 When I say modern physics, I am talking about these chapters, dual nature, atoms, nuclei and semiconductors. So again I am reiterating that these chapters are probably the most important ones when it comes to community exams perspective because it is very difficult to make a tough question from these chapters and definitely there will be 25% of questions from modern physics only. So make sure that you do this properly, it's very easy to master and there are lot of such topics. For example, I think even thermodynamics, thermo-properties of matter, then current electricity, then you have EMI, then optics, especially wave optics, there are so many, at least 60 to 70% of physics if you carefully strategize your preparation is extremely easy. You can easily score 70-80% marks if you strategize properly. But what happens is that sadly students, they tend to spend more and more time in mechanics and if there is a difficult question in the paper, most probably it will be from mechanics because it is easy to make a tough question in mechanics. So that is why I am not going to finish modern physics today only in a hurry. What I will do, this is just a part one, so there are at least 80 questions. So we will be taking few questions right now and there will be another 3R class on just problem solving for modern physics. That's why it is written part one. So let's immediately take up questions. I think I said work by energy for today's session but I have taken up modern physics because this is more important. All right, do these two questions quickly. Ramcharan, where is your friend, Kondanya? Okay, anyone? Okay, so I will do the first one. The half life of Radon is 3.8 days. We need to find time at the end of which 120th will remain a decade. So basically we need to find time where number of nucleus from N0 becomes N0 by 20. So you need to be very careful with the words of the question. It might have written that time so that 1 by 20th of the Radon sample will decay. Then remaining will be whatever is decayed, you have to subtract that from the initial number. So all that you should critically read each and every word from the question. Because in this modern physics they will try to make question tricky by just playing with the words. So we need to find time and we know that ln of N by N0 is equal to minus of lambda times t. So ln of N by N0 is nothing but 1 by 20. This is equal to minus lambda t or t will come out to be 1 by lambda times ln of 20. Now ln of 20 can be written as ln of 2 plus ln of 10. So you need to use what is given over here. So now this will be equal to 1 by lambda times ln of 2 is 0.693 and ln of 10 you can write this you know ln of 10 this is ln of 10 to the base 10 ln of e to the base 10 like this you can write. So this will come out to be 1 by lambda 0.693 plus 1 divided by 0.4343. Now lambda is lambda how you can find out 0.693 divided by the half life which is 3.8 is basically lambda. So use this lambda and then you will get the answer. Now question number 2. So it talks about shortest wavelength of x-ray is emitted from an x-ray tube. Now all of you have gone through that video which I have circulated for x-ray please type in yes or no have you gone through that video? You must go through that video because x-ray is one of the important topics so I have taken a lot of efforts to make that video you know a lot of diagrams involved there to explain. So make sure you watch it okay I will just remind me if I forget but I will try to remember myself and circulate that video immediately after the class okay. So the answer for this one is the voltage applied to the so I will just briefly explain how x-ray is generated actually. So there is this material okay so let us say a material has atomic number z so there will be electrons revolving around this atom okay there are many electrons that are revolving around. Now what happens is that you accelerate one electron from outside and just bombard this atom okay what happens when this electron comes in the influence of the atom okay because there will be electrons revolving around so these electrons will ripple this electron and then the nucleus will try to attract it so basically the kinetic energy of this electron will get changed getting it so if kinetic energy of the electron changes then whatever is the difference of the energy that happens between initial energy of the electron and the final energy of the electron when it goes away from the atom that energy difference will create a photon fine. Now the minimum possible wavelength will have highest possible frequency okay that happens when electron loses all its kinetic energy and you will get the shortest possible wavelength or maximum frequency okay so if you talk about the shortest wavelength then voltage applied to the tube is the one which is accelerating the electron or creating its kinetic energy okay so this kinetic energy only when it get decreased on that when it get lost creates a photon so that is why okay so this is called continuous x-ray and then there is something called characteristic x-ray characteristic x-ray gets created when this accelerated electron knocks off electron from KL or M shell of the atom so when the electron gets knocked off let's say from the innermost shell then there is a void that get created so electron from the outer shell will jump to the inner shell because that is a lower energy so when they jump they create a frequency because of this jump in the shells okay so this jump do not depend on what is the accelerated voltage or what is the kinetic energy of the electron that is coming from outside it only depends on this energy difference only depends on what is a material okay because it depends on the atomic number right for example for hydrogen it is 13.6 by n square similarly so the characteristic x-ray frequency depends only on the characteristic of the material which is what is the atom okay apart from that there will be a continuous wavelength that comes because the accelerated electron will lose its energy okay so now I'll circuit the video after the after this class please go through that it is important many theoretical questions get asked from modern physics keep keep on answering them if you if you can see the question start answering them okay so the third one is option number three beta rays emitted by the radioactive materials are of course charged particles emitted by the nucleus so option three fourth if elements with principle quantum number n equal n greater than four when not allowed then the number of possible elements would have been see you just write down like this 1s to 2s to 2p6 3s to 3p6 then you will have 3d10 okay then you have 4s to there is no a principle quantum number which is 5 right so just 4s to 4p6 4d10 and 4f how many are there in f 14 right 4f 14 so when you count 2 plus 2 plus 6 10 then 10 20 30 36 46 46 and 14 how many 60 how come you're counting 64 10 20 30 36 46 60 it's option one right try solving the fifth one no one for fifth energy in the nth orbit is what minus 13.6 by n square into z square z is the atomic number okay so if energy difference is more okay if energy difference is more then the wavelength will be less a frequency will be more okay so whichever have the highest atomic number will create the shortest wavelength okay this is not helium this is lithium okay so lithium has the highest atomic number so option three option four sorry otherwise option three if you doubly ionize the helium there is no electron left so there is no question of making transition from n equal to 2 to n equal to 1 okay there is no electron itself if it is doubly ionized helium okay sixth one I mean there is nothing to think so much it's a fusion reaction fusion means joining of two nucleus okay fusion means splitting so here you have four hydrogen they're joining all right so let's move to next one start solving these okay seventh you're saying first yes we have several application of this we have full wave rectifier we have half wave rectifier okay we have studied this someone has taught you this right question number eight x-ray beam coming from an x-ray tube will have all wavelength larger than certain minimum wavelength okay so the plot if you remember this is energy for a particular wavelength and this is wavelength so the plot goes like this it just keeps on going okay ninth 10th is yeah 10th is four where fusion is combination of two lighter nuclei what about ninth negative beta decay means the electron comes out when electron comes out what happens neutron in the nucleus neutron in the nucleus it converts to proton and because charge has to get conserved one electron get generated okay so neutron is decayed decayed means that it has lost its identity okay so neutron got converted into proton it also means that neutron decayed so all these two I'm here watch it on your laptop or desktop you'll be able to see all of you having same problem then I'll enlarge enlarge it 11th one first which one should have highest energy think of can a thermo energy of air molecule be mega lepton volt no photon energy of a visible light that is also not I mean it could be in terms of electron volt in fact to make sense for D okay and thermal energy of air molecules be very less okay otherwise we will feel the heat of the air molecules hitting over this our skin okay so it should be very very less so that is why for a option one make sense so you are fixed a for one and you have fixed D for two okay and then you look at the B and C binding energy of heavy nuclei per nucleon okay now we know that when we plot that graph you remember that plot so that plot we make binding energy per nucleon it that plot goes like this and this is in MeV isn't it and here is the mass number okay so that is why binding energy of a heavy nuclei should be in terms of MeV okay so we have locked option 4 for B okay so if you look at only option one here make sense not 12th a freshly prepared radioactive source has a half life of 2 hours okay so T half is 2 hours it emits radiation of intensity which is 64 times the permissible level okay so permissible level let's let's say I0 right now the intensity is 64 times I0 okay you need to find minimum time after which it would be possible to work safely we know that the intensity is proportional to number of particles that are emitting per second and that is also proportional to number of nuclei radioactive nuclei okay so in a way we have ratio of intensity I0 by I representing how many nuclei are left okay so this is 1 divided by 2 raised to power 6 okay so basically intensity should become half 6 times or it should happen after 6 half lives so if the half life is 2 hours so 2 into 6 after that time the intensity will become 1 by 64 times so if it is 64 time permissible level then 1 by 64 times will give you permissible level of radiation okay so for 12th option 2 is correct alright we move to the next one so I'm skipping the very easy ones alright okay this will take some time try solving this ideally it is good to I guess leave this question for the end okay if this kind of question comes where a lot of calculation is there but you should definitely attempt it because you know how to solve this right okay should I do this fine so you have two reactions okay now whenever you have a chain reaction like this you can just add these two equations alright when you add these two equations you'll get three times 1H2 giving you 2H4 plus P plus N okay so basically you require three deutrons in one reaction fine and we know that the average power radiated by the star is 10 to the power 16 watt okay so 10 to the power 16 joule every second must be released this much energy must be released okay now first I will try to find out how much energy is released in this reaction okay in order to find energy release in this reaction I'll use mass defect okay so three times mass of deuterium minus mass of helium okay then minus mass of proton minus mass of neutron okay this is the mass defect so this multiplied by 931 MeV is the energy now that is still MeV only right so this into 1.6 into 10 is so minus 19 this much joule is emitted in every reaction per reaction okay now you can find out how many reactions are happening per second how can you find out let's call this as y and let's call this as x so x by y will give you how many reactions should happen per second fine and every reaction will consume three deutron right so three times x by y deutrons are consumed per second okay so let's say it takes time t to consume all the deutrons so three times x by y into t should be equal to 10 is power 40 okay so t will be equal to 10 is power 40 divided by three times x by y all right so like this you can get time in seconds fine now here it is only asking you the order okay so if you are you know don't break your head in terms of what is the exact answer if you just judge what is the order of 10 by doing all this calculation that is good enough okay but then you have to be very good at calculations you cannot be like all the time you are using calculator and then you will just fall flat when you see such question in exam if you are not if you don't have a habit of doing calculation by yourself all of you clear about this any doubts see what kind of theoretical questions are asked from this modern physics section you cannot ignore the theory okay 14th is 2 you just have to know this okay because that nuclear reactor is there right in your syllabus if you if you would have read it you know it is like this you have to slow down the fast moving neutrons. Neutron will get unaffected by neutral be unaffected by electric field okay and yeah if you have collisions then they will probably you know they will not be moving in a direction which you want them to move anyways question number 15 now many of you what what is the natural tendency you find a momentum momentum is under root 2m into kinetic energy okay and then you compare whichever has a higher momentum which have will have higher penetrating power but that is not correct okay penetrating power and ionizing power are two different thing okay so ionizing power write down this thing ionizing ionizing power depends on momentum what is the amount of momentum okay and penetrating power or penetration yeah penetration depends on what is the velocity okay so we know that gamma ray has the highest velocity so gamma will have highest and beta then we will fast option one 16th 16th okay I guess this is again with respect to the definition and little attention to detail let's say this is your Pn okay this is your P and this is your N okay in case of forward bias it is like this right so this is how the current should flow okay this direction also shows the direction of the direction of the positive charge or the reverse of this direction of current shows the direction of the flow of electrons okay so if I look at this depletion layer okay then I'll have electrons getting accumulated from here and positive charges getting accumulated from P side isn't it that that's how the depletion layer is so now diffusion current is what diffusion current is a current or diffusion itself is a phenomena that happens from higher concentration to lower concentration okay so if charge or if a positive charge flows like this you can say that is diffusion and if a positive charge flows like that okay then that is not diffusion because right hand side majority carriers are electrons so diffusion always happens from higher concentration to lower concentration right so you can see that when it is forward bias you have diffusion current that is playing role okay and when it is a reverse bias when the reverse bias the current flow reverses okay and if current flow reverses then it cannot be diffusion it has to be then drift and there there is a limit up to which drift current can exist because drift will happen only when there are extra holes in the n side and extra electrons in the p side then only that side of current will get developed fine so that is the reason why the reverse bias has almost zero current because it forces it forces the holes from the n side to flow to the p side and there are no I mean as such ready I mean there are no free holes in the n side and there are no free electrons in the p side that is why now diffusion or sorry drift current limits the flow of current in the reverse bias okay so that is the reason why here auction 2 is correct okay fine so that's it for today we will take up modern physics in the next session also whenever we'll be having okay and in case I know that many of you have seen this question before you might have solved this in case that is there let me know I'll send you another set of questions okay no point doing same thing again and again if you have seen or done these already all right so that's it for today and see you soon