How to (Re)Magnetize a Permanent Magnet

in this video we're going to make a permanent magnet or more accurately we're going to re-magnetize this magnet which has lost its magnetism unlike this other one which is still good now you say how does he know this magnet is bad and this one is good well luckily I have an extremely high tech magnet tester yep a Jar full of screws and if I take the supposed bad magnet and put it in the screws well I'm lucky if I can pick up what does it look like three screws now if I take the good magnet let's see what happens well look at that a whole pile of screws clearly this is a much better magnet so we have a defective magnet here and we want to re-magnetize it so how do we magnetize something well when you were in grade school you probably did this you took a nice big nail and you took a magnet and you moved it along the nail like that a whole bunch of times and if you were lucky you've got a weak magnet let's see if we were lucky I've got some iron filings here and can we pick them up with our nail well a few there we go not bad but certainly not great and this is really barely a magnet like I can pretty much knock them off so that's not really the way to magnetize something certainly not the way this magnet would have been magnetized in the factory and to understand the best way to magnetize things we really have to understand what makes them magnetic so if you have a chunk of a magnetic material like iron or Steel here's a chunk of that material it turns out that there are a whole bunch of little regions in that material like this and they're usually about a millimeter in size or smaller that's maybe something like an eighth of an inch if you're in the United States and all these regions are actually like little magnets so they have a Direction south to North like that and as I'm drawing them here what you can sort of see is in most cases they are almost pointing in random directions and doing so almost evenly and you might be wondering why well it's as if these were individual magnets and if they're just sort of thrown together they will naturally Orient like this to keep the amount of energy in all the magnetic fields the lowest and that's sort of a very typical physics type phenomena you always want the energy to be the lowest now if you take your magnet and move it across like that if you're lucky you will end up with more of these regions called domains pointing in the same direction and if a significant number point in One Direction you end up with a total magnetic field now the way these domains move is it's not like this domain suddenly turns and points to another Direction what actually happens is the corners or the boundaries of these domains move and the domains that are pointing in the direction we would like tend to get bigger in this case this boundary would move over here so that this domains area gets smaller so if we look at the total I guess it's volume not area of all the domains if on average they're pointing in this direction we have a nice magnet and that's what happens when we apply an external magnetic field when we move our bar magnet along here the edges of these domains move slightly and we get an average magnetic force in this direction but it doesn't work very well because you can sort of see as I move the magnet across some of the domains will tend to point in this direction some will point there and it's not really very perfect what we'd like to do is subject this whole thing to a big magnetic field and that will essentially force all the domains that are pointing in the right direction to get bigger and all the ones that are pointing in the wrong direction to get smaller there's actually a very nice microscopic video of that happening so the question would be how do we do that well in the previous video what we did was we made an electromagnet and we put a coil of wire around our steel bar and when we passed current through the coil the coil made a weak magnetic field going in this direction but what that weak field did was it aligned the domains in our magnetic material which was a bolt to also point in that direction and if you want you could sort of say the bolt Amplified the magnetic field and we actually have a way of expressing that and that is with this formula here B the field or actually it's the the flux density equals mu times H H is the magnetic force field the field that is as a result of the current going through the loop and B is the magnetic field that we normally associate coming out of the edges or the ends of the magnet and that's what we really want to have be nice and big and again we put a core in and we get that amplification happening now in the case of an electromagnet when we take the current away the edges of these domains all move back to the original locations or close to it and very little magnetism remains but in a piece of magnetic material they stay pretty much locked in the same place and it's because magnetic materials are chosen to have defects in the crystal lattice that basically hold the magnetic domain walls in place and makes them fairly hard to move and that's what makes a really good permanent magnet so the way we will charge a magnet is we will subject it to a nice big magnetic field so how do we do that well the simplest way is we make a nice big electromagnet shaped something like this this is a steel core right here and we put a loop of wire around it like that that's going to magnetize this core so now that we have our steel core what we do is we take our unmagnetized piece of Steel and we put it right in the middle like that and then we apply a current through the coil and that creates a nice big strong magnetic field through this magnetic circuit and if we get that field strong enough it will take all the domain walls in this piece of Steel and move them so that the bulk of the magnetic domains are pointing in the correct direction and then we have a magnet now you will notice that I said magnetic circuit and we actually think of magnetic circuits and we even draw them much like we draw electrical circuits so this might represent the magnet and the Magnetic term for resistance is reluctance so we'll do an RM here for this Magnet or magnet to be and then the rest of our circuit over here is in fact another resistor or reluctance in this case like that and we'll call this the reluctance of the core and then of course we have our magnetic force over here which we could even represent as a battery so you can see what I'm getting at here we can generally model magnetic circuits as if they were electrical circuits the resistors in this case are reluctances and the force which in an electric circuit would be the battery is in fact the magnetic force field from the coil over here and the reason we use a core like this is we want to have this resistance or reluctance as small as possible so we get as much flux flowing through the magnet to be to magnetize it as possible so there you have it let's actually try it so I don't have a perfectly shaped core but what I do have is this core here that we've used in previous experiments now normally the coil would be over the center of the core like that but because of what we're going to do I'm going to put it on one leg like this and the first thing we need to do is figure out whether this is north or south so that we don't inadvertently put the magnet on it the wrong way so what I'm going to do is use my big old battery charger as a current source and hook it up to the leads of this coil and off screen or maybe even on screen I'm going to put an ammeter around one of the wires so we can just see how much current is Flowing the first thing we need to do is put a small current through our coil which has 300 turns to just see which way the magnetic field is if I use our little field tester you can see that well the north side of the field tester is in this direction so that's the direction we will want to put our magnet to be re-magnetized North on this side so here we go now what I'm going to do is actually just try and magnetize this section of the magnet and then we will reverse the polarity and magnetize this section and that's because I don't really have a big enough gap on my core over here and I should also point out since this core connects with no wires through it from here to here like that it essentially shorts the magnetic field on this side here so we should really get virtually no field flowing along here other than what's coming from the remains of this permanent magnet itself so with that said I'm going to turn on some nice high current I think seven or eight amps if I can do it and that times 300 turns should give us about 2 000 amp turns which will hopefully be enough to magnetize the magnet or at least this half of it let's try it that was even more than I expected that was more like 100 amps in fact it started melting a bunch of things let's try that again that's like 100 amps and I'm going to tap it a few times and the tapping is just to help those magnetic domains move now what we'll do it's all turned off is turn this around and we will repeat the process having changed the polarity of the magnet everything is hot from that big current that's really way more than this is ever designed to take but we can do that just for a few seconds and there we go that was somewhere around 50 or 60 or 70 amps so multiply by 300 and that's a vast amount of current going through here and the coil is hot so with a bit of luck now we have a magnet Let's test it and here we go there's our magnet tester and oh look at that didn't that work well so we really nicely re-magnetized this magnet so for a slightly more in-depth look as to what makes a good bar magnet we need to look at what is known as the b h curve and we saw this in some previous videos about Transformers and what this curve shows is when you apply a certain magnetic force in other words the current through the coil we get as that current increases more and more flux flowing till it sort of bottoms out at the saturation level over here a nice strong magnet has a very high saturation level and that is typically around one Tesla and that's what we would expect from things like alnaco and rare earth magnets that we use today if we have weaker magnets like these ones which are from made from ferrite we end up with only about a third of a Tesla at the saturation point now that's the strength but the other key thing is how hard those domain walls are locked in place by Crystal defects and if they're not locked in at all or just barely what we end up with is as we decrease the current the magnetic force the magnetic field drops pretty much straight down to zero but if they are locked in place through some very strong defects in the crystals as we decrease the current a lot of those domains don't change direction and we end up with a remaining magnetic field the field of the permanent magnet and in fact we need to have current flowing in the other direction to demagnetize it and how big that current is really depends on the strength of the defects holding those domain walled in place and on this graph it would mean that this dotted line gets further and further away from the zero point so the bottom line is those really good neodymium magnets that we have today have a nice strong flux density up here of about a Tesla but they also have a very wide curve in the BH curve so that it really takes a lot of force in the other direction to demagnetize them and that's what makes them so great so we've looked at using a very strong magnetic field to magnetize a magnet and also stroking it but there is another way and to show you that I have a piece of lava and this came from near Whistler in British Columbia and it turns out that when you take magnetic material and you heat it up to a very high temperature called the Curie temperature the boundaries in the magnetic domains break down and the material becomes non-magnetic and then if you cool it in a magnetic field you get most of the domains pointing in the direction of the field and that's what happens when you have a chunk of nice hot molten lava that cools down and it has some bits of iron type material in it it will assume a magnetic field and I don't have the instruments to see if this piece of material has a very weak magnetic field but it probably does and this has actually been used to date things like the materials on the rift zones in the middle of our oceans based on the magnetic field that is captured in the Rock and by looking at the direction of the field and knowing where the North and South Poles were at various times in history you can magnetically date things so that's another way to magnetize something and overheating a magnet is also a way to demagnetize something and some of the newer fancier magnets actually have relatively low Curie temperatures in the few hundred degrees Celsius so you've got to be a little bit careful that you don't overheat them well that brings this video to an end I hope you enjoyed it next video will be all about demagnetizing things see you then

Channel: Electromagnetic Videos

Views: 86,840

Keywords: Permanent Magnet, Magnet, Magnetize, Magnetic Domain

Id: S21rlwrY74I

Length: 19min 10sec (1150 seconds)

Published: Sat Feb 04 2023