Permanent magnets and energy question

[Glenn G]

TL;DR Summary

Loss of magnetic stored energy over repeated use

Hi,
I have read some threads on this but I still have some confusion.

1) I get that work has to be done to produce a permanent bar magnet (work to align the dipole moments to give a net field, say in a solenoid/electromagnet) so we can say that the magnet is a store of magnetic potential energy.

2) Imagine I had a huge number of unmagnetised steel bolts and I place them one by on against the magnetic pole of the permanent magnet. I get that the bolt, due to magnetic induction will stick to the permanent magnet.

3) say I have a lot of time on my hands so I wait until the steel bolt becomes permanently magnetised

4) I now pull this steel bolt off and set it aside (I get that I would have to do work to separate the bolt from the magnet, not sure if this has anything to do with the real thread of the argument.)

5) I then keep repeating with load of other bolts.

6) Seeing as these bolts are only being magnetised due to their interaction with my original bar magnet IS IT TRUE that overtime the bar magnet will become less magnetic?? If this is not the case then I'm a bit confused as an initial amount of magnetic stored potential energy seems to be able to store magnetic potential energy in an infinite number of initially unmagnetised bolts.

I'm sure I'm missing something would love some input please.

 

[Keith_McClary]

Summary:: Loss of magnetic stored energy over repeated use

do work to separate the bolt from the magnet

Work is also done as you move the unmagnetised bolts towards the magnet. This may be different than the work to separate. I am not saying that this answers your question, just that it is part of the total energy budget.

 

[Glenn G]

Work is also done as you move the unmagnetised bolts towards the magnet. This may be different than the work to separate. I am not saying that this answers your question, just that it is part of the total energy budget.

Was thinking about that but the bolts would be attracted so that work would be done by the magnet itself

 

[sophiecentaur]

Summary:: Loss of magnetic stored energy over repeated use

I'm sure I'm missing something

The work done (as mentioned above) doesn't have to come from the permanent magnet, although any disturbance can reduce the magnetic energy in it (a second order effect). The work is done by the input of the hand / arm.
I think a suitable analogy is to dig a hole in the ground and then to stretch a spring with a ratchet by hanging a mass on a spring down into the hole. You then remove the permanently stretched spring and mass from the hole. You can repeat this with another mass and spring. Each process leaves the 'negative GPE' that the hole made available and just transfers energy from the arms to the energy storage spring.

 

[Charles Link]

The permanent magnet is a stable state that is lower in energy than the unmagnetized state. For this reason, permanent magnets that are part of electric motors usually have very long lifetimes. For the case of iron bolts that are not permanent magnets, the magnetized state may be higher in energy than the unmagnetized state. The case of the iron bolts in close proximity to a permanent magnet seems to be somewhat more complicated in regards to what is energetically favorable, but the magnetized state appears to be more energetically favorable in many cases. I haven't seen the details of this written up in the literature though, and I'm somewhat at the drawing board on this particular item.

 

[alan123hk]

Summary:: Loss of magnetic stored energy over repeated use

I'm a bit confused as an initial amount of magnetic stored potential energy seems to be able to store magnetic potential energy in an infinite number of initially unmagnetised bolts.

The permanent magnet is placed in a fixed place and will not move, and its own magnetic field will not change, so the energy does not come from this permanent magnet.

When you move countless bolts one by one to the permanent magnet to magnetize it and then move it away from this permanent magnet, you has indeed successfully stored energy in an infinite number of initially unmagnetised bolts. But this infinite energy must be provided by your hand when you move the bolt by hand.

When the bolt moves to the permanent magnet, the force exerted by your hand on the bolt is opposite to the movement direction of the bolt, so your hand will not provide energy to the system, but when the bolt moves away from the permanent magnet, the force your hand exerts on the bolt is in the same direction as the bolt's movement, so your hand provides energy to the system.

You can try to do an experiment to verify whether my statement is wrong.

 

[alan123hk]

You can try to do an experiment to verify whether my statement is wrong.

Even if my statement is not wrong, I admit that my explanation is only a preliminary view and does not touch the core of the problem.

Obviously people will ask, since our hands actually get energy from the system when the bolt gradually approaches the permanent magnet, then we only need to store this energy and use it to move the bolt away from the permanent magnet, so the net energy input from our hands to the system is zero.

However, as long as we think about it carefully, we know that this may not work, because we have assumed that after the bolt is magnetized, its magnetic force will not disappear as it moves away from the permanent magnet (In an ideal situation, after the bolt leaves the permanent magnet, its own magnetic field always remains near the maximum magnetization obtained when it is closest to the permanent magnet.), so it is obviously that when the bolt moves away from the permanent magnet, the force it feels should be greater than when it is close to the permanent magnet. Therefore, the energy required to move the bolt away from the permanent magnet should be greater than the energy obtained when the bolt is close to the permanent magnet.

I also want to do an experiment to prove whether my imagination is correct, but unfortunately I don’t have the tools I need.

 

[sophiecentaur]

since our hands actually get energy from the system

I think it's time to bring in the notion of Hysteresis. Putting and taking a steel bolt will involve a net loss of energy (work) and that lost energy will turn up in the magnetisation of the bolt.
I like to work from the principle that conservation of energy has to apply and then look for mechanisms that account for energy flow. If you did the experiment and plotted a Force/Distance graph over the cycle, there would be a 'hole' in the middle.

 

[alan123hk]

I think it's time to bring in the notion of Hysteresis. Putting and taking a steel bolt will involve a net loss of energy (work) and that lost energy will turn up in the magnetisation of the bolt.
I like to work from the principle that conservation of energy has to apply and then look for mechanisms that account for energy flow. If you did the experiment and plotted a Force/Distance graph over the cycle, there would be a 'hole' in the middle.

Yes, what you said is very important, because in this case, hysteresis loss will inevitably occur, especially this loss is very large in materials such as iron or steel. Therefore, hysteresis loss must be considered in the operation described by OP.

 

[sophiecentaur]

Yes, what you said is very important, because in this case, hysteresis loss will inevitably occur, especially this loss is very large in materials such as iron or steel. Therefore, hysteresis loss must be considered in the operation described by OP.

So, I suggest that the hysteresis curve would have a smaller area in subsequent cycles as the magnetisation energy may not be involved after the first insertion. All you'd have is 'resistive' losses.

 

[Charles Link]

One other thing that needs to be addressed is the magnetic domains. In a high quality permanent magnet, the domains are all aligned for the most part, and the stable configuration is the completely magnetized state. In a magnet of lower quality, you may get domain formations with magnetization in various directions. I believe this is what happens in soft iron when it is in an unmagnetized state, but perhaps someone else can confirm or refute this=On some of these items, I'm still on a learning curve.

 

[votingmachine]

I'm having trouble with the initial statement that a magnet is a store of potential energy. I don't think it is. Is a planet a store of gravitational potential energy? It simply has mass. A rock on a mountain and the Earth have potential energy, due to the separation. But neither has potential energy without the other.

Put another way, a magnet doesn't lift an iron thing. It has an attractive force, and if YOU lift the magnet, you also lift the iron. YOU are the energy potential. You could have used velcro. You could have used any attachment. None would be "energy".

 

[gary350]

Steel & cast iron will stick to a magnet but not all with store up and hold a strong magnetic field. I have done a lot of research with magnets. Magnetic technology has gotten much better in the past 20+ years. The chemical make up of a magnet is what makes a good strong magnet that holds a strong magnetic field for many years. Magnets made 100 years ago were not so good they slowly lost their magnetic field. NOVA TV SHOW did a show about magnets 20 years ago you might find it on YouTube. Magnets are charged by discharging a capacitor bank into a coil the magnet pulse charges the magnet. Take a permanent magnet generator for example, if a gas engine turns the generator the gas engine is doing all the work of producing electricity.

 

[rude man]

You have to be careful applying classical physics to permanent magnets.

A permanent magnet does not lose magnetization when it transfers magnetism to unmagnetised objects. e.g. those exhibiting large levels of ferromagnetism.

Permanent magnets owe their magnetism to two phenomena: mostly to the spin of aligned electrons about their own axes, a small rest to the revolutions of those electrons about their nuclei. These "Amperian currents" see no resistance and so continue indefinitely unless heated or jarred (causing electron axes to partially or wholly unalign).

This at any rate is what was theory in the 1960's. If any later description is at hand I've never seen it. Perhaps some of our high-powered solid state physicists know better.

 

[alan123hk]

I'm having trouble with the initial statement that a magnet is a store of potential energy. I don't think it is. Is a planet a store of gravitational potential energy? It simply has mass. A rock on a mountain and the Earth have potential energy, due to the separation. But neither has potential energy without the other.

Put another way, a magnet doesn't lift an iron thing. It has an attractive force, and if YOU lift the magnet, you also lift the iron. YOU are the energy potential. You could have used velcro. You could have used any attachment. None would be "energy".

My own understanding is as follows, but I am not sure if I am correct.

If the material is attracted to the magnet and moves closer to it, the magnetic potential energy goes down. That’s just like when something falls toward the Earth and the gravitational potential energy goes down. Now if the object is stopped somehow (say by friction) that energy turns into thermal energy, just like when a ball gradually stops bouncing.

If something (e.g. you) then pulls the object away from the magnet, then that’s what supplies some more energy to the system- the same as how you have to supply energy to lift something up in a gravitational field.

However, since the magnetic field is generally considered to be a non-conservative field, even if there is no energy loss due to friction or impact in the path of motion, the work done along the path in the configuration space may not only depend on the end points of the path. it may depend on the actual path taken.

But what is interesting is that the system mentioned by OP contains ferromagnetic materials and permanent magnets, so the energy conversion process can include the magnetization and demagnetization of ferromagnetic materials, and the ferromagnetic materials may have hysteresis loss in some form of motion path.

 

[hutchphd]

The motion of a charged point particle in a uniform field (in say x direction) is a helix down the x-axis. If the otherwise uniform field increases in the x direction the translation speed $v_x$ can be made to change sign (this is a "magnetic mirror"). The total KE of the particle remains unchanged and no magnetic work is done.
This would seem the classical nature of this force and the rest is bookkeeping I think.

 

[256bits]

'm having trouble with the initial statement that a magnet is a store of potential energy

The energy is stored in the magnetic field.

Take an air coil connected to an EMF.
Current will flow setting up a magnetic field.
Disconnect the EMF and the stored energy of the magnetic field can be recovered as it collapses.
Under 'ideal' conditions as much energy as was initially put into the system, can be recovered.

If the coil has a ferromagnetic material within, the field is 'easier' ( commonly referred to as such ) to set up, in so far as that the field lines can be more concentrated within the ferromagnetic material. But which contribution to the field comes from the current and which comes from the self magnetization of the material.
See http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfield.html

Most people would be used to seeing the β = μ_mH , where u_m is given in tables, but not the formula with the M term ( true for me also ).

The free space field can be recovered, leaving the M field of the permanent magnet.
We had to add extra energy, though, to move around some material ( the domains and electric dipoles ), resulting in a stressed material, with dimensional changes, and heat, ( If I recall, permanent magnets are a bit longer than their uncharged cousins, maybe fatter too in some cases ).

Can that energy be recovered - not sure.
Thinking thermodynamically, in that the switching back to an uncharged state would result in the heat thing again for a ferromagnet.

Although, ...
Although magnetic cooling is a thing - paramagnetic salts for example.
https://en.wikipedia.org/wiki/Magnetic_refrigeration