 So what do you guys have done? Can you tell me one by one exactly what you did atoms and nuclei? So I finished atoms and I did half of nuclear. From HCWM? Yes sir. Great. Ramcharan what you did? So same same as half of nuclear and I finished atoms. Okay so you must be having lot of doubts right? Kondena what you did? Yes sir. So I did like little of atoms and nuclear. Little little means what? What is little? I know little means that only but how much little? Number? Sir from 20 or 25. Okay fine. Okay I got one from I think this is Amal. Yes sir. Fine so let us take up this question. So this question says the average lifetime of hydrogen atom excited to N equal to 2 state. Many a times you will see that you know the question says like that hydrogen atom is excited but actually the electron is excited. Yes sir. So the average lifetime is 10 is for minus 8 seconds. It means what? From N equal to 2 it goes to N equal to 1 state in 10 is for minus 8 seconds. Okay you have to find number of revolutions made by the electron on the average before it jumps to the ground state. Right before it goes to the ground state where it was we can assume that it will be at N equal to 2 state only for this much time. Sir. Okay now if I know how much time it takes to revolve the second orbit time period. Then time period if I divide time period from T average will I get number of revolutions? Yes sir. Right so this is what you have understood or you were not. Sir I found velocity and then I found the how many I found number or I found 2 pi R basically and then there is the same thing. Yeah the same thing you have to find out 2 pi R by velocity this will be the time period. Okay and you know that MvR is equal to NH by 2 pi and Mv square by R is K times E square by R square. Getting it? Yes. Now see you can either solve these two equations and get the value of radius in the second orbit when you put N equal to 2 and velocity in the second orbit to get V when N equal to 2 or you can just remember the formulas like for radius I think it is 53. Sir I remember I used the formula for velocity. Yeah velocity and radius also I guess 53 M strong into N square I guess. N square by Z. It is hydrogen atom only so Z is 1 so 53 N square M strong so you will get radius and similarly you can get the velocity so this is how you get time period of revolution. Okay and then just divide that time period revolution from this you will get the number of revolutions. Any other doubt? Okay Ramcharan send me two doubts. Sir I have one more. You just send me okay don't wait. Fine so in an imaginary world the angular momentum is quantized to be even integral multiple of H by 2 pi. Basically Mvr which is equal to N times H by 2 pi will be actually 2 M times H by 2 pi. Are you getting it? Where M is equal to 1 2 3 like that. I have to make sure it is even so that is why I am multiplying with 2. Okay now everywhere in the derivation wherever integer N is all I have to do is to substitute value 2 M. Okay now this particular question talks about visible range right so we know that the visible spectrum is there in bomber series and what is bomber series? In bomber series this should be the formula 1 by 2 square minus 1 by N square. Okay where N should be what? 3 4 5 6. Okay we keep on substituting values you will get wavelengths in the visible light. Fine now 2 is possible or not in this world. Is 2 allowed? Number 2? Yes sir. Right so 3 is not allowed so you will be taking next possible wavelength next possible quantum number which is 4. Just put N equal to 4 you will be getting 1 by lambda minimum. 1 by lambda will be minimum when this thing is least 1 by 2 square minus 1 by N square and 1 by 2 square minus 1 by N square will be least when 1 by N square will be the largest when N equal to 4. So from here you will get lambda max. Fine so 46 question is on Doppler's effect that I will be taking up little later. Let me move to the question which Amov has asked. Any other doubt you just keep on sending me. Okay sir I will send. Okay calculate the magnetic dipole moment corresponding to the motion of electron in the ground state of hydrogen atom. Amov do you remember magneton? The magneton. Yes. Yes sir. That is what it is. Sir how do you find it? Don't you find N I A as a magnetic dipole? Yes so that is what we derived that point in time. See magnetic moment will be equal to N I A. Now when it comes to the hydrogen atom there is no question of number of terms. So it is simply current into area. Now current is what? Current is charge which is electron charge divided by time period of revolution. This into area which is pi r square. Okay now time period of revolution is what? 2 pi r divided by velocity. Fine. So this pi n pi goes and one r also goes away. I will be getting here as e v r divided by 2. Fine. And Bohr's model says that m v r is equal to n h by 2 pi. So v into r is n h by 2 pi m. Just substitute here and put the value of n as 1. You will be getting magnetic dipole moment when n equal to 1. Okay. Fine. Sir is confusing concept of area of an atom? Not area of the atom it is the area of the loop which electron is taking. Okay. Right. Next question is from Ramcharan. A neutron moving with a speed v strikes a hydrogen atom in the ground state moving towards it with the same speed. So this is a neutron. Okay. Neutron moving towards hydrogen with velocity v and hydrogen atom that is also moving with the same velocity v. Okay. They both are moving towards each other. Find the minimum speed of the neutron for which inelastic collision may take place. Mass of neutron is taken as mass of hydrogen. Now tell me what is the initial momentum of this neutron plus hydrogen together? What it is? If I take this as one system and I want to conserve momentum, what is the total momentum initially for this system? Sundar you can type in your answer on the chat box of YouTube, I will be able to see that. Okay. Tell me. What is the initial momentum? Mass of hydrogen is same as mass of neutron, initial momentum of the system. 0. Yeah. 0. So final momentum should be? 0. Ideally should be 0. Okay. And if it is inelastic collision, both of them will come together and they will stick, will come to rest. Yes. Right. Now tell me what is the initial energy of them together? Half mass of neutron into its velocity square plus half mass of hydrogen into velocity square. Right. This is the initial counting energy. And what is the final counting energy after inelastic collision? 0. So inelastic collision there will be some loss of energy or something. Right. Right. So I am talking about that. So this is what, this is initial energy, this is final energy, where the energy went? Defense energy. Where it went? So I excite the electron. Correct. If this is equal to, if this is equal to 13.6 into charge of electron, you just need a minimum amount of energy like one, from one to two, if the electron jumps and if this energy is sufficient to supply this energy, then this energy will be used up to do this. Otherwise, the electron will reject this energy and it will not be perfectly inelastic collision. Okay. Yes. Fine. Another question? Sir, after this what do we do? After this when I got stuck, I don't know how to simplify this. Okay. What is asked? Minimum speed of neutron. So see, the mass is same. Right. So you just write M into V square. Is this. M is known, you just find V. That is what the minimum speed is. Any lesser energy will not be utilized. You need at least this much energy to excite the hydrogen atom. Fine. Next is, okay. In a decade, like till which question number you did for nuclei? Sir, 22. Sir, I did around till like 23, 22, 20. Okay. Do you have access to the solutions? Yeah. So there are solutions. Yeah, but do you refer to them? No, sir. Only if I'm stuck on it. If I'm not getting it for a lot of time then. A lot of time as in? Sir, that. See, ideally don't look at it. You should ask me. Then I'll be giving you hints and that's how you learn. If you directly look at the solution, you may learn that particular question. The most important is learning the concept. So follow that. Don't hesitate to send me the doubts at any point in time. And I guess I have circulated a Google form also where you can upload your doubts. You just have to take a snap and upload it. Fine. So in this particular reaction, the maximum kinetic energy carried by positron. So kinetic energy maximum that is carried by positron is 0.650 MeV. What does it mean maximum kinetic energy for the positron? What does it imply? That means no energy is shared to the nutrient. Sir, so that will just be the subtracted. There is a mass defect between copper and nickel and positron. So if you add up the mass of nickel and positron, this mass will be less than the copper's mass. So some part of the copper's mass was utilized to generate energy. And that energy is distributed among all these three particles. And if all the energy is taken by the positron, then we can say that that is the maximum kinetic energy. Now we need to find what is the momentum of this neutrino in kg meter per second. Use a formula applicable to the photon. Is it a follow-up of part A as in what is the energy of neutrino which was emitted together with the positron of kinetic energy? So yeah, it will be a follow-up. Yeah, it is a follow-up. So you just use hc by lambda. So hc by lambda is the energy for the neutrino. And momentum should be what? h by lambda which is what? Energy divided by speed of light. You just divide speed of light from the energy and get the momentum.