3
$\begingroup$

I saw a young man's science project. He asked this question in a different way "does magnetizing something reduce the magnetic field potential of the original magnet?" His answer experimentally was "no". This begs the question, was he right?

Magnetizing an object clearly requires energy. It is now placed into an ordered state from which there are many other possibilities. Therefore, it has moved from a state of relative disorder to a state of order. The laws of thermodynamics say that this requires energy.

Where does the energy come from?

  1. Does the magnetic field strength of the original "permanent" magnet decrease?
  2. Is there "work" performed (e.g. Force x distance) and therefore the energy comes from the work it takes to bring the non-magnetic material close and then tug it away from the "permanent" magnet?
$\endgroup$
2
  • $\begingroup$ What you are wondering about is called the magnetocaloric effect. The answer is yes, there is in fact a very small amount of energy change associated with magnetization of a material. See, for example, en.wikipedia.org/wiki/Magnetic_refrigeration. $\endgroup$ May 2, 2014 at 12:25
  • $\begingroup$ The energy should be quantifiable. It should be at least the amount of energy required to "order" the material (e.g. the change in entropy) in a statistical mechanical sense. I'm hesitant to say that the magnetic field has energy...but it must. There must be a potential energy that is "stored" in the creation of the second magnet. This is not necessarily a temperature. Are you suggesting that the transfer is small because it requires very little energy to magnetize a material or that a permanent magnet cannot give up much of its field to create another field? $\endgroup$ May 5, 2014 at 4:18

4 Answers 4

1
$\begingroup$

Have a look at this video .

An iron bar is used to try to pick up some paperclips or thumbtacks. It is not able to do this because it is not magnetized. The rod is placed in a long solenoid and DC power applied. The rod becomes magnetized and is able to pick up some of the paperclips or tacks. The rod is again placed inside the solenoid and 120 VAC applied. This demagnetizes the rod and it will not pickup any tacks.

In this video the energy is supplied by the DC current for magnetization, and the AC current for demagnetization .

If you use another permanent magnet for magnetizing, the energy will be supplied by the motions of the experimenter. If a magnet and a non magnetized iron bar are brought into contact for a long time the energy will be supplied from the kinetic energy of the molecules.

It is a re arangement of existing tiny magnetic moments in random directions into an ordered one.

$\endgroup$
9
  • $\begingroup$ The case of the electromagnet implies that at some point, the electrical power consumption should drop. For example, the circuit should consume electricity until the ferromagnetic material becomes magnetized. Then the power consumption should decrease. This amount that the power decreases should be equal to the energy transferred to the material that was magnetized...right? $\endgroup$ May 5, 2014 at 4:11
  • $\begingroup$ You do not state what the young man was doing to produce magnetization nor give a link. $\endgroup$
    – anna v
    May 5, 2014 at 4:15
  • $\begingroup$ usc.edu/CSSF/Current/Projects/J18.pdf Project number J1827 $\endgroup$ May 5, 2014 at 4:22
  • $\begingroup$ well, it doe not say how he magnetized the screw, probably moving the magnets over it, or the screw over the magnets. $\endgroup$
    – anna v
    May 5, 2014 at 4:53
  • $\begingroup$ I spoke with him. He said he just let the magnet attract and become in contact with the screw. So the screw was brought to the magnet and then removed from the magnet without repetitive motion or rubbing. So, it appears to me that the permanent magnet aligned the spin direction of some electrons in the screw. I would expect that the magnetic field strength the screw would grow with time and saturate at a certain field strength. Therefore, there are only two places to get the energy: (1) the magnetic field of the PM decreased or (2) from work done moving the screw to and from the magnet. $\endgroup$ May 5, 2014 at 5:09
0
$\begingroup$

Why does the PM not loose his magnetisation? Look at it this way, lets say this is not one PM but billions of little PM-s that are closely together. Each of them generates a field, if you add up those fields they will create a pretty strong field that keeps the all the small PM-s in line. When I now think about it, I conclude that it is a bit like human conformism. The ones on the fringes are less affected by the field so they can create pockets of resistance to the central field.

Now lets say that you change the magnetisation orientation of one little PM, what happens is that the others will force him back! As long as the bulk macroscopic PM isn't overpowered the microscopic PM-s(on the inside) will be remagnetized to the same direction. It can however happen that a large enough part of the macroscopic PM gets demagnetized(boundaries), so that the total magnetisation falls off. This is of course a simplification, but it isn't so far off.

In short: The force will be mechanical if the other object is not magnetized so strongly that it would overpower the PM.

I must add that the realisation that human society can be pretty well described by a permanant magnet(while writing this) made me feel awkward.

Maybe you should look at the demagnetisation processes.

$\endgroup$
4
  • $\begingroup$ So from your perspective the student was unable to measure the effect, but it should exist. I believe that this is likely the case. So, perhaps a better experiment would be: First, magnetize a piece of steel. Then measure the magnetic field strength. Next, use that steel to magnetize another piece of steel. Given that the first piece of steel has a weak magnetic field, the loss of magnetic field should be obvious. Or perhaps, magnetizing something with another magnet can only create a field much smaller than the original? $\endgroup$ May 5, 2014 at 4:16
  • 1
    $\begingroup$ There must be some engineering factor that describes how efficiently a material can be magnetized (e.g. field strength, temperature, time, and final field strength). Looking at Electromagnetics and the Calculation of Fields (Nathan Ida) [book on E/M FEA] there are examples of diamagnetic and ferromagnetic calculations. However, there is no "factor" describing exactly how close to a perfect ferromagnet any given material behaves. Don't chrome, iron, nickel, ... all exhibit different amounts of ferromagnetism? For example, water is described as "weakly" diamagnetic. $\endgroup$ May 5, 2014 at 4:31
  • $\begingroup$ You are on the right track, it has to do with the hysteresis curve of the material in question. I'll add something about that to the answer a bit later, you can look at Remanent magnetic induction and Coercitive magnetic field. For example in this brief description, you can see a hysteresis curve. The PM is used in the 2nd quadrant(look at the arrows) so the field produced depends the point in the saturation curve where you stopped increasing the field(you'll get different return paths for different maximal fields). $\endgroup$
    – WalyKu
    May 5, 2014 at 12:22
  • $\begingroup$ Hmmm... getting warmer. I'm not a big fan of mysterious explanations on the web. However, you've identified one with sources! I'm re-reading Solid State Physics by Kittel and States of Matter by Goodstein. Perhaps there is an answer in there. I am the only one with books on my shelf? $\endgroup$ May 6, 2014 at 16:04
0
$\begingroup$

Unmagnetized iron (Fe) rods can become magnetized because of iron's atomic structure

This book does a nice job and even has pictures: (The Foundations of Magnetic Recording by John C. Mallinson, page 4)

1

If you study electrodynamics then you know that an electric current generates a magnetic field (this is separate from the magnetic dipole moment "spin" of electrons... any moving charge must generate a magnetic field this is simply due to special relativity)... when you pass current through a solenoid a static and straight magnetic field (like N-S magnet) will be generated within the internal of the solenoid.

Now, no one can give you nice formula of the force / counter-force of a pair of electrons (one involved in current and the other stuck in an iron bar). But we do have the very simple magnetic dipole moment potential energy equation: Take the derivative with respect to displacement $(x, y, z)$ and you've got yourself force.

So force is being applied to the electrons in the iron bar. And a counter force is being applied to electrons in the solenoid: electrons in solenoid lose energy, and the electrons in iron bar line up together, and hence the iron bar becomes "magnetized". You should be able to measure a nice voltage drop, until the iron bar becomes fully saturated.

Another way of looking at it is that, you have a time-varying magnetic field being generated in the solenoid, against which the current has to push, hence the voltage drop (in essence you are charging up an inductor when magnetizing an iron bar).

The "lost" energy isn't lost. It is present in the form of a magnetic field. Which if permitted, can release energy in form of current (or a nasty shock as in igniters).

$\endgroup$
0
$\begingroup$

As you said, magnetising a material does require energy. It might seem counterintuitive that a magnetised material can magnetise other ferromagnetic materials (objects) without losing its own magnetic strength (it doesn't), but there's a catch.

If you were to put an unmagnetised object near other ferromagnetic unmagnetised objects, they would all technically be at equilibrium. If, though, you used some current to magnetise this unmagnetised object (assuming it is permanent, we do not need to sustain the current), it suddenly needs to attract all the ferromagnetic objects around it to reach equilibrium, magnetising them in the process.

The thing is, that it cannot magnetise more and more objects forever. A magnetised object still has the potential to magnetise more objects, and if surrounded by them, it will keep doing so, but the strength will exponentially decrease till the value becomes ignorable. It won't continue forever, hence breaking the law of conservation of energy.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.