How is a magnet not perpetual motion?

[Modelworks]

Originally posted by: Onceler

If you were hanging by one hand off a cliff then you are doing work just by hanging on.

That would only be a valid comparison if you are pulling up at the same rate gravity is pulling down. If you pull up harder than gravity then you are doing work. Magnets don't do work. Motion at the electron level isn't considered mechanical work, you need to move the whole atom for that. Otherwise I could claim every atom in my body is a perpetual motion machine doing work because they all have electrons that move around the nucleus.

If the magnet physically gave away parts of itself to hold itself in place that might be considered work. But its a stretch.

Now if you really want to get confused there are studies that show super cooled magnets that can cause aluminum cans to levitate inches above the magnet surface.

 

[SagaLore]

It took energy to produce the magnet. Energy to lift the magnet up to the fridge. Energy to peel the magnet off the fridge.

The term "perpetual motion" is often part of "free energy" ideas. You can't use the magnet and fridge in anyway to produce more energy than what was put into it.

 

[JTsyo]

Originally posted by: 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."

Let's look at this from the energy balance point of view. You use a magnet to lift a 1kg piece of metal 1m. That needs PE= (1kg)(9.81 m/s^2)(1m) = 9.81 J. So how does the magnet lose that 9.81 J?

 

[Born2bwire]

Originally posted by: JTsyo

Originally posted by: 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."

Let's look at this from the energy balance point of view. You use a magnet to lift a 1kg piece of metal 1m. That needs PE= (1kg)(9.81 m/s^2)(1m) = 9.81 J. So how does the magnet lose that 9.81 J?

That is like asking how does a link in a chain give up energy as a chain lifts the load? The magnetic field, like the chain link, in that situation is not doing any work.

 

[madh83]

If a chain is linked to the load, the force pulling the chain is doing the work. His analogy, the magnet is doing the pulling. So the force exerted by the magnet is greater than gravity, therefore the object is pulled up. That is work being done. As to how a magnet exerts this force continuously, I have no clue = p I think some of the things about electron spin makes sense, but I haven't looked at physics in a very long time.

 

[bsobel]

Originally posted by: madh83
If a chain is linked to the load, the force pulling the chain is doing the work. His analogy, the magnet is doing the pulling. So the force exerted by the magnet is greater than gravity, therefore the object is pulled up. That is work being done. As to how a magnet exerts this force continuously, I have no clue = p I think some of the things about electron spin makes sense, but I haven't looked at physics in a very long time.

See earlier 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." So there is your net work

 

[madh83]

Originally posted by: bsobel

Originally posted by: madh83
If a chain is linked to the load, the force pulling the chain is doing the work. His analogy, the magnet is doing the pulling. So the force exerted by the magnet is greater than gravity, therefore the object is pulled up. That is work being done. As to how a magnet exerts this force continuously, I have no clue = p I think some of the things about electron spin makes sense, but I haven't looked at physics in a very long time.

See earlier 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." So there is your net work

Well, when the objected is pulled, I assume the magnet does not immediately lose a fraction of its magnetism. It occurs over time and is not directly related to the work done at any short length. I'm assuming magnet's losing their properties is like decay, which can be approximated by some equation dB/dt where B is the magnetic strength. It doesn't seem like it is from any short period of time where energy has been used.

 

[Born2bwire]

Originally posted by: madh83
If a chain is linked to the load, the force pulling the chain is doing the work. His analogy, the magnet is doing the pulling. So the force exerted by the magnet is greater than gravity, therefore the object is pulled up. That is work being done. As to how a magnet exerts this force continuously, I have no clue = p I think some of the things about electron spin makes sense, but I haven't looked at physics in a very long time.

Nevermind, I thought he was attaching a magnet and then lifting, not placing a magnet above an object and having the object being attracted up.

 

[JTsyo]

Originally posted by: madh83

Originally posted by: bsobel

Originally posted by: madh83
If a chain is linked to the load, the force pulling the chain is doing the work. His analogy, the magnet is doing the pulling. So the force exerted by the magnet is greater than gravity, therefore the object is pulled up. That is work being done. As to how a magnet exerts this force continuously, I have no clue = p I think some of the things about electron spin makes sense, but I haven't looked at physics in a very long time.

See earlier 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." So there is your net work

Well, when the objected is pulled, I assume the magnet does not immediately lose a fraction of its magnetism. It occurs over time and is not directly related to the work done at any short length. I'm assuming magnet's losing their properties is like decay, which can be approximated by some equation dB/dt where B is the magnetic strength. It doesn't seem like it is from any short period of time where energy has been used.

Then conservation of energy wouldn't hold. As the object gains potential energy, the magnet has to lose of kind of energy.

 

[BrownTown]

"Highly Technical" is pretty much a big fucking joke at this point. I mean this is a 9th grade physics question and it should take 1 replay to clear up the answer and then nothing more, instead we have a bunch of bull crap that doesn't amount to much.

Work = force * distance, a magnet applies a force to a refrigerator, but its not moving any distance, so no work is done, and no energy is created or destroyed. Its no different than if you just welded the magnet onto the fridge. If you really think about it, its not like 2 atoms in a solid are physically connected to each other, it is also the electromagnetic force holding them together.

 

[bobsmith1492]

Originally posted by: BrownTown
"Highly Technical" is pretty much a big fucking joke at this point. I mean this is a 9th grade physics question and it should take 1 replay to clear up the answer and then nothing more, instead we have a bunch of bull crap that doesn't amount to much.

Work = force * distance, a magnet applies a force to a refrigerator, but its not moving any distance, so no work is done, and no energy is created or destroyed. Its no different than if you just welded the magnet onto the fridge. If you really think about it, its not like 2 atoms in a solid are physically connected to each other, it is also the electromagnetic force holding them together.

Next up: plane on a treadmill? It's a legitimate question with a simple answer which you have provided in a succinct way.


About the magnet pulling up a piece of metal: that would seem to be work done by the magnetic field. However, when you pull the metal back away, you are putting work back INTO the field. So, the net work is zero. Since entropy always applies, I would assume this pair of actions weakens the overall magnetic field.

 

[BrownTown]

Originally posted by: bobsmith1492
I would assume this pair of actions weakens the overall magnetic field.

I wouldn't...

 

[Cogman]

Man, we might as well be asking the question "How is gravity not perpetual motion?". Yet that question seems ridiculous to discuss or debate and yet here we are...

 

[Snezzer]

Net force equals zero.

 

[dflynchimp]

Originally posted by: Snezzer
Net force equals zero.

XD...yup.

Perpetual motion is when an object is in motion with a state of zero resistence in relation to its environment. Anything that exerts a force in any vector that effects the objects net inertia would put it out of perpetual motion.

For the OP's fridge example, if we zoom in far enough to any seemingly unmoving object, we will find some form of motion. Electrons never take coffee breaks afterall... The little vibrations and energy transfers (physical and otherwise) you would definitely feel even without magnification in any part of a fridge would prove this point.

 

[Mark R]

Originally posted by: JTsyo

Then conservation of energy wouldn't hold. As the object gains potential energy, the magnet has to lose of kind of energy.

Yes. An object in a magnetic field can be thought of as providing 'magnetic potential energy', in the same way as an object on top of a well has 'gravitational potential energy'.

When the object gets attracted to the magnet, it falls down the 'magnetic well', as magnetic potential energy gets converted into kinetic energy and/or gravitational potential energy. So, the magnet has provided energy to the object.

When you pull the object off the magnet - just like lifting the object out of a well, by replenishing it's gavitational potential energy, so you must provide energy to the object, replenishing the magnetic potential energy. No energy is created or destroyed.

So, where does the energy in the magnetic field come from - it comes from within the magnet. This is energy that was captured when the magnet was created. When a permanent magnet becomes permanently magnetized by an external magnetic field, the external field is weakened - it's magnetic potential energy is drained and transferred into the new magnet.

Let's say you make an electromagnet out of a coil of wire, and a metal that does not retain a magnetic field, e.g. 'soft iron'. When you apply voltage to the wire, the current ramps up gradually as the magnetic field builds up - the gradual ramping of the current is due to electrical energy being converted into magnetic potential energy. When the voltage is disconnected, the magnetic field begins to disappear and it's energy is converted back into electrical energy, which gets forced back into the electrical circuit. This electrical energy can be seen as sparks in a switch, or it can destroy electronic components like transistors, unless the circuit is able to dump the energy when the magnet is switched off. (This is a common mistake for beginners making electronically controlled relay circuits. If you don't put an energy dump diode in the circuit, the relay's magnet discharging will fry the transistors).

Now repeat this coil experiment, but instead of 'soft iron', you use a magnetic material such as magnet steel. The magnet has been magnetized and has retained some of the magnetic energy, now when the electrical current is switched off, less energy gets released, because some of the energy has been retained as magnetic energy.

 

[bobsmith1492]

Originally posted by: BrownTown

Originally posted by: bobsmith1492
I would assume this pair of actions weakens the overall magnetic field.

I wouldn't...

Do you mean a magnet is a perfectly efficient energy storage device? Or energy is dissipated elsewhere?

 

[dflynchimp]

Originally posted by: bobsmith1492

Originally posted by: BrownTown

Originally posted by: bobsmith1492
I would assume this pair of actions weakens the overall magnetic field.

I wouldn't...

Do you mean a magnet is a perfectly efficient energy storage device? Or energy is dissipated elsewhere?

IMO a magnet is at most an efficient energy transfer device. It doesn't store energy or create it, but when put in motion via a turbine generator can turn physical energy caused by the power source into electrical energy.

 

[BrownTown]

Originally posted by: bobsmith1492

Originally posted by: BrownTown

Originally posted by: bobsmith1492
I would assume this pair of actions weakens the overall magnetic field.

I wouldn't...

Do you mean a magnet is a perfectly efficient energy storage device? Or energy is dissipated elsewhere?

its jsut like gravity, are you saying that me jumping up and down against the pull of gravity weakens the earths pull on me?

 

[silverpig]

Originally posted by: BrownTown

Originally posted by: bobsmith1492

Originally posted by: BrownTown

Originally posted by: bobsmith1492
I would assume this pair of actions weakens the overall magnetic field.

I wouldn't...

Do you mean a magnet is a perfectly efficient energy storage device? Or energy is dissipated elsewhere?

its jsut like gravity, are you saying that me jumping up and down against the pull of gravity weakens the earths pull on me?

So THAT's how people lose weight! Do a bunch of jumping jacks and weaken the Earth's gravitational pull, then you weigh less!

 

[bobsmith1492]

Originally posted by: BrownTown

Originally posted by: bobsmith1492

Originally posted by: BrownTown

Originally posted by: bobsmith1492
I would assume this pair of actions weakens the overall magnetic field.

I wouldn't...

Do you mean a magnet is a perfectly efficient energy storage device? Or energy is dissipated elsewhere?

its jsut like gravity, are you saying that me jumping up and down against the pull of gravity weakens the earths pull on me?

Hm, that sounds like a good analogy. So, in a magnetic storage system, losses would come from friction->heat as opposed to changing the structure of the magnet, assuming the magnet wasn't overheated or impacted...

 

[bwanaaa]

A magnet is the result of a spinning charge (like a gyroscope). When in the presence of another magnetic field, it will precess (like a gyro scope under the influence of gravity). This is the basis of magnetic resonance. Now if a magnet is stuck to a fridge, the atoms experience a force - why dont they precess? or do they but VERY slowly?

 

[KIAman]

Ahem...

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

 

[mindless1]

To put things in simpler terms, if one tried to take the magnet off the fridge, cause motion, it would require more effort than if it were not attracted. Additionally the magnet is not defying gravity, you would find the increased mass of the fridge plus magnet cause even more effort to lift, or the same as the sum of the two individually. What others have already mentioned is the need to put more power into the system than you get out to perpetuate motion even if magnetism makes something behave differently than if there were none.

 

[Matt1970]

Originally posted by: BrownTown

Originally posted by: bobsmith1492

Originally posted by: BrownTown

Originally posted by: bobsmith1492
I would assume this pair of actions weakens the overall magnetic field.

I wouldn't...

Do you mean a magnet is a perfectly efficient energy storage device? Or energy is dissipated elsewhere?

its jsut like gravity, are you saying that me jumping up and down against the pull of gravity weakens the earths pull on me?

Yes and no. The pull of gravity is always a constant. There is nothing known to man that defies gravity, only ways to counteract it. When a plane flies, it is not defying gravity. The pull of gravity is always there. It just has enough force, or lift to counteract it.

Distance however does weaken it?s pull, so you could say that when you are at your highest point of your jump, the gravity is at it?s weakest.