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Q & A: magnetic work

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Most recent answer: 03/13/2017
Q:
I know how a magnet works, but what is the power source of a magnetic field? ie: why does a magnet NEVER come of the fridge? PS: Please don't spout some nonsense about the magnet doesn't move so no work is done and no energy is needed. Thanks.
- Jake
England
A:
The magnet doesn't move so no work is done and no energy is needed

Mike W.

(published on 10/22/2007)

Follow-Up #1: magnetic work

Q:
If magnets do not do work (because their affect on charged particles is at right angles and Cos 90 is zero so no work is done) then how do magnets make each other move? For example if there is a N side magnet and S side fixed magnet and the N side slides across a table to the S side of the magnet then isn't work done? And since we both know work is done, how is it done? Do the two magnets create an electric field? What about on the microscopic level? What's happening to each of those individual charged particles? And are these principals the same thing that govern the work of a compass? Thank you so much!
- Heather (age 25)
Watkinsville, Ga
A:

This is a really nice question. There are a couple ways in which work is done, i.e. in which the energy stored in the magnetic field is converted to kinetic energy. One is that particles such as electrons are not simply charged particles whose magnetic fields come only from their motion. They also have intrinsic spin magnetism even when they are at rest.  So there's a term in the potential energy which depends on the product of this intrinsic magnetism and the magnetic field. If the electrons move the right way in the field, that potential energy drops and work is done on the electrons.
 Even in the case of magnetism due to currents flowing, not spin, work can be done indirectly.
For a fuller discussion, see 
https://van.physics.illinois.edu/qa/listing.php?id=17176
(https://van.physics.illinois.edu/qa/listing.php?id=17176)


Mike W.


(published on 07/23/2008)

Follow-Up #2: work and motion

Q:
I found you answer to the magnetic work question most unsatisfactory. Assuming that the principal behind "the magnet doesn’t move so no work is done and no energy is needed" is true for all situations means that to hold an apple in the air completely still requires no energy to be used in the muscles. Since everybody knows that holding an apple up requires energy how do you explain the "the magnet doesn’t move so no work is done and no energy is needed" argument?
- Muhammed (age 17)
Sir George Monoux College, London
A:
Now that's a nice question.

If you hold an apple up in certain ways, it feels like you're doing work but in other ways it doesn't. If you lie on the ground with an apple on your belly (or hand) it doesn't feel like you're doing work. If you hold the apple out in your hand, it does feel like you're doing work. There must be some difference.

The feeling of doing work is a way of registering what's going on in your muscles. If they're using up chemical fuel to make one muscle protein (actin) move with respect to another (myosin) then you sense that you're doing work. Yet if the apple is just staying at a fixed height, no work is being done on it, in the physics sense. Why do you need to use energy in your muscles just to keep the apple up?

If you're using your muscles to hold up the apple, it would slip a little because the actin and the myosin don't stick perfectly. You have to use up some energy to make up for that slippage.

Here's an analogy. Say you have a car parked on a slope, with very high friction between the tires and the road. The car doesn't need to do any work to stay up. You can leave the engine off. Now say that the tires slipped a little on the road. The car will fall unless you turn on the engine and get the wheels turning, expending fuel. Keeping the car in place will feel a lot like accelerating it, since the motor will vibrate, fuel will be used up, exhaust products made, etc.

Now back to the magnet. There aren't any little molecules slipping when the magnet sticks well. It's like the car with high-friction tires. So no energy has to be expended.

Mike W.

(published on 10/22/2007)

Follow-Up #3: magnetic perpetual motion machine?

Q:
I have a different question related to gravity and magentism. I bought some neodymium magnets for work (about 3/16" dia. x 3/32") for a project. After finishing, I put them on my door to hold papers up. I then used one to hang a steel rod from the cieling tile channel (about a 2 foot x 3/32" rod). It's been hanging there for months. Energy has to be expended counteracting the gravitational energy pulling it down to the ground. Not only is the magnet holding the weight of the rod, but also the weight of the magnet and holding both up against the steel ceiling grid. The energy needed to hold this rod suspended above the ground seems infinite at this point. Since the energy is much greater than the gravitational pull, there should be some way to capture this energy and convert it to another form of energy such as electricity. I'm sure you see where this is going, a perpetual motion machine that would generate energy. I remember seeing a wheel using these magnets on the internet that once it was started, continued to run. I am sure there are bearing friction losses and wind resistance that it has to overcome. If you reduced these by placing it in a vacuum and using magnetic levitation frictionless bearings, you could concevably generate excess energy? Can you tell me where this idea fails?
- Tim McCreary (age 46)
Roaring Spring, PA USA
A:
Tim- I've switched the thread of your follow-up to one that provides a better introduction.  Your argument goes wrong right at the start- the magnet is expending no work.  As we argued before, it's just like a car parked on a slope. If the wheels aren't slipping, the engine doesn't have to do any work.

Mike W.

(published on 04/18/2013)

Follow-Up #4: magnets sticking forever?

Q:
If you put a 5 lb magnet on a wood ceiling, it falls to the floor. If you put the 5 lb magnet on an iron ceiling it stay there. What keep the magnet on the iron ceiling? I know by definition "work" is not being done but energy surely is required. Will the magnet remain for eternity?
- Paul (age 73)
Akron, Ohio
A:

Our arguments above about how no work is done apply exactly to this situation. 

Will the magnet stick forever? Oddly, energy is required for something here- for the magnet to come loose. It sticks because there's no path for it to come off that doesn't require increasing the energy some on the way. Once it's loose, that extra energy and more can convert to kinetic and ultimately thermal, after it lands. First, though, it would have to pick up that extra energy to get loose. If it's a big enough magnet to see, and if the only energy around is ordinary room-temperature thermal energy, the time required to accidentally get enough energy to knock the magnet loose is, for all practical purposes, infinite.

Mike W.


(published on 10/17/2014)

Follow-Up #5: magnets, glue, and energy

Q:
Although it is an old thread, seems interesting enough to be seen again and again, and I think I can contribute a little to clarify what is happening there to all the people that get puzzled by the fact that the magnet that is fixed with a load attached is no doing work at all.What happens there is pretty much the same than what happens if one used some kind of glue to fix the very same load to the very same surface to which the magnet is attached. Nobody would think that the glue is storing energy and/or using it in order to hold the piece in place: It is just bond now! It is part of the structure and nobody would expect it to fall unless the structure was broken by some force!Well, while there are some glue technologies that actually modify the chemical structure of the surface and/or the piece in order to make it "truly part of the structure", there are indeed many other glue technologies that do not work in that way, but that work just like magnets: they hold things together by electro-magnetic links, they act as if they were billions of microscopic magnets between the atoms of the glue and the surfaces that it is bonding. Therefore you don't think about the magnet that is holding the load as something that is actively using energy to hold it, but as the glue that fixed the piece to the surface and will keep it there until it is distrubed by some force. Now, there is no doubt that gravity is pulling and that the resulting weight of the object is a force that tends to make it fall; however, that force is not resisted by some sort of stored energy inside the magnet; but, just as the glued piece, or as the weight of the metal plate to which it is attached, for that matter, the pull of gravity, by the third law of Newton, is generating a reaction of the same magnitude, but opposite direction... at the structure that holds the magnet and the metal plate together. It is then that whole structure, and is attachment to the ground, the one that is bearing the load of the piece, just exactly in the same way that it would hold it if the piece was glued.BTW, in the case of an electro-magnet that is performing the very same trick, we do not need to feed energy continously to it in order to provide an active force that bears the load, but to replenish the current that creates the magnetic field. Energy that is not being consumed by the weight of the load (we already know that the whole structure is bearing that load as any other structure bears its loads), but is being consumed and tranformed to heat by the resistance of the wire that we used to create the electro-magnet.If, instead of a standard copper wire, we had a superconductor wire, then it would not have resistance and the energy would not dissipate in the form of heat, the current would continue to flow there and the magnetic field would not decrease, allowing the set to stay together without providing external energy for very long, just like it happens with the permanent magnet or with the glue!
- Eduardo
A:

Yes, that's a nice way to think of it. One of your points, however, is incorrect. You write "the pull of gravity, by the third law of Newton, is generating a reaction of the same magnitude, but opposite direction." That the glue force on the weight is equal and opposite to the gravitational force is not a consequence of Newton's 3d law. The law is entirely general, so if it accounted for that equality it would be impossible to have a case where the glue force was smaller and the object fell. Anybody who has tried home repairs knows that it's quite possible for that to happen.  The force pairs that Newton's 3d law says are equal but opposite are:

1. The gravitational force of the weight on the earth is equal and opposite to the  gravitational force of the earth on the weight. 

2. The force of the weight on the glue is equal and opposite to the force of the glue on the weight. 

Whether the glue force and the gravity force are equal depends on details, not general laws.

Mike W.


(published on 03/09/2017)

Follow-Up #6: forces in static arrangements

Q:
Mike: Thanks for your clarification, it seems I was not clear enough in that point. I was not trying to state that the strenght of the glue bond is somehow "created" by the third law of Newton. But rather I was trying to describe what is known as "support reactions" in structural calculus, where those reactions describe the loads that a part of the structure will have to sustain, and are therefore used to define things like the required section of a beam in a bridge. As you described it, the strenght of the glue bond can be lower than those forces, and therefore the union will not stand and the object will fall. But it can happen too that the strenght of the bond (or the strenght of the magnetic attachement, for that matter) is greater than those forces, and the union holds... but the weight becomes too much for a rod that is supporting the whole assembly at another part of the structure and that rod bends or fails (breaks), making the entire assembly and not just the attached object to fall (many of us have experienced that too!!). I insist on it because this example of a rod that can be broken because of the additional load generated by the attached object, can help people understand better the solution to the "mystery" of "who is holding" the weight of the attached piece, regardless if it was attached by glue or by a magnet.
- Eduardo
A:

Thanks for the clarification.

Mike W.


(published on 03/13/2017)

Follow-up on this answer.