# Physics question (magnetism)

Magnets can pick up metal objects and hold them indefinitely against the force of gravity.

Since it must take energy to do this, where does the energy come from? I’m talking about permanent magnets here, rather than electromagnets which have a power source. But a permanent magnet is just a hunk of iron with no “fuel”.

Can anyone with a better grasp of physics than me explain, please?

No, it doesn’t take energy to do this, any more than it takes energy for a table to hold up a book indefinitely against the force of gravity. Both gravity and magnetism can apply a force, but neither is a source of energy.

OK, forget about the “holding up” aspect and consider the initial picking up of the object.

Hold a magnet a short distance above an iron object and it will jump off the ground and cling to the magnet. This is a force acting at a distance, which means work has been done. And as I understand it, it takes energy to do work. What am I missing?

What type of magnet are you referring to? I understand you mean to dismiss “electro-magnets”. Which can be turned on/off by cutting the electricity. Are you then referring to a modern “permanent” magnet which is made by running an electric current through steel and polarizing it to become magnetic. This is a man-made and temporary process. The force being derived from the electricity that was passed through the metal. It will eventually depolarize.
Like Flex was saying, magnetism is a force but it does possess energy in the form of stored energy in the alignment of the atoms which give it the properties of magnetism. Like the book on the table. It tokk energy to put it there and thus has potential energy stored in it.
A magnet… any magnet is the force produced in an object by an electrical charge aligning the atoms along polar lines. The electrical charge can be natural or man made. Permanence depends on the nature and intensity of the charge and properties/materials in the object.

The OP has already disqualified elecro-magnets. The OP simply refers to your everyday fridge magnets, for example. Aren’t they doing work against gravity to lift iron filings, or to hold themselves 3-ft from the floor when clinging to your fridge? What about one magnet hovering 1" over the other by repulsion?

of course they are. I didn’t mean to be confusing about the type of magnet he meant. It’s just that all magnets are electrical in nature. I wanted to clarify that he did mean what most of us understand as an electromagnet. One which ceases to magnify when the current stops.
Again… the force found in a magnet is the result of potential energy stored in the object when it was polarized. In a man-made/permanent magnet the amount of electricity used to create the polarization was used to align the atoms in the ferrous/steel and was stored in the atoms to maintain their alignment. In lodestone/natural magnets the force is generally much weaker but still measurable and is man’s original source of magnets.
I could go on but I feel I’m being redundant.

Yes, that is what I meant by an “electromagnet”. Thanks for the explanation.

Again, it’s no different than if you take a rock to the top of a building and let go. It “jumps” to the ground - the energy came from your carrying the rock up (stored potential energy). With a magnet it’s the same - the energy comes from separating the magnetic object from the magnet, even if you didn’t do the separating.

When you pull the two objects apart to reset the system to its initial configuration, you’re exerting some force over some distance and therefore adding the energy back in to the system. So the same amount of magnetic potential energy was converted into kinetic energy, then into sound and heat, and then added back in by whatever external agent pried the two things apart. Total net energy loss/gain of the system on this closed cycle: zero.

Or, what flex727 said.

I really don’t know much about the topic but no, the magnet holding the paper on the fridge is not doing work. Just like you pushing the wall that doesn’t move is not work (no matter how tired you get). If there is no movement, there is no work.

That said, I don’t know the answer to the OP.

• if I hold it a short distance away from a metal object (such as a fridge), it jumps across the distance. That takes energy - is the magnet degrading in power?

• the magnet stays on the fridge despite gravity. Is this using up the magnetic energy? Will it eventually fall off?

First of all, the type of magnet you’re using is irrelevent. Magnetism is magnetism, and whether it’s generated by an electric current, by a permanent magnet, or electrically charged angels dancing around the head of a pin matters not, it’s still the same force. Both gravity and electromagnetism are conservative forces; that is to say, an object in their field has a constant amount of potential and kinetic energy. The only way this changes to to impose an additional, outside force on the system (this being both the object in question and the mass or magnet attracting or repulsing it).

In the case of a lowering a permanent magnet to pick up a ferromagnetic object (say a steel ball bearing) from a table, the work is being done on the magnet by its attraction to the ball bearing; as the magnet gets closer, the holder has to restrain it from jumping to the magnet, until it gets close enough that the magnetic force overwhelms gravity and it jumps up to the magnet. The work done on the system comes from resisting the pull of the magnet, up until the point that you pick it up, and then it can be measured by the weight of the ball bearing and the vertical distance it moves.

In truth, with respect to the magnet, every ferromagnetic object has a given level of potential energy based upon the strength of the magnet, even though it will never come within an influential proximity of the vast majority of these objects. So, in the larger system (ball bearing, magnet, and Earth) energy still remains constant, save for whatever energy you input or remove by moving the magnet around. Picking up a ball bearing with a magnet is really no different than picking it up with your fingers, and in fact electromagnetic attraction and repulsion is involved in both actions (see the previous thread: [post=5771403]Why can’t my hand go through my desk?[/post]).

Now, some smart-alec is going to come along and ask why electromagnets take so much damned energy to generate a field while permanent magnets seem to take nothing at all. The answer to that is that electromagnets generate a field by moving electrons around in a big loop, and in most common materials at normal temperatures this causes a great deal of loss. You can hear this in the disturbing humming of big electromagnets in a crane. However, a superconducting loop would (in theory) have no losses at all, and so all you’d have to do is charge it up and seal it off from ground. The reason permament magnets work is conceptually more complicated and requires quantum theory to explain, but essentially it’s a result of the combination of electron spin and (atomic) orbital angular momentum resulting in a dipole moment. So instead of going around in a loop it’s just spinning frenetically about itself. (The last statement is, strictly speaking, totally wrong, 'cause that’s not the way things work on the quantum level, but it’ll do as an analogy until you crack open Volume III of Feynman’s Lectures.)

I hope that makes things clearer than a Teamster’s pension plan audit. Now, if only this kind of stuff impressed the chicks…

Stranger

Ouch, my head hurts! : )

Very interesting. I guess the idea of “potential energy” always seemed like “hand waving to me.” I mean, does everything have potential energy, i.e., it will fall if what’s under it is removed? (Starting to sound like a Monty Python skit: “the society of people who put things ontop of other things”, or something like that…)

To explain further: A system composed of a magnet and some other things can have energy stored in it, and that energy can be used up. But just sticking to something is not one of the ways that it gets used up. For instance, if I have a powerful magnet and a piece of iron held by clamps some distance apart, there’s energy stored there. If I let go of the clamps, the piece of iron will start moving towards the magnet. While it’s doing so, it’s using up the magnetic potential energy, and turning it into kinetic energy (which will in turn get turned into heat or some other form when they collide). But after they collide, you’re not adding any more kinetic energy, which means that you don’t need to take away any more magnetic energy.

So do “magnets” (naturally occuring ones or man-made) lose power over time? Would a magnet in a compass in the old days have eventually become dysfunctional for no other reason than time?

To add to this, assuming an elastic collision (i.e. no permanent deformation or hysteresis in either of the two bodies) the kinetic energy of the system remains constant. If one or both of the bodies does deform or in some other way convert the kinetic energy into some other form (sound, light, heat) then the collision is inelastic. Energy is still conserved in the total system, but now that system has to include the ambient environment into which the energy was radiated. (All real collisions are in some way inelastic on the macro level.) Either way, momentum is always conserved.

drhess, consider this from a cosmological perspective. In the beginning, the Universe was created. This made a lot of people very unhappy and has been widely regarded as a bad mo…oh, sorry, wrong text. In the begining, the Universe was comprised of a very tiny ball of all energy and matter (in the form of fundamental particles, i.e. quarks and gluons). As space expanded, the energy stored therein pushed everything apart, and the fundamental unified force degenerated into electromagnetic, gravitational, and nuclear forces we know and love today. Although these forces have strong attractive potentials, there was also a heck of a lot of kinetic energy that caused the frenetic particles to distribute across the universe; nonetheless, the particles with mass or like charges are still attracted to each other and have a resultant potential (gravitational or electromagnetic) energy with respect to one another. So at one time all the players were stuck to one another, and now they’ve been seperated like two ends of a spring, but they still have the intervening coils trying to pull them back together and the energy stored therein.

Stranger

Magnets don’t really have “power”, which is yet another one of those terms which has a very specific technical meaning different from its common meaning. But in more proper terms, your question would be, can permanent magnets lose their magnetization? The answer is yes, they can, in a variety of ways, but it’s not a matter of the magnetization “running out”. If a magnet is struck hard (such as by dropping it on a hard floor), or heated too much, or exposed to an opposing field that’s too strong, it can be demagnetized (but won’t necessarily be; it’s somewhat random). But absent any of those conditions, it could just go on forever. In much the same way, a boulder can be broken up by an impact, or melted, but if it isn’t, it can stick around in one piece forever. It doesn’t ever use up its boulder-ness and just fall apart on its own.

Perhaps a better analogy might be that of a brick supporting a stone weight. The brick isn’t using up any energy by supporting the weight; it can go on holding it up forever without using up any ‘supportiveness’.