 Let us take up a question, a solenoid has, there is a solenoid, there is a solenoid that has a core of material, it has a core of material, what does it mean? It means that inside the solenoid, there is a material that is filled with it. This is core of a solenoid. Inside it, you are filling a material, fine. This material has relative permeability of 400. What is relative permeability? Mu R. Mu R is 400, okay. And the windings of the solenoid, they carry, okay. So, Ritvik is asking, solenoid is the cause and the iron gets the effect, perfect. That is correct. Because of solenoid, because of the solenoid only, the material inside the solenoid get magnetized, okay. So, magnetization happens inside the material, okay. And the cause of the magnetization is magnetic intensity that is there on the solenoid, fine. So, let us proceed. So, the current is 2 ampere, number of turns per meter in the solenoid is 1000, okay. You need to find these things quickly. First is the value of magnetic intensity H, then magnetization M, and then total magnetic field B. All of you try to solve it. Any doubts with respect to question, feel free to ask. All of you have to try this out. Okay. Shweta is getting H equal to 2000. Anybody else got 2000? Hmm. See, the value of H is equal to N into I, number of turns per length into current. So, that is 1000 into 2, which is 2000. Value of M, how much, Kushal directly telling the magnetic field, no. As in, it could be correct, but get the exact value Kushal. I know you are not using calculator, you can tell me the value now. Get the value of M first, M, how much it is, Shweta, find the value of M first, that is B, you are saying. What is M, others, what is M, okay, okay, so the value of M, how will you get any idea? M is related to H, like this, M is X times H, remember this, M is X times H and mu R is what, mu R is 1 plus X, right, so X is mu R minus 1, okay, so I will get M as mu R which is 400 minus 1 times 2000, right, so like this only probably you guys have done, so this is coming roughly equals to 8 into 10 raise to power 5, okay, and the units will be what, see, moment, magnetic moment unit is ampere meter square, this into volume which is meter cube is the unit of H or M, so it will be ampere per meter, okay, so this is M, you guys were more accurate, 7.98 into 10 raise to power 5 that is like more accurate but I am writing approximately which is 8 into 10 raise to power 5 ampere per meter, okay, magnetic field is what, mu naught M plus H, isn't it, it is just mu naught M plus H, we have already, we have already got it, right, so this will come out to be approximately 1 Tesla roughly, so mu naught by 4 pi is equal to 10 raise to power minus 7, so this will give you mu naught equals to 4 pi into 10 raise to power minus 7, okay, so magnetic field will come out to be 4 pi into 10 raise to power minus 7 which is mu naught into M plus H which is 2000 plus 8 into 10 raise to power 5, okay, roughly it will come out to be 1 Tesla, getting it all of you, now the last part of the question, D, the last part is this, note down, you need to find magnetization current IEM, what is magnetization current, I will tell you now, are you guys ready, have you done A, B and C, okay, so magnetization current is the extra current needed, it is extra current needed in solenoid to have same magnetic field B without the material inside it, without the material, so M is not present, if, okay, we have one answer, 76.5 ampere, any other answer, that's not correct Vaishnavi, please check any silly errors you might have done, any other answer we have, 765, that's from Kushal, it's around that only, but still it's not the correct one, okay, let me solve it, see if there is no material inside the solenoid, magnetic field, okay, so I mean 790, close to the answer, but still needs little bit more refinement, so magnetic field if there is no material inside will be what, mu naught n i, okay, this should be the magnetic field, if there is no material inside, there is no question of mu r, the value of x will be 0 and mu r will be just 1, so as if nothing is there, so magnetic field should be mu naught n i, this should be equal to 1 tesla, okay, so when you equate this to 1 tesla, you will get current is equal to 1 divided by mu naught n, which comes out to be 796 ampere, okay, so how much extra current is required, already there was 2 ampere of current, so extra current required, which is magnetizing current I m will be equal to 794 amperes, okay, so you need 794 ampere if you are not using the material to achieve the same magnetic field, which is tremendous amount of current, you know your wires can just literally melt away, okay, so now you can appreciate how you know strong the effect of material can be on the magnetic field, fine, so that is the reason why you use soft iron core whenever you are using you know things like transformer where the magnetic field role is very important, okay, any doubts till now, anything, whatever we have done till now since start, you can ask any doubts and we can discuss that, anything, just type in yes or no, okay, I assume no doubts from any one of you, so there is still a half an hour or 45 minutes discussion that is required on this chapter to close it and I hope you have learned something today and you know you did not feel the, I just wanted a quick feedback of all of yours, like you know did you feel that I mean how I mean this online session, whatever we are taking these online session, is it like as effective as the classroom session or it is you know almost equivalent, better, how it is, just you know feedback on it, Saim here asking, Saim here, I will send a worksheet on this so that you can you know practice a lot of questions, meanwhile you can do one thing, all of you please solve, start solving the NCRD back exercises, okay and this chapter is there in J mains but it is not in J advanced, okay, but then you should you know pay good attention to this chapter because the concepts are not very difficult in this chapter, if you do it properly there is a guarantee that you will get all the questions right in the exam, okay, so do it properly and I guess we are I think we are done with the syllabus leaving apart those chapters like communication systems and EM wave, okay, so since we are almost done with the syllabus now what we will do is that we will take up you know every week I will send you a list of chapters, there will be like one chapter from class 11th or two chapters from class 11th and probably one chapter from class 12th, okay, so we will be revising two or three chapters together every week, okay and then we will be taking tests also on those chapters, so like that we will be able to cover up entire syllabus and remember this thing that entire December month we will be taking mock test, mock test means we will be doing we will be doing entire syllabus mock test, okay, so do not count December as your month for revising concepts, okay, because mock tests are very very important and we have to learn from our mistakes what we do in the mock test and the mistakes need not be on the concept only, the mistakes could be on your strategy, how you tackle the questions or how you you know if you don't get a question right what you do, so there are so many other things that should be taken care of and of course the speed also matters, okay, so our target is to finish the revision by end of November, okay and right now right now we have what this is middle of September, so October number we have two months, okay, so in these two months we will be revising the entire physics, okay and two months means that we will be having approximately 8 or 10 classes, fine, so we have 10 classes before we start taking mock test, so 10 classes for entire physics, okay, so do not when I ask you to come with a revision of suppose let us say I asked you to revise kinematics and electrostatics for next class, so what you should do is that you should you know not only read this chapter yourself, okay, also practice at least 50 to 60 question yourself and then come to the class, if you start expecting me to teach kinematics again for example, then we have hardly you know 10 classes left, so let's not spend lot of time only about talking theory part of the chapter, so what I expect you to is whenever I tell you that next class whatever the topic of the revision make sure you do it best possible way from your side, so that when I do it in class you are ahead and able to do you know beyond what is normal routine things, otherwise if you come without preparation for revision classes then we will be stuck with some routine stuff only, okay, so let's do one thing for next class there are seven days, okay, of course I understand you have your midterms going on but it is difficult, I know but still people are doing it you also have to do if you want to do well in JMAX, okay, so do one thing come up with a revision of full kinematics motion in 1D and motion in 2D kinematics and we'll take one lighter chapter here we will be taking optics, is it fine all of you for next week revision of these two topics and then end of the week we'll be taking test, is it fine message on the group, Kushal we will be revising rotation soon but we can't ignore other topics also, okay.