 Okay, fine. So I think we have Nafla's exams started on 14 September. Okay hmm, so I Think we can start now. We have good enough audience Right, so I think last class we have already started this chapter magnetism and matter. Okay, so this this particular chapter is More basic than the chapter which we have already discussed like moving charge and magnetism was the Chapter that was related to magnetism. We have done already Okay, so this chapter comes before that and it assumes it does not assume you have any prior knowledge of magnetism Okay, so that's how it starts and it it actually assumes that you have no prior knowledge of Magnetism. Okay, so, you know when magnetism was first discovered it must be, you know One of those magnets must have been found Somewhere thousands of years back. So from there on the story started, you know The thing about magnetism. So they they understood I think we have already discussed that, you know, whenever you put a magnet in air and it is free to rotate It always aligns at a particular direction that happens because the magnet which you take Gets affected by the magnetism of the earth itself All right. So just because a basic property of the magnet That North Pole will attract the South Pole and South Pole will attract the North Pole because of that the magnetism of earth will always Magnetism of earth will always affect the permanent magnet in a certain fixed manner and always the magnet will get aligned in a particular fixed Direction. Okay. So that was the first usage of magnetism, right? They they started using The the magnet as a navigation device. Fine. So we have already discussed in In a great detail last class we have spent I think half an hour on just talking about What is the history of magnetism and all? All right. So The way we'll start today's session is that we'll just list down the basic property of of the magnetism like whatever we know about the magnetic Substance or whatever it is the basic knowledge will first write down and then we'll build this chapter from there on Okay, so We'll write down few of the very very basic things that are true about the magnetism write down the first one Earth behaves like a magnet. Okay. Now earth behaves like a magnet then Where is a magnet? Is it inside the magnet somewhere? Sorry. Is it inside the earth somewhere? There is no magnet as such inside the earth Earlier times people used to think that there is a huge magnet hidden inside the earth But that is not what is creating the magnetism of the earth. Okay? It's a molten lava that is flowing because of that magnetism is coming up and They there is no as such, you know, very clear understanding also exactly how it get develops Okay, but then how part if you leave it aside What it is is well known Okay, so earth behaves like a huge magnet and the magnetic field write down the field of the earth it points from Geo my Geographic south to geographic north Okay, so we have learned about Magnetic field already in moving charges and magnetism chapter So magnetic field is a vector, right? So it has a direction. It has sense of direction So earth behaves like a magnet and the direction of magnetic field. It goes from geographic south to geographic north All right Now this is the first Observation the second observation related to magnetism is When a bar magnet, okay, when you suspend a bar magnet if bar magnet is Suspended like this you have a string Let us say this is a string and you suspend a bar magnet like this When you suspend the bar magnet it always points in north south direction it always points in north south direction Fine, so I mean that that should be understood because suppose this is North pole of the magnet and this is south pole of the magnet Automatically north pole of the magnet will get influenced by one of the poles of the earth magnet Isn't it so earth magnet will be able to influence it and in such a way That the force because of the earth magnetic field, okay because of that Force if the torque on this magnet becomes zero then only this bar magnet will stop Rotating otherwise it will always feel a torque whose value will be m cross b we will discuss this in greater detail a little bit later right now Just understand this that torque is m cross b right this we have this formula we have already discussed so when the magnetic field When magnetic field becomes parallel to magnetic moment Then torque becomes zero and that is the direction in which the bar magnet will always Want to be get aligned to Okay All right, so let us talk about the next property So next property says that I mean it's pretty straightforward. It's just an observation that like poles will ripple and Unlike poles Attract Okay, so this is like the basic that like poles will ripple and unlike poles will attract there. I mean there People are not able to find out why it is happening exactly Because like poles ripple and unlike pole attract, but then this is the clear-cut observation And there was no exception to it. Okay, so we are not going into why it is a happening okay next Next property is that we cannot Isolate north and south poles No matter what you do North and south pole. They will together exist Okay, whenever there is a magnetic field That magnetic field if it is there there has to be a north pole and a south pole both will be there okay and The last observation is We can create magnets or destroy magnets fine, so you can take a matter and You can make it a magnet Fine, there are different ways of creating a magnet, but This is what it is like. It's not like energy Where you cannot create or destroy so magnets can be created or can be destroyed also fine So these are the basic property of any magnet alright, so The the center point of the study for this entire chapter is Actually a bar magnet or a permanent magnet. Why because this chapter Focuses on magnetism inside the matter Okay, so we are not focusing on magnetism due to a current our focus is How magnetism comes inside the matter Okay, and how we can put it in some sort of formula or analyze it, right? so that is what the focus is about and one clear good example of You know of a matter having a magnetic field is a bar magnet Okay, so that that is why you will see that we'll be taking examples of bar magnet again and again now Let us take a bar magnet Let's take a bar magnet like this I'm sure you might have done experiment in your lab Where you have to plot the magnetic field lines for a bar magnet. Yes or no kindly type in You have done this experiment Okay, fine, but then one basic knowledge is there that Feel lines will always start from north and it will end in the south pole Okay, so the way you plot magnetic field lines Around the bar magnet is what you do you take a compass? okay, this compass is Nothing, but it's a needle which is free to rotate about this axis Okay, it's it's a magnet itself. Okay, so if this needle rotates and this particular direction is Showing this way then this way is north and this way is south Okay, so like that if you move the needle you can trace the path of the Magnetic field lines. Okay, so like that if you do this experiment You'll see that magnetic field line from the north will start like this Okay, all of you please draw this It will go like that Okay So like this you will see that Magnetic lines of forces Sorry, just magnetic Lines it's not a line of force because the force because a magnetic field is not along the magnetic field It is perpendicular to it. So You can't say line of force So like this you will see there will be you know like that and Not only that there the magnetic field line They are Forming closed loop. So they go inside the magnet also. Okay, so it is like this Okay, so if you visualize the magnetic field line Due to a bar magnet it will be always in a form of loop Okay, and it will always start from north and Then end at south if you talk about externally how the magnetic field lines are okay But since magnetic field lines they form closed loop, you know, if you look at entire loop You can't make out where is a start and where is the end? But if you look at from outside you can feel that okay from outside it starts on north and ends at South Pole so like that you can Draw the magnetic field line for a bar magnet and whenever you see Magnetic field lines you can actually think of a wire Which has a current how the magnetic field line was magnetic field lines where in a form of circle Isn't it concentric circles it where? Okay, so whichever situation you take magnetic field lines will always Always they will always form a closed loop Okay, so if I talk about property of Write down if I talk about properties of magnetic field lines Then the first property is what? The first property is that the magnetic field lines They always exist in Loop they will form a closed loop. Okay, and the second property of magnetic field line is that the tangent to the field line tangent to the field line gives the direction of the Magnetic field fine, is it clear Can you think of any other property? There are two more properties that we have already discussed in you know In the previous chapter on magnetism. Can you just you know write down in the chat window any Property of magnetic field line that we might have missed Any other properties can magnetic field line intersect? Yes or no? Exactly, they cannot intersect they don't intersect any other property Any other let's keep the class interactive like if you have any doubts feel free to put it on the chat Okay, and if you have anything to add feel free to write it down Okay, they don't intersect because if they do intersect Suppose these two field lines they intersect at the point of intersection you see they are two tangent one tangent in this direction other tangent in that direction, so I Mean a physical quantity cannot have two direction because because of point B Tangent to this line is in this direction tangent to that line in that direction So you cannot have two direction for the magnetic field. So that is why they cannot Intersect fine any other property that you can think of please message What about magnitude of magnetic field? There's something related to magnetic Okay density of field line Decreases further from the magnet that is correct. Okay. That is an observation For with respect to only bar magnet that is correct. Okay right now our discussion is Property of field lines in general, okay in general exactly so the Magnitude I am not talking about direction now. Okay. I am talking about magnitude. So magnitude of Magnetic field which is the magnitude of this vector is proportional to is proportional to number of field lines Number of field lines per unit area per unit area Okay, so it's a special kind of vector whose magnitude is not related to the length of the vector Okay, the magnitude of this vector is the density of the lines. It's not the length of the vector. Okay Fine, so these are the few Properties of magnetic field line which we need to you know keep in our mind