   Thanks for all your comments Yor_on.   The scope of this thread is growing beyond anything I imagined.  The links you provided are interesting but I can't respond to all of them today.  I also think it will go far beyond what was originally described as a "school-level question" in my title.

So what are you thinking of ES?.....  I thought you might refer to laws and properties but if it is something else it's time for you to state it, as nothing seems to fit?

    It probably is time to say something about what I think and perhaps explain why I don't think gravitational potential energy is required.

1.   Potential energy (of any kind) is something that tends to be invented (in the sense of given a name) whenever existing forms of energy are insufficient to account for the total energy in the system.
      Alancalverd presented one example of this in his/her earlier posts.   For simple Newtonian systems with gravity it is noted that
   = constant.
and then the quantity mgh was called gravitational potential energy.

[see reply #22 and #25 on this thread for more details]

  There are many examples where the term "potential energy" is introduced just because it's convenient.  It represents the most abstract part of a conserved quantity that exists in some systems.   Specifically, since Total Energy is conserved, we tend to sum all the forms of energy that can be seen and then subtract this from the total energy.  This "bit left over" is given the name "potential energy" but it is abstract and (in my opinion) quite misleading to suggest that this potential energy is any sort of fundamental or tangible thing that exists in the universe.

2.   Historically, the existence of a conserved quantity that could be considered as energy was a conjecture (a generalisation or extrapolation) based on what we knew for only a small number of special cases.

3.   The first mathematically rigorous proof that systems have conserved quantities which we could call Total Energy  (and also momentum and angular momentum) was provided by Noether's theorem circa. 1918.  This assumes that a "system" can be described by a Lagrangian and a least action principle governs the evolution of the system.  I am not aware of any proof that all things we might want to consider as a physical system can be formulated in this way (with a Lagrangian).  However, many of them can and it is now so widely accepted that this is the correct way to model physical systems that we will just go along with the idea that all physical systems can be described with a suitable Lagrangian for the remainder of this post.
      Noether's theorem showed that symmetries were required in the physics of the system to produce these conserved quantities.  There is a correspondence between the symmetries and the conserved quantities.  Given a symmetry we can find a conserved quantity but also we can reverse this - given a conserved quantity we can find a symmetry that the Lagrangian must obey.
       This is all inherently abstract mathematics.  It describes energy as a mathematical expression involving several canonical variables that were sufficient to describe the Lagrangian of the system.  These expressions for Energy don't always break apart into easily identified components.  It is just a relationship between some canonical variables that are unique to the individual system.
        In particular, Noether's theorem does nothing to identify fundamental forms of energy that may exist in the universe.  In many systems there isn't a quantity that looks like or behaves like gravitational potential energy.  Furthermore, Noether's theorem does not define energy as the "ability to do work", it is just an abstract relation between some variables.

4.     Our best theory of gravity is still General Relativity (but I'm biased because I like it).   In this, it is important to identify all sources of energy-momentum since this will be the source of gravitation.  In particular, the energy density due to the content of the manifold must be specified. 
  (i)  You are not required to include gravitational potential energy as a form of energy-momentum that contributes to the stress-energy tensor.    (This was discussed in an earlier reply).
  (ii)  You are not able to include gravitational potential energy as a source of mass-energy even if you wanted to.  Some of this was also discussed in earlier replies.  There is a problem knowing where in space this energy would be located (for example, is it in the earth or in the apple, or spread out in the space between them).   You don't actually have a working model of gravity as described by GR yet so any method of calculating gravitational potential energy would need to be based on some other theory available to you - such as Newtonian gravity.   There is also a problem trying to determine how much gravitational potential energy there would be due to each particle in your system.  For example, if you were using Newtonian gravity to calculate the potential energy then you have this formula to work with:
   Gravitational potential energy =       so that grav. p.e. → ∞ as r→0.
  We can then place two particles of matter close together and provide arbitrarily large (large negative) energies.   If we assume that this energy is located in the vicinity of those particles then strange things would happen.  It would make two small particles of mass m that were close together the single most important source of gravitation in the manifold,  far more important than say one particle located further away with a mass of 1000m.  Now, it seems that this is not what happens in reality.  Newtonian gravity describes the situation well enough for most purposes.  If we put a test mass half-way between  the  1000m  mass particle and the  pair of smaller particles of mass m that are close together,   then the pair of close particles just behave like one particle of mass 2m.  The test particle is not attracted to the close pair of small particles, instead it is pulled toward the more massive 1000m particle.  Just in case you were concerned that the energy is a large but negative in the vicinity of the two close particles,  the test mass isn't repelled by those two close particles either - it just sees them like one particle of mass 2m.
     We have already discussed the idea that gravitational potential energy should NOT be included as a contribution to the stress-energy tensor but this makes gravitational p.e. stand out from all other forms of energy.  In the theory of GR all forms of energy are sources of energy-momentum for the stress-energy tensor and we've got to ask why gravitational p.e. wouldn't be included - perhaps it is not a fundamental form of energy after all.
    Another reasonable possibility is that if gravitational potential energy exists and should be included as a source of energy-momentum for the stress-energy tensor then it is not localised in or around two objects that are a certain distance apart.  Instead that energy is spread uniformly throughtout the manifold.  We already acknowledge that we cannot measure the absolute value of most forms of energy that contribute to the stress-energy tensor.  The electric and magnetic fields are a common example since day-to-day physics only involves the differences between these potentials not the absolute value of those potentials.  Anyway, if gravitational potential energy is a form of energy that is spread uniformly throughout the manifold then it would seem to be a component of the vaccum energy.
    OK,  I've over-run my time here again.   Let's just say that I don't think gravitational potential energy is required, we already have vaccum energy in GR.

    (iii)   Finally, I should mention that, unlike the Newtonian theory of gravity, gravitational potential energy does not emerge as well defined quantity after GR.  There are some situations where time-like Killing vectors can be identified and symmetry conditions like those required in Noether's theorem are met - but there are many situations where this can not be done.
In general relativity gravitational energy is extremely complex, and there is no single agreed upon definition of the concept. It is sometimes modelled via the Landau–Lifshitz pseudotensor....

[Taken from Wikipedia  https://en.wikipedia.org/wiki/Gravitational_energy#General_relativity]

Summary
1.     Energy is an extremely complicated thing.
2.     Gravitational potential energy is more likely to be an emergent property.  Some systems have an abstract relation between some variables that we can identify as and call gravitational potential energy but this is a property of that system.  Gravitational potential energy is unlikely to be a fundamental form of energy that exists in the universe. 
3.      If we step back and re-examine something that @Halc mentioned in reply #5 we can start to make sense of it another way.  If we consider a system that is the earth with all it's apples on the trees  compared  to a system which is earth with all it's apples on the ground,   then we can attempt to consider how much force would need to be applied to these entire systems in order to start to move them.  The system with the apples on the trees, has a slightly higher inertial mass then the system with all the apples on the ground.  So the increased potential energy of the system seems to show up as a change in inertial mass of the system.  This is one interpretation for the quantity Halc described as "system mass".  It's perfectly sensible but only required because we were considering the earth-and-apples system as if they were one combined particle.  If we take a suitable reductionist approach to analysing these systems then gravitational potential energy is not there to be found and indeed neither the earth nor the apples have changed their mass at all just because their positions have changed.
4.      I only started the thread to discuss some basic science that we start teaching children at school and mainly just to pass a few evenings in discussion.  If the thread has made anyone pause to reconsider what they thought they knew about Energy and how they should present to children then that'll be a bonus.

Thanks to everyone who has spent some time here.  You are all free to continue commenting of course and indeed you can rip apart anything I've said apart if you want to. 

Best Wishes.

[AJ_WWE]

‘How to be a magnet’. It depends on the size of ferromagnetic and it’s crystal domain alignment.
In a half-field shell electrons are not paired and their tiny magnets are pointing in the same direction. This is intrinsic magnetism of electrons.
But if an atom is magnetic, it doesn’t need that the material made of lots of these atoms are magnetic.
Crystals. Ferromagnetic - a bunch of atoms are aligned in the same magnetic direction. Aligned domains (bunches of bunches) of atoms. This is a quantum property - aligned to macro size.
Domains of material can point in different directions.
Piece of iron may not have a magnetic field at all, because all domains are pointing to different directions.
However if you apply a strong magnetic field from outside the material, you can make one solid unified piece of magnet.
Magnetism is a quantum property magnified to the size of the object.
These criteria are difficult to fulfill. There are few materials that can do that: Fe, Co, Ni, Gd.

Wikipedia:
The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units A/m. A good bar magnet may have a magnetic moment of magnitude 0.1 A·m2 and a volume of 1 cm3, or 1×10−6 m3, and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter.

Thanks for the link. But this rule does not always work. In our university laboratory, we used a press to draw stainless steel. After this process, part of the material was magnetized, and part was not. I can not understand how the material properties could change so much.

[JimmyW9]

‘How to be a magnet’. It depends on the size of ferromagnetic and it’s crystal domain alignment.
In a half-field shell electrons are not paired and their tiny magnets are pointing in the same direction. This is intrinsic magnetism of electrons.
But if an atom is magnetic, it doesn’t need that the material made of lots of these atoms are magnetic.
Crystals. Ferromagnetic - a bunch of atoms are aligned in the same magnetic direction. Aligned domains (bunches of bunches) of atoms. This is a quantum property - aligned to macro size.
Domains of material can point in different directions.
Piece of iron may not have a magnetic field at all, because all domains are pointing to different directions.
However if you apply a strong magnetic field from outside the material, you can make one solid unified piece of magnet.
Magnetism is a quantum property magnified to the size of the object.
These criteria are difficult to fulfill. There are few materials that can do that: Fe, Co, Ni, Gd.

Wikipedia:
The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units A/m. A good bar magnet may have a magnetic moment of magnitude 0.1 A·m2 and a volume of 1 cm3, or 1×10−6 m3, and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter.

Thanks for the link. But this rule does not always work. In our university laboratory, we used a press to draw stainless steel. After this process, part of the material was magnetized, and part was not. I can not understand how the material properties could change so much.

It depends on the steel grade. Google the article Behavior of steel as a living organism. There this phenomenon was described. My university supervisor was one of the co-authors of the article. Using plagiarism checker for students https://fixgerald.com/plagiarism-checker-for-students [nofollow] I took some of the material for my term paper. So it might be useful for someone else. Interesting analytics and research.