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Current time:0:00Total duration:8:53

Coercivity & retentivity (Permanent & electromagnets)

Video transcript

ferromagnets come in two flavors ones that can be permanently magnetized and these are used in our magnets and ones that can be temporarily magnetized these are the ones that we use as a core inside electromagnets like maybe a solenoid or inside a transformer the idea is when you pass a current through it only then it gets magnetized it became behaves like a strong magnet but when you stop passing the current through it it stops behaving like a magnet but they're both ferromagnets so there must be some difference in their properties right and that's what we're going to explore over here what's the difference and in practice how do we figure out the difference using the hysteresis graph to explore the difference in their properties we need to look at what makes a ferromagnet ferromagnet and we've talked about that it's magnetic domains we've seen that in in ferromagnets atoms behave like tiny magnets because they have unpaired electrons but it's not just that we've seen that there are groups of atoms that are all completely aligned spontaneously aligned in the same direction and you know that when you have atoms aligned in the same direction these tiny magnets align in the same direction the magnetism adds up and so each domain produces a very strong magnetic field but the domains like you can see over here can all be randomly aligned so one domain can tend to cancel out another domain's magnetic field and so on so if you have randomly aligned domains like what you see over here then their magnetic fields can all cancel out and the whole ferromagnet still does not behave like a magnet but let's see what happens when i close the circuit and pass a current through it so when i pass a current through it there's an external magnetic field that is generated this is the field that is generated by the solenoid and we've seen before that magnetic domains have a tendency to get lined up in the direction of the field so all these domains we'll find if the field is strong enough will get lined up in the direction of the vacuum field generated by the solenoid and now the ferromagnet is super strongly magnetized it behaves like a strong magnet but what happens when you switch off the current when you switch off the current the vacuum field the field generated by the solenoid disappears b not disappears and in these ferromagnets what we'll find is that once that external field disappears almost all of those domains which got aligned they will go back to being random it's like all these undisciplined kids once you get rid of the field they'll all go back to being you know whatever they were doing earlier and as a result they stop behaving like a magnet but what happens over here well even here we can start by thinking about the domains and let's say we currently have an unmagnetized ferromagnet over here but over here what will happen is when you switch on an external magnetic field just like before you'll find all the domains get aligned in the direction of the magnetic field but the difference is if you get rid of that external magnetic field most of the domains stay they don't turn back okay maybe some of the domains might turn back so i'll just show one turning back but most of the domains just stay as it is and that's why these are permanently magnetized so what's the big difference we find between these two we see over here this has low retention meaning it can't retain its magnetization this one has a very high retention it can retain its monetization and so we write this has low we call it retentivity return retentivity and this other one has high retentivity high retentivity and such magnets which have very low retentivity such ferromagnets they're called soft ferromagnet it doesn't mean that they're soft it's like a pillow it basically means magnetically soft okay and these are called hard ferromagnets so steel is an example for a hard fedora magnet and there's something called a soft iron which is an example for software magnets soft iron is basically iron that has been heated to an extremely high temperature and then cooled back down we call that as annealing okay now comes the question how do we figure out whether something has high retentivity or low retentivity and maybe other properties practically one of the best ways of doing that is by drawing or plotting a hysteresis graph in previous videos we've seen hysteresis graph is a graph of vacuum field generated by say a solenoid versus the magnetic field inside the ferromagnet now my question to you is based on what you've just seen and based on your knowledge of hysteresis graphs that you've gotten probably from previous videos i want you to make a prediction of what would be the difference in the hysteresis loops that you would find for a hard ferromagnet versus a software or magnet so can you pause the video and think about make a you know make a guess of how the hysteresis graph would look like all right here we go if you were to experimentally plot hysteresis graphs you would get something like this this would be the big difference you would see the hard ferromagnets would tend to have a fat hysteresis graph and the software of magnus would have a very slim very thin physteresis graph but let's see how does it make any sense well if i start from over here this is the point where we have a very strong magnetic field and we have all the domains completely aligned in the same direction now when i go back and i decrease the vacuum field notice the magnetic field inside pretty much stays the same this is the one that's representing that most of the domains stay aligned and as a result even when the external field is zero look the field inside the magnetic field is super strong and so this represents your high retentivity this point represents your retentivity and we can say the same thing over the other direction as well you can see when the magnetic field on the other direction also when we reduce it and make it zero the magnetic field inside stays pretty high so these two points represent this length you can say in the graph represents retentivity and you can see high retentivity what happens in this slim graph well if you look carefully you can see over here again if we start from this point as i decrease the magnetic field the vacuum field you can see the field inside pretty quickly drops almost to zero not exactly zero because all ferromagnets have some retentivity not zero retentivity they will have some retentivity but very very low retentivity look at this very low compared to this one again the same same is the case from the other direction so again this represents this represents the retentivity there is another point of interest for us and that is this point this is the point where the magnetic field inside has gone to zero which means this magnet has the ferromagnet has been demagnetized and how did we do that by putting a magnetic field in the opposite direction this is the point where some of the domains have aligned to the left and about half of them are aligned to the right and the magnetic field has cancelled out so the amount of magnetic field that is needed to be put in the opposite direction to demagnetize the ferromagnet this this thing is often what we call coercivity or coercive force core cvt and you can see that heart ferromagnets should have very high coercivity why because they should be hard to demagnetize right these we want them to be permanent magnets so if you keep them close to another magnet we don't want it to its magnetism to change and so we need this to be very large but look at the coercivity of your temporary magnets you can see it's very very tiny very very almost zero which means it's you need almost zero so low correct let me write that so you you get low coercivity over here so this means temporary magnets not only have very low retention of magnetization they can be very easily demagnetized as well permanent magnets not only have very high retention but they're also very hard to demagnetize as well so that's the basic difference as we see but here's your similarity one major similarity you see is in both of these cases we want to make sure that when you have some external field when you put an external field we want the magnetic field inside to be much much larger than the magnetic field outside that's the defining feature another defining feature of our ferromagnets and we want that to be true in both of them we want both of them to generate very strong magnets you know magnetic fields and so that is often called permeability so both of them we want both of them to have high permeability permeability this basically means you want both of them to allow the magnetic field lines to easily pass through them you want both of them to get magnetized to a very high value that's what it means in both cases you want that property so these are the major differences and similarities between the two