Science Questions

Power of a Magnet

Sun, 9th Dec 2007

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Question

Brian Starkey asked:

From where do permanent magnets get their energy or power? I can put a fridge magnet on a fridge, and it seems as if it will stay there forever with no sign of any power source. Also, if I try to push the like poles of two bar magnets together, my arms will grow tired long before the magnets grow weak, yet again there is no power or energy source. Can we not harness this invisible and seemingly endless source of energy?

Answer

Alastair Rae, University of Birmingham

At this time of year, many of us decorate our fridges by attaching magnets carrying pictures of Christmas puddings, holly, Father Christmas, snowmen and so on. One advantage of these magnets is that they are easily removed and replaced when Christmas is over. Brian Starkey asks, Ďhow can they stay on the fridges when there is no obvious power source?í

Magnetic lines of force of a bar magnet shown by iron filings on paperThe first point to note is that we donít need any energy to stand still! A stationary car with its engine turned off doesnít use any petrol!  Power is required only when the engine starts turning and the car starts moving.  What we have in the case of a fridge magnet is a magnetic force pulling the magnet against the iron door; this then leads to a frictional force that stops the magnet sliding down under gravity, but once the magnet is in place, no energy or power is consumed keeping it there.  Itís not very different in principle to sticking the magnet onto the fridge using glue.

When Brian pushes the two like poles of a magnet together, he has to apply a force and use energy.  If they are then allowed to move apart, this energy is released and converted into motion.  However, if he holds them together without letting them move, no more power is needed.  Itís perhaps easier to understand this if we think of the magnets being supported by a rigid frame instead of by a person. Why then do Brianís arms grow tired if he is not doing any work?  This is all to do with biology and the complex way our muscles work: chemical energy has to be burned to keep them stiff and able to exert pressure.  But magnets are not like that: they exert a force pushing each other apart and do not consume any power as long as they donít move.

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Every electron in the metal acts as a magnetic dipole.  Dipoles are the simplest sources of magnetic fields.  In order to generate a permanent magnet form a metal, you force all these dipoles to line up, so the tiny fields they each generate all add together to make a big field. 

If you stick this magnet to a fridge, I think the fields from the magnet should cause the electrons in the field to line up in an opposite direction.  This means if you have the north pole of a magnet near a fridge, the fridge will start to act like a south pole, and viola--your magnet will stick to your fridge.  If you try to push two like poles of magnets together, they repel because the electrons in each magnet are already aligned to repel each other and won't change direction easily. 

Finally, in terms of energy.  In physics, change in energy is defined as a force applied over a distance.  If you hold two magnets near each other until your arms get tired, you're not actually putting any energy into the magnets.  Similarly, by holding the magnets near each other, you're not getting energy out.  To get energy out, you'd probably have to let the magnets fly apart and use that motion to generate electricity or move something.  The simple reason why that isn't practical is that you have to put the same amount of energy into the system in the first place in order to put the magnets near each other!  jpetruccelli, Fri, 7th Dec 2007



I think this question can be addressed two ways: are we asking about the nature of magnetism itself, or about the "energy" source?

You could ask almost the same question but replace "magnet" with "spring". e.g. "If I squash the spring between my fingers it keeps pushing back; where does the power come from?" Or "If I pull the spring it keeps pulling back, seemingly forever without getting any weaker." As already mentioned, "work" (ie energy) is only done when something actually moves against (or as a result of) the force. You cannot extract any energy without there being motion, and since the force weakens as the distance increases there is only a finite and rather small amount of energy available. Forcing two repelling magnets together, or pulling an attracting magnet away from something merely stores energy (temporarily) in the field. You can only get back the energy which you put in previously. techmind, Fri, 7th Dec 2007

About "their energy" it depends on what you mean: every magnetic field has an energy associated with itself: U = 1/2 μ H2 where U is the energy density (energy per unit volume), so, to create a magnet, which in this case is equivalent to align those magnetic moments of which jpetruccelli wrote, you certainly have to give some energy to that system.

Let's make an example: you magnetize a piece of iron with a magnet; you approach the two bodies, and you don't have to make work for this, then (let's say the magnet magnetizes slowly the iron) at the end, they attract each other; now, if you want to take them apart, you have to make work against that attractive force, and that work goes to the system of the two bodies. lightarrow, Fri, 7th Dec 2007

You have beaten me by 20 seconds! lightarrow, Fri, 7th Dec 2007

We do harness this energy, its called a generator (or dynamo).  Check electromagnetic induction, Lenz's law and Faraday laws. How does the magnet get its magnetism? In the case of permanent magnets, there are magnetic domains where various numbers of molecules of the material "line up" in a particular direction. (These are not molecular magnets.) These domains exist in many materials in random directions but in certain materials, eg. nickel cobalt and iron they can be made to line up forming a permanent magnet. Fridgemagnet, Thu, 13th Dec 2007

I notice that nobody has actually answered the question yet.

It comes from electricity.

Magnets get their energy from the factory that they are made. Making a magnet involves immersing the magnet in a strong magnetic field, usually generated by an electromagnet. After this is done, the permanent magnet has stored some of the magnetic energy from the electromagnet in the form of its own magnetic field. The energy in the field of the electromagnet,  came from the electricity flowing through the coils.

Once it is stored in the magnet, it's fairly hard to make the magnet lose the energy, although a strong repetitive shock will do so, as will a sufficiently strong alternating field, or heating the magnet up to its 'Curie point'. However it is lost, the energy usually ends up as tiny increase in the temperature of the magnet.

The amount of energy in a typical bar magnet you might have on your fridge is not usually very much, just a few joules. This would be theoretically enough to run a 100W lightbulb for maybe a tenth or a hundredth of a second or so; and it would be possible with the right equipment to do this, by using the magnet to move itself towards a piece of iron, through a coil connected to the lightbulb for example.

Because magnetism is what is called a 'conservative force' the energy you need to remove the iron from the magnet would be as much as it took to light the lightbulb, so the magnetic energy is not used up by doing this, and you can get it back again and the magnet can be reused.

Other conservative forces include gravity- gravity never needs recharging either! wolfekeeper, Mon, 17th Dec 2007

These replies all seem to have come from a textbook which was written by someone who reads textbooks. The "irrational" mind would serve to at least stimulate here. Is it possible that our definition of "work" may be flawed. Is it possible that merely "acting" on a body, either in motion or at rest, does indeed constitute work? Is it possible that the energy is actually input into the magnet at a rate equal to the energy "lost". Is it possible that we humans have all the answers regarding physics, and no modification to what we "know" will ever be made? Is it possible that man cannot and will not ever "fly like a bird"? I think we had all better preface our statements with "I am told" or "I read" until we actually figure out some of our other seemingly unexplained mysteries, like where 75% of the matter (energy) in the universe resides. Wayne, Tue, 26th Aug 2008

hey there. i'm 18 years old and i stay in south africa... and i had a idea of building an engine out of magnets. does anyone think that this will be possible? and how can that work? i have had a few ideas already but they dont work properly, now im studying magnets so that i can find a way that magnets can change the world and no petrol or any energy is used to power things... if anyone has any ideas can you please help me out. my e-mail: michalecosta90@gmail.com thank you, Michael. Michael, Fri, 14th Nov 2008

Thank you for the answers to this question which has been bothering me for a while. I see how magnets are made, how they work through aligned mangnetic domains and how they form feilds. But some questions remain... A parked car uses no energy, yet I cannot stick it to a metal wall and hang it there in defiance of gravity for years at a time like I can with a fridge magnet. Where does the energy for this come from? You easily stick a magnet of several kilos upside down to a metal plate, that requires watts of power to hold there. If it were glued there, no power is used, but a feild requires energy. Neil, Tue, 27th Jan 2009


Where is the answer to this question? Chemistry4me, Tue, 27th Jan 2009

I don't pretend to know any better than anyone else the answer to this question, however I do have an idea. The Earth has a magnetic field, much like a magnet does. Hence the reason the compass points north and so on. Obviously a magnetic field has its own energy, anyone who would chose to argue that just doesn't know what they are talking about. You hear people talk all the time about renewable energy, and how it doesn't exist, and how endless power supplies don't exit. It would seem to me that the magnet is the answer to both of those, and mankind has chosen to ignore it thusfar. Its not too hard to see that mankind has a recent history of rejecting the obvious answers just because it requires a change in our way of thinking, and god forbit we figure out something that disproves anyones theory, that would just make them look stupid. So, magnets must be an endless source of renewable energy that we just haven't opened our eyes to. It would be my guess that it could be used as the greenest source of energy possible, provided by nature, which causes no known harm to the environment or humanity in any way. That being the case, I would also say that the reason we haven't decided to look into that in more depth, is because humanity is so stubborn, and would rather argue and fight over it for another 1000 years instead of just going with the flow. Timmy, Sun, 1st Feb 2009

Wow, I would agree. And here are some examples. MAGLEV, this is a magnetic levitation teqnique used to levitate and often propell trains. Perfectly clean, renewable energy as the magnets produce no exhaust, and use no energy. Harmless, Sun, 1st Feb 2009

To see an example of a working device you should check out youtube and search for the Minato magnetic motor. I find that Magnets are fascinating. I have two stacks of 1" disk magnets levitating inside of a transparent glass tube. They have sat on my desk for the last ten years without any loss in repulsion. The two stacks of 1" disks were set into repulsion mode and were carefully placed inside of a 1" diameter glass tube. Daniel Pearson, Thu, 5th Feb 2009

Some of the answers here can be summed up as "I don't understand the answer the scientists give me, therefore the scientists must be wrong". This is fallacious logic. It is however very understandable since the physics at work in magnets is very counter-intuitive. It seems only natural that magnets have their own energy source since you - among other things - can use magnets to transform physical movement into electricity. But they are just passive mediums. I have some science education from university and still have a hard time wrapping my head around this topic. However, I don't claim that scientists are wrong just because _I_ don't fully understand the theory. By production and implementation scientists prove daily that the theory of magnetic force is correct and accurate. This might help you understanding the principle of magnetic energy without the difficult physics: Magnetic fields have properties that resemble gravity. A car is attracted to earth just like two magnets are attracted to each other. A car standing on the ground doesn't use up earth's gravity, neither does lifting the car out to space outside the earth's gravity field. Pulling two magnets apart is similar to lifting a car from the ground: When you pull them apart you add potential energy equal to what is released when you let them fall back together. The anology of a spring is also quite good. It can be used for absorbing and releasing energy, but the spring itself doesn't have it's own power source. The fact that you use a lot of muscle energy failing to pull two strong magnets apart is equal to failing to pull a solid piece of rock in two parts. The muscle energy used is radiated into the air as heat, no energy is put into the either the magnets or the rock. JÝrgen BÝckman, Thu, 26th Mar 2009

i don't think so from falling rock raghavendra, Mon, 6th Apr 2009


Where is the answer to this question?

Thanks for pointing it out!  It's now in the second post of the thread. BRValsler, Mon, 6th Apr 2009



What?? Madidus_Scientia, Mon, 6th Apr 2009



What??

Beats me!

-------

Where is the answer to this question?

Thanks for pointing it out!  It's now in the second post of the thread.

Not worries. Chemistry4me, Tue, 7th Apr 2009

Magnets do indeed seem to defy mr Neuton in a way. I bought a magnetic clamp for my table saw a while back. This device clamps onto the table with the simple twist of a knob which removes a simple baffle between the magnet and the table. The torque required to do this can easily be done with ones baby finger what 2 inch pounds over 90 degrees of twist, yet when in the clamp position I can literally lift hte whole table-- nearly 100 pounds . Thsi makes no sense whatsoever. I am exerting less than 1/50th of the force required to clamp the device. Something is missing no equal opposite reaction applies here...So there is no answer here (sorry) but a suggestion that we have to look deeper and with more humility and more of an open and more determined mind than science offers at this time.... lichardi, Fri, 17th Apr 2009

It doesn't take much force to apply glue either, which could have the same effect. Madidus_Scientia, Mon, 11th May 2009

It seems to me that the real question is about potential energy. When two magnets are set up in such away that they will try to pull or push each other, it's much the same as a heavy weight being positioned where it can fall downwards towards the Earth. The energy is stored as potential energy and can be converted to kinetic energy if one of the magnets or the weight is allowed to move. By raising a weight we can turn kinetic energy back into potential energy. When you compress a spring you also convert kinetic energy into potential energy which can later be recovered and turned back into kinetic energy.

When you store energy in chemicals, you make electrons move more energetically and the potential energy is manifested as this extra kinetic energy like a flywheel. In the case of hydrogen and oxygen gas, their bonds involve energetic electrons which hold potential energy in their speed: when these molecules are burned to make water, the electrons lose energy which is converted into movement of atoms (heat and travel).

I assume that magnetism and gravity store potential energy in some other way, but I've no idea what the mechanism is. David Cooper, Sun, 5th Jul 2009

the power comes from the magnetic force feild of the earth gurpal, Wed, 22nd Jul 2009

No matter what the question was this "the power comes from the magnetic force feild of the earth" was never the answer. Bored chemist, Wed, 22nd Jul 2009

I am trying to affiliate myself with people like you here to develop maybe not perpetual motion but something close to develop electricity, there are hundreds of ideas already in existance but not much on the market, perhaps I can intrigue someone to get in touch with me to help me develop further my ideas Michael L, Thu, 17th Sep 2009

So... I understand how all the static magnets behave. What I don't understand is how can this: http://www.youtube.com/watch?v=7PDeK6rprA4&feature=related work? Considering the friction and air drag, this fan shouldn't be working. Yet it does. Mike, Sun, 14th Oct 2012

I still don't get it. Suppose we had a weight on one magent that was suspended below another magnet and the weight was just enough to keep the weight suspended, but to keep the magnets apart. There must be some energy being expended to keep them in this position? amster, Fri, 22nd Feb 2013

I believe the magnetic force is caused by the accumulation of the aligned atoms within the magnet. these aligned atoms have electron 'clouds' which are polarized (north/south). the electrons are essentially 'speeding' around each atom (more or less. i don't feel like getting into quantum physics at the moment) creating and electron field or cloud. when all the atoms within a material are aligned with one another, the electron fields are shared as a common whole, more or less. electricity and magnetism are two aspects of one whole, the photon. as electricity flows, a field of magnetism is produced around it, and vise versa. so basically, the atoms all align, therefor creating one large electric flow of sorts, and thereby creating one large shared, magnetic field. So, basically what i'm trying to say is that the magnetic force itself comes from the nucleus of the atom, holding the electrons in orbit, compiling the fields together, creating a magnetic force. essentially, it's powered by nuclear force. electromagnetic force, and strong and weak nuclear forces (which bind the atom together) are all different representations of the photon. so the answer is basically that the magnet gets its power from the universal energy that binds all things together, the same source that causes fission and explosions, and chemistry, and teddy bears, and the sun, and...carpet ants. just another form of the energy, condensed into a specific orientation, combination, variation, etc ( you get the point i'm sure) to come together to form what we call a magnet with it's magnetic force. I hope that helps. malaki, Wed, 15th Jan 2014

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