How is a magnet not perpetual motion?

[Titan]

For the record I got straight 100's in physics in HS and college and learned mechanics and electricity and magnetism. I can look at a lot of things and understand the physics of them, like when I teach martial arts and start rapping about conservation of momentum.

From a purely experimental perspective, I just don't understand this. Consider this like a word problem on a physics test, maybe in the extra credit section.

When you have a magnet attached to your refrigerator, the magnet exerts a force on the fridge and the normal force back from the fridge results in friction which keeps the magnet from sliding own. How does the magnet supply continuous force to the fridge indefinitely? In other words, how is a magnet not perpetual motion (except for that it isn't moving this case, but it is constantly fighting gravity).

What is the answer?

 

[masteryoda34]

This example isn't really unique. Lots of things constantly exert a force. The walls in your house are constantly exerting a force on your roof. Are they perpetual motion devices too? The air pressure in your tires constantly exerts a force holding up your car.

The magnet is not doing any work and thus not violating any laws.

 

[Titan]

Originally posted by: masteryoda34
This example isn't really unique. Lots of things constantly exert a force. The walls in your house are constantly exerting a force on your roof. Are they perpetual motion devices too? The air pressure in your tires constantly exerts a force holding up your car.

The magnet is not doing any work and thus not violating any laws.

Fair enough, I can see that.

I guess what bewilders me is that the beams in your house and the air in your tires are matter, exerting a counter-force as per Newton's 3rd law. Whereas a magnet holds itself with a continuous magnetic force, that is energy, and naturally the fridge is fine staying still.

Does the magnetic field weaken with time?

 

[masteryoda34]

The magnet exerts a force on the fridge and the fridge exerts a counter force on the magnet. The net force on the magnet is zero.

 

[Paperdoc]

You said the answer without realizing it: "it isn't moving this case". Exerting a force does not consume energy. Exerting a force to cause movement over a distance does work - force x distance - which is another way of saying the action converts energy from one form to another. Remember, (excepting E=mc^2) energy cannot be created or destroyed - it can only be converted. So lifting a weight inputs kinetic energy of muscles (originating in chemical bond potential energy) to increase the potential energy of the weight. Sliding a block across the table against a frictional resistance converts muscle energy input into waste heat in the block and the table. But a block sitting stationary on the table is not doing any work, and there is no energy consumed - or rather, converted - and no work done. Likewise a magnet that is not moving is doing no work.

Maybe the confusion is in drawing a parallel to human actions. It I push on a fridge wall so that my hand does not fall down, but it stays in place because the frictional force between hand and fridge prevents its falling, then my arm actually does do work. It does that because my muscles constantly convert nutrient energy into waste heat energy to keep my muscle cells contracted. I do not do any work on the fridge or on my hand. But my muscle cells do work that escapes a heat to my surroundings. A magnet does not work this way. The attractive force between itself and a suitable substrate does not depend on ongoing chemical reactions, and the attraction does not fail without supporting activity.

 

[KIAman]

I think I see where you're going with this question. Did you really mean to ask "Where does the energy come from to keep a permanent magnet magnetic?"

 

[TuxDave]

Doesn't perpetual motion imply motion?

 

[ShawnD1]

Originally posted by: KIAman
I think I see where you're going with this question. Did you really mean to ask "Where does the energy come from to keep a permanent magnet magnetic?"

I think one of the reasons people find magnets to be confusing is that it's usually described as being similar to electricity. The problem is that electricity doesn't have anything that resembles a permanent magnet. Batteries and capacitors are not like magnets because their stored charge is dissipated through moving particles (electrons). Magnets do not lose their charge because there are no moving particles.

A permanent magnet has a net magnetic field because a bunch of tiny magnetic fields inside the material are all facing the same direction. The reason the magnet stays this way is the same reason a flammable chemical like propane stays as propane until something causes it to change - it needs initial energy before it can change. You can find all kinds of energy illustrations if you google search "reaction kinetics". Magnets and chemical reactions share this property because in both cases the change involves moving particles around when they are already sitting in a relatively stable position. We can't just move the little magnetic dipoles into random positions without weakening bonds within the material or by applying a stronger field of opposite polarity. In the case of weakening the bonds, a magnet will lose its magnetic properties if you weaken the bonds through heating. This is called the curie point.

 

[silverpig]

The actual answer is that it is more energetically favourable for the magnet to stick to the fridge.

It's sort of like asking why we all stick to the earth when we should be pulled into the sun (yeah I know we're in orbit...).

 

[Born2bwire]

Originally posted by: silverpig
The actual answer is that it is more energetically favourable for the magnet to stick to the fridge.

It's sort of like asking why we all stick to the earth when we should be pulled into the sun (yeah I know we're in orbit...).

Velcro, lots and lots of tiny microscopic velcro.

 

[Titan]

Originally posted by: KIAman
I think I see where you're going with this question. Did you really mean to ask "Where does the energy come from to keep a permanent magnet magnetic?"

Yes, you're getting what I was stuck on. I guess "energy" isn't the word, nor is "work" but "force."

Obviously no work is being done as all forces cancel each other out, magnet - fridge_normal, and gravity - friction.

My question is how can the magnet supply a continuous force, as the earth supplies continuous gravity? If it were a remote-controlled electromagnet which I switched off, the magnet would fall as it no longer exerts enough force.

So I guess that's what I meant, sorry I got the terms all wrong at the outset.

How does a magnet supply continuous force? Will it lose force over time, or if there is work applied to it, or nether?

 

[herm0016]

you can apply work to an object to make it magnetic. hammering on the end of a steel bar will eventually change its structure to make it magnetic.

 

[Mark R]

Work = Force * Distance moved in the direction of the force.

So when you move the magnet towards the fridge and it starts pulling, in the direction it is moving, the magnet is doing work. When the magnet gets stuck and stops moving then there is no more work done.

If the magnet is weak, and won't stop the magnet falling to the floor - that doesn't matter. The direction of falling is at right angles to the force of the magnet - so no work is done, as the movement in the directino of the force is zero.

When you pull the magnet off the fridge - the magnet continues to exert its force, but you are pulling it away in the opposite direction (i.e. a negative movement in the direction of the force). In this case, you are doing work, and the magnet is absorbing the energy.

IN effect, the magnet gave you a *loan* of the energy that was charged into it when it was made, when it pulls itself onto the fridge. To get the magnet off, you have to put that energy back, restoring the magnet's energy.

A force does not require energy to exist. Gravity exerts its force purely by the presence of matter. Electrostatic forces exist due to the presence of an electric charge. Magnetic forces exist due to the constrained movement of electrical charges (a property of the material).

Electromagnets may require a supply of energy to remain magnetic, but that is not due to their magnetic nature, but due to electrical resistance. If you build an electromagnet out of a material with no electrical resistance i.e. a superconductor (e.g. an MRI scanner), then you don't need to keep the magnet connected to an electrical supply. (Indeed, MRI scanners require such a precise magnetic field that they require the magnet to be disconnected from its PSU when running to give optimum performance, otherwise voltage fluctuations in the PSU could disturb the magnet).

 

[KIAman]

Ok, here is the short answer; magnetism is caused by (get this, you're assumptions about motion are partially correct) electron spin. Each electron spin creates a dipole moment which a very small force. When you get enough of these electrons spinning identically, their respective dipole moments add up incrementally to a large enough force to hold the magnet in place.

Unlike an electromagnet which relies on the movement of electrons from atom to atom, the permanent magnet relies on the spin of an electron and its associated angular momentum to create its magnetic force.

Now... the natural follow up question is "what powers an electron spin?"

 

[PowerEngineer]

Originally posted by: KIAman
Now... the natural follow up question is "what powers an electron spin?"

Or maybe to ask what exactly electron spin is? We'd need to head back into quantum mechanics for an answer to that question...

 

[Born2bwire]

Originally posted by: PowerEngineer

Originally posted by: KIAman
Now... the natural follow up question is "what powers an electron spin?"

Or maybe to ask what exactly electron spin is?

Pfftt... that's easy, it's an operator.

 

[silverpig]

Originally posted by: KIAman
Ok, here is the short answer; magnetism is caused by (get this, you're assumptions about motion are partially correct) electron spin. Each electron spin creates a dipole moment which a very small force. When you get enough of these electrons spinning identically, their respective dipole moments add up incrementally to a large enough force to hold the magnet in place.

Unlike an electromagnet which relies on the movement of electrons from atom to atom, the permanent magnet relies on the spin of an electron and its associated angular momentum to create its magnetic force.

Now... the natural follow up question is "what powers an electron spin?"

Magnetism that we see is caused by the electron's orbital angular momentum, not its intrinsic spin.

 

[Biftheunderstudy]

Exactly silverpig, intrinsic spin from the electron would mean all the magnetic phenomenon would need to be multiplied by a factor of 2 -- it would be 42cm radiation from hydrogen etc. This can be used to argue against magnetic monopoles somehow...
Also keep in mind that the magnetic field doesn't do work.
Electromagnetism is a fundamental force, when you get to the smallest common divisor that's it, the force doesn't degrade from that source. For simplicity think of the electron, the Coulomb force from that doesn't degrade over time. Magnetic fields are the same way.

 

[KIAman]

Originally posted by: silverpig

Originally posted by: KIAman
Ok, here is the short answer; magnetism is caused by (get this, you're assumptions about motion are partially correct) electron spin. Each electron spin creates a dipole moment which a very small force. When you get enough of these electrons spinning identically, their respective dipole moments add up incrementally to a large enough force to hold the magnet in place.

Unlike an electromagnet which relies on the movement of electrons from atom to atom, the permanent magnet relies on the spin of an electron and its associated angular momentum to create its magnetic force.

Now... the natural follow up question is "what powers an electron spin?"

Magnetism that we see is caused by the electron's orbital angular momentum, not its intrinsic spin.

It's not one or the other. It's both, which I accidentally left off in my first paragraph.

 

[Matt1970]

I I see what you men by the guestion. The the energy a magnet will produce, basicly if you built a generator off the pull of the magnet against the fridge, will never be more than the energy it takes to make the magnet. The same would apply to a magnet pulling on the blades of a fan or wheel hooked to a generator.

 

[bsobel]

My question is how can the magnet supply a continuous force, as the earth supplies continuous gravity? If it were a remote-controlled electromagnet which I switched off, the magnet would fall as it no longer exerts enough force.

Answer:

"Modern magnet materials do lose a very small fraction of their magnetism over time. For Samarium Cobalt materials, for example, this has been shown to be less that 1% over a period of ten years."

 

[polarbear6]

ahhhhh physics I HATE IT

 

[polarbear6]

ahhhhh physics I HATE IT

 

[bsobel]

Originally posted by: ganesh1
ahhhhh physics I HATE IT

Dont troll in HT.

 

[Onceler]

There is motion on the subatomic level. Magnetism is caused by the movement of electrons - a bunch of them with their spin aligned with each other. So it is motion.
There has to be some work done if the magnet is resisting gravity.
If you were hanging by one hand off a cliff then you are doing work just by hanging on.