Consider the DC current electromagnet hanging off a filing cabinet.
The power is spent to maintain the magnetic field, because that’s caused in this case by the wholesale movement of electrons… (permanent magnets are not the result of such movement of electrons.)
There is no power spent by the magnetic field to hold the magnet to the cabinet.
You could trace this by looking at where heat is generated. It is generated in the electromagnets coil and not in the filing cabinets metal …
The question of how permanent magnets can make magnetic fields and not consume power is really the same as “how come atoms don’t collapse ?”.
The answer to the question “Where is the energy coming from?” is not “Fundamental force”. The answer is “What energy?”. Technical terms like “energy” have actual meaning in physics: They’re not just something you can throw around willy-nilly. By the actual definition of energy, there is no energy being expended by that magnet. It’s not just “energy that we ignore because it’s always on”, it’s no energy being expended at all.
In the case of an electromagnet hanging from a piece of metal, you still can’t say that the energy is maintaining the magnetism. The energy is leaving the battery, and is being converted into heat in the wire. Yes, the wire is also producing a magnetic field, but it’s doing that without consuming any energy at all. The consumption is just the heat.
Energy does not equal Force. There are four Forces. Electromagnetism is one of them. And just like you don’t need to expend any energy to fall. Magnets do what they do because it takes less energy to do so than not to. Gravity is very weak but its reach is far and cumulative. E-M is much stronger but is only close range. Strong and Weak nuclear Forces are stronger still and even much closer range. So much so that we don’t notice them on a daily basis. Solids, liquids, gases, electricity, light, magnetism and chemistry are all effects and aspects of the Electromagnetic Force.
Something I learned from another thread is that what is colloquially referred to as the strong force is really a residual artifact of the color force. The color force is actually infinite and invariant.
edit: ‘invariant’ meaning that its strength does not decrease as a function of distance.
Huh. I think I get it, but it still seems off. I can’t quite get my head around it.
When I was in college I took a higher level math course, and at one point we were doing something involving infinity. The exercises we had to do involved various calculations using infinity, which I was able do simply using what we had been taught, but it didn’t make any sense. I read more in the book we were using, then some in other books, but it still sound right to me. So I went to the professor and asked him about some of the things I didn’t get. We talked for over an hour, and filled a blackboard up. In the end I understood more, but it still didn’t feel right. The professor smiled at me and said to just let it go, and I quote “The concept of infinity has driven more than one mathematician to insanity.”
I think this is going to end the same way. I just don’t know enough physics to think about it correctly.
Toward helping you get there, could you answer the following question:
If I place a rock on a table, it sits there forever and it takes no energy for it to do so. Are you okay this claim?
If so, then we should explain more about how the magnet situation is physically identical. If not, then we can leave the magnet aside for a moment and explain more about energy itself.
IDK, I think it depends on how you approach it. If you come at from the idea of it being unending and unfathomable, well, yeah. You’re going to have problems. But if you come at from being infinitely tiny and insignificant, it makes a lot more sense - at least to me.
For example, something that always made a lot of sense to me was one of Xeno’s paradoxes. It’s the one about never being able to go from point A to point B. Why? Because to do so, you first have to move half of the distance. Then half of the half. Then half of the half of the half and so on. You keep dividing by half off into infinity - but the infinitely small.
Of course the paradox is complete bullshit for obvious reasons, but I think it puts the concept of infinity in its place very nicely.
Yup. The structure of the table holds the rock, it provides more resistance(the normal force?) than gravity (assumed to be the only force acting on it) can apply.
A magnet hanging under a metal cabinet does not have anything physically holding it up. It is not physically attached, it has no normal force acting on it, it stays up only through the power of its magnetism. In the case of the table & the rock, the system won’t fail until the molecules of the table degrade. With the magnet, it doesn’t have molecular cohesion (the strong/weak nuclear forces?) to hold it up.
So I guess one question would be, does magnetism behave like molecular/atomic cohesion, but only under certain circumstances?
(In case it’s not already obvious, IANAphysicist. I just can’t resist trying to make physics make sense.)
But it does take energy. Gravity is pulling it “down” harder than centrifugal force and the moon/sun’s gravity are pulling it “up.” (or sideways or slantways depending upon the time of day etc.)
There is energy holding the atoms of the table together, which keeps them in between the rock and the strongest force currently working on the rock - gravity. There si kinetic energy keeping it all moving at the same astounding pace as the Galaxy within the Universe, and the solar system within the galaxy, and the Earth within it’s orbit. And there is energy keeping the atoms/molecules of the rock in close interaction with each other, thus preventing the rock from simply disintegrating and dispersing in the air.
There is all sorts of energy involved, pulling all the elements, molecules and etc. in all sorts of different directions. But the energy that holds the atoms together is tapped at such a vanishingly infinitesimal level, that is appears to be untouched from our observation.
The “work” involved is that of resisting these tiny outside forces.
Take water molecules. They don’t hold together as strongly or as densely as silicon dioxide molecules. That’s why the ocean responds to the movements of the Moon, but the shore doesn’t. (Except as acted upon by the movement of the water.)
The pull of Gravity is not energy, it’s a force, which is how it can, as you write, balance the centrifugal force and the force from the table. The rest of your post is a continued misuse of energy when force is what’s relevant. This is very much clear in your last few paragraphs:
“pulling” is what forces do, and resisting a force isn’t work or “work” and doesn’t use energy.
No. There are forces binding the atoms to each other. Those forces are electromagnetic in nature, and are stronger than the gravitational force, which is why the table doesn’t fall apart. There are also forces holding the protons and neutrons inside the atomic nuclei together. Those forces are much, much stronger than either electromagnetism or gravity. Gravity is the weakest force operating on the table.
There is also a binding energy intrinsic in the bonds between atoms. This energy is only ‘tapped’ if you break those bonds, such as by breaking or burning the table. Likewise there is a much, much greater binding energy intrinsic to the bonds between nucleons inside the atomic nuclei. Those aren’t tapped unless you take those atoms and break them down in a particle accelerator beam or something.
No. Work only occurs if something is actually moved over a distance. If something is just sitting there, no work has occurred. Resisting a force isn’t work and doesn’t require energy.
Ah, but it does, in the same way as the rock on the table!
Consider a coat hanger. If you set the coat hanger down on the table, it’s just like the rock. What if you hang the coat hanger on the rod in your closet, where the hook is the only thing touching anything, with the rest of the hanger dangling underneath? This still requires no energy to stay put, just like the rock. (Do you agree with this claim?)
The hanger, though, is made up of atoms all holding on to one another. Somewhere on the neck of the hanger (between the hook part and the rest of the hanger), you could picture an imaginary plane slicing through. The atoms above this boundary are holding on to the ones below this boundary. The entire weight of the bottom part of the hanger is being supported by the attraction between the atoms above this plane and the atoms below.
In this case, the attraction is primarily electrical in nature. What if we cut the hook off the hanger, welded little strong magnets to both ends, and then stuck them back together? We’d have a working hanger again, only now the top and bottom are held by magnetic attraction rather than electrical attraction, at least at this one slice. (The other multitude of possible slices are still electrically attracted). The switch to a magnetic support doesn’t change the energy story at all. In both cases, there are pulling forces holding things together.
It is possible that this just makes you question the energetics of the rock-on-table or hanger scenarios rather than providing satisfactory evidence that magnetic support takes no energy. We can see where to go next based on your reply.
So the magnetic force does behave like the nuclear attraction force (in certain circumstances), and the magnet can be considered to be part of the cabinet for purposes of gravity, etc.
The strong force is mediated by the exchange of different force carriers - gluons. Electromagnetism is mediated by the exchange of photons. the weak nuclear force by W and Z particles. I think I got all of that right.
Yep, although I would have said something like “So the magnetic force does behave like electrical attraction, and the magnet can be considered to be part of the cabinet for purposes of gravity, etc.” (The atoms aren’t held to one another by nuclear forces, per se, but by electrical forces. If the nucleus wasn’t intact, the atoms wouldn’t be there, so the nuclear forces are involved indirectly, but the electrical attraction (or magnetic attraction) is the proximate force holding the atoms together.)
To be sure: the “part of the cabinet” way of thinking works, but suggests something that isn’t a requirement. The rock isn’t necessarily a part of the table, and the magnet isn’t necessarily a part of the cabinet. The real underlying idea is that there are electrical and magnetic forces contributing to the stationary positions of the objects, and in some cases these forces are pushing and in others they are pulling and usually there are both, but in all such stationary cases these forces are just there doing their thing and are not using up any energy.
Any decent intro to physics will cover this. I’m partial to Tipler (Paul, not Frank) as it’s stood the test of time. It was my text thirty years ago and it’s now in its sixth edition (2008).
If you want to get started now, Khan Academy has a pretty good introductory series: Physics library | Science | Khan Academy
Forces are covered in the third tutorial and Work & Energy in the fourth.
