I understand the OP. Magnets do work. Magnets can pull objects across a table, for example.
Why then isn’t a magnet’s resources depleted eventually?
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Magnets can be used to pull objects across a table. So can string, and the string won’t be depleted any more quickly than the magnet.
But perhaps it’s better to think of it as similar to gravity. Once the magnet is there, then it’s as natural for a ball bearing to ‘fall’ towards the magnet as it is to fall down towards the earth. It’s easier to pick up and move the magnet, and its fields, than it is to pick up a source of gravitational fields, but it’s the picking up the magnet and moving it that’s the source of energy for any work done, not the magnetic fields themselves.
Fundamental forces don’t work that way. They’re properties of matter. As long as the matter exists, the forces exist. An object can’t run out of electromagnetic attraction the same way it can’t run out of gravitational attraction.
And dickishness. Chronos is getting cranky in his old age. The OP seems perfectly valid to a non-physicist like me. People ask questions here to learn. Not to be mocked by those in higher learned professions. There was a much better way to answer the OP.
Magnets do work only when they’re put in an unstable position. It requires an input of energy to get them there. Imagine two magnets in contact with each other. They’re pulling on each other, but no work is being done. Now pull them apart and let them go. They slam into each other. Work was done. Where did the energy come from? It came from you, when you pulled them apart.
In soft iron (which can be magnetized by electricity), the atomic/electron magnets are all lined up into magnets. The North end of each atom to the South End of the next. The the magnets are all laid nose-to-tail so that the magnetism in the total chunk of iron cancels out.
Why? because if the atoms weren’t nose-to-tail, magnetism would spin them around. But if all the atoms in the entire chunk of iron were lined up the same way, the large object would be so magnetic that it would flip /some/ of the atoms the other way.
Put these two constraints together, and you get a piece of soft iron that is all magnetised, but split up into regions that are magnetised one way or the other.
When you apply a magnetic field to the soft iron. one set of magnets (lets say the North-South magnets) get fatter, and the other set (the South-North set) get thinner.
When you mix up other things, you get crystals and/or defects in the material, across which the magnetic regions can’t grow and shrink. So if you magnetise the material while it is hot, then let it cool, the magnetic regions are stuck in size and shape.
Textbooks from the 60’s understood little more than this. And a bit less: the standard theory was that the magnetic regions were randomly organised, and gradually become more organised under the influence of external magnetism. By the time I was learning about it in the 80’s, it was well known that the magnetic regions are /not/ randomly organised, they are nose-tio-tail as described above, and grow and shrink in response to external magnetism.
Consider a system that doesn’t directly involve magnetism OR gravity, since both of these can be ripe with misconceptions.
Take two bowling balls into deep space. Connect the two balls together with a rope and give the entire system a nice gentle spin so that the centrifugal force of the spinning puts tension in the rope.
This system will continue to spin for eternity(*) with no energy input. The rope, however, is definitely under tension. That is, it is applying a force on both balls. The molecules in one bit of rope are pulling on the molecules in the next bit of rope and so on all along the rope. Forces are everywhere, yet there is no energy expended. The forces happen to be electromagnetic in nature.
I think the perpetualness of this system will be taken as obvious for most. (Or not?)
Now cut the rope in half and affix magnets to the cut ends. Then stick the magnets together to “repair” the cut in the rope. Again spin the system. Nothing has really changed. There is tension in the rope, all the forces are still electromagnetic in nature, and there is no energy expended.
If the spinning/centrifugal aspect is distracting, you can put a system in compression instead, say with a C-clamp tightened down on a block of wood. Then replace part of the C-clamp’s rigid structure with two strong magnets stuck together.
The above isn’t meant to be an explanation. Rather, I’m hoping it sparks some new intuition or mental connections.
sub Please don’t bring up GR. It’s not pedagogically relevant.[/sub]
Somewhere, it needs to be mentioned: the relationship between force and energy.
As explained a number of times above, a force where nothing is moving involves no energy transfer.
This is counter-intuitive for us fleshy beasts, as we do expend energy just holding something up. Standing around holding a steel ball at arms length sure feels like we are expending energy. But that is all about inefficiencies in how we operate internally. Replace your arm with a solid beam, and no energy is needed. So to, holding the steel ball up with a magnet. Indeed there is actually no difference in terms of the underlying force carriers. The same underlying core physics is holding the ball up.
But if the force moves something, then work is done. Picking the steel ball up, either with your arm or a magnet expends energy. Waving a permanent magnet about and using it to attract metal objects causes energy to be moved about. Indeed, the formula is trivial. Force times the distance the object is moved by the force is the energy.
Pushing on something when it doesn’t move does no work, and no energy moves about. (Again, us poor meat bags are not well constructed for this, so we expend effort due to internal issues, not against whatever it is we are pushing.) Push on something and it moves: energy is the force times the distance. Your chair does not normally move you upwards to the ceiling. So it does no work.
Pushing or pulling a magnet about when there are external magnetic objects (say in a generator) and the magnet does work. Usually this will end up creating a current in a conductor somewhere. If your system isn’t designed to usefully use this current, the energy will typically just dissipate as local heating.
Since all the science-y stuff tends to give me headaches, I just think as magnets as being made of very, very small velcro.
I’m impressed this thread has gotten this far without an Insane Clown Posse reference.
Yes, that’s Newton’s Third Law, and is true of all forces, of any type, anywhere in the Universe. I’m not sure what you think makes this different from the magnet.
That wasn’t an answer, but it wasn’t mocking, either. You’re right that there was a much better way to answer the OP, but I didn’t know what way that was. The question showed evidence of misconceptions, and so it was necessary to determine exactly what misconceptions were at play before giving an answer, and any answer in turn must be preceded by a correction of those misconceptions. So I was asking follow-up questions to determine what the best answer would be.
So is the answer that magnetism doesn’t “run out” the same way that gravity doesn’t “run out”?
It’s probably more similar to the way that an electron never runs out of it’s -1 charge. Somehow an electron emits a electric field to the ends of the universe without depleting its energy.
Yes. That’s it in a nutshell.
Gravity and magnetism exist for different reasons, and do some different things, but they have that feature in common.
Not really. The OP was really asking about permanent magnets. Permanent magnets do eventually run out. As they say in the classics, “entropy requires no maintenance.” A permanent magnet is a macroscopic arrangement of microscopic magnets (domains) - themselves being arrangements down to the atomic level. It is the regularity and order of that structure that makes it a magnet in the macroscopic sense. That order can and will break down over time.
You can cause a permanent magnet to lose strength by a range of abuses. Striking it hard can loosen up the domains enough that some will flip and the overall order reduces. Heating can do the same thing. Sufficient heat and the magnet won’t recover. An intermediate heat (to the Currie point) and the magnet loses effect temporarily. You can overpower the domain’s alignment with an external field, and by careful staged reduction of the external field strength, leave your permanent magnet de-magnetised. And so on.
Because gravity only comes in one kind - attractive - it is always the same, no matter which way you arrange the building blocks of the matter. So you can’t make the attraction less or greater. It just is what it is. Becuse magnetism comes in two flavours - (N and S for want of a better labelling) you can arrange a whole pile of magnets to form one big magnet, or arrange the same pile of magnets so that from a distance they all cancel out. A run down magnet is that whole pile of cancelling out magnets.
hmmmmm…
Is the chair on a treadmill?
No contradiction. LSLGuy was replying about the fundamental atomic magnet. I was talking about a macroscopic permanent magnet, the one built by stacking heaps of those little ones all in neat bundles pointing the same way.
The atomic ones will stay magnetic whilst the atom still exists - so pretty much until the universe runs down. But any macroscopic arrangement of those magnets is subject to degradation.
They do weaken over time, but not because it has a finite amount of energy stored in it that gets used up (which seems to be what some people are thinking).
[Quote=Quartz]
With an electromagnet, the magnetic field turns off with the electricity.
[/quote]
Only because the resistance of the wire stops the current flow. A superconducting magnet (i.e. electromagnet made of superconducting wires) can work in persistent mode, needing no external power supply. The current just keeps flowing.
Of course if you take a superconducting magnet and let it do work (e.g. bring a piece of metal nearby and let it get lifted up by the magnet), then the current should decrease slightly. But I think the original current will be restored once you undo that work (i.e. take away that piece of metal).
OK, imagine a magnet stuck to the refrigerator. Is the magnet working to remain stuck to the fridge? No, it is not.
Then think about the handle of the fridge, just sticking out there in empty space. Is the handle doing work to not fall to the floor? What keeps that handle stuck to the fridge? The same fundamental force that keeps the fridge in one piece is the same fundamental force that keeps the magnet stuck to the fridge.
The fridge doesn’t need energy input to stay in one piece. The magnet doesn’t need energy input to stay stuck to the fridge.
But what about the pulling force you feel when you put the magnet next to the fridge? Isn’t that work? Yes, but it’s not the magnet doing the work…it’s you. The work was done by you moving the magnet near the fridge. Just like if you lift up a ball and the ball falls, the ball isn’t doing work by falling, the work was done by lifting up the ball in the first place.