 So in this question, there should be what kind of collision it should be, perfectly inelastic can we say? Yes sir. Or can I say that entire kinetic energy should be converted into ionization, can I say like that? It's not possible. That's not possible. Maximum loss that can happen is by converting, by having an inelastic collision. And in the inelastic collision wherein one atom collides with the other atom, it is mass m and m. Momentum m into v will be equal to m plus m to v1. So v1 is equal to v by 2. Okay, so the initial kinetic energy is half m into v square. The final kinetic energy is half into both the masses will stuck together and move together half this. So this will be 1 by 4 m v square. So the loss in kinetic energy, which will be used for ionization is initial minus final that is 1 by 4 m v square. This should be equal to how much? The ionization. How much? 13.6. That's it. 1.6 into 10 is power minus 19. So you need to write the mass of the atom also over here, which is this. Once you do that, you'll get velocity which is, if you do your calculation properly, which is 7.2 into 10 is power 4 meter per second. This is what you'll get. Okay, this one. Every physics question, you must draw a diagram. Without drawing, I'll not even look at what you have done. Okay, and there's a high chance you'll be making a sillier. Even if you get a very, very simple physics question also, draw a diagram. That is a must. Put a thread like this or a ball as a tango here. Like that, put a ball. Also, you know that J mains high chance that it will get postponed by at least two months. So you are at a very, very good position that syllabus got over quickly. And you have a lot of time to do everything else in physics. Okay, Chaitanya, Vikas and Ruchir, all three got different answers. Definitely one of you are wrong. One of you are right. No, need not be right. At least two will be wrong. That is a better statement to make. Chaitanya changed his answer. Others, Hia, what are you getting? Dhruv Bhatia. So here, what is happening? Okay, I can see many of you have answered here. Silver ball of ultraviolet wavelength is this. Total energy that has been imparted on the silver ball is given. Total energy is 1.1 into 1 into 10 is minus 7 joules. So I can find out number of photons that has hit there because if I get number of photons, I will be getting number of electrons that ejected. Okay, and electron that will be ejected will create a positive charge. So potential will be positive. Okay, it will not be negative. If you're getting negative, that is wrong because electrons are ejecting. So it will become positive charge, isn't it? Now, I think it is a unitary method question. There is no formula as such till the final formula of the potential. You have to do it as a unitary method. So first you have to find out number of photons hitting. That will be 10 is power minus 7 divided by 1242 by 200 electron volt. So that is 1.6 into 10 is power minus 19. So I hope you remember this formula. 1242 by wavelength in nanominter will give you electron volts for the photon. Yes or no? So here if we do that 6.626 into 3 into 10 to power 8, that 6.626 into 3 we can take as 20, right? Then it will directly go off. Whatever it is, you can do that. You can do this way also. That's what I'm saying. Calculation I'm not teaching anyways. Okay, so number of electrons that will be ejected will be equal to number of photons divided by 10,000. That is 10 is power 4. You get number of electrons. Okay, once you get number of electron, you will get total amount of positive charge will be number of electrons into charge of one electron this. So the potential that will get developed is 1 by 4 pi epsilon naught Q divided by the radius which is 4.8 into 10 is minus 2. So once you do your calculation correctly, the final answer you will get it is 0.3 volts. Not this. That is a good question. Do this, all of you. Draw a small diagram also. Okay, Ruchir got something. Minimum kinetic energy is how much? Minimum is how much? Both the Ruchir got something. That photoelectric experiment, you remember? That kind of tube was there. This thing. This is the experiment they are talking about. Between these two, 2 volt is there. The light is hitting here. Electrons are getting ejected and going there. Now this is the emitter. This is the collector. The minimum kinetic energy that can come out of emitter will be how much? From emitter. Minimum kinetic energy is how much? Zero. Zero. K minimum is zero. And for collector, the same kinetic energy will become how much? How much by the time it reaches collector? EV. That is 2 electron volt. Fine, so this is the minimum kinetic energy. So even if kinetic energy is zero, because of the potential difference, by the time it reaches collector, its kinetic energy will be too EV. All of you getting it? Now the maximum kinetic energy. Maximum kinetic energy from emitter. You can get it from Einstein's equation. K is equal to h mu minus work function. Now h mu you can write in electron volts. That will be very comfortable. 1242 divided by 200 electron volt minus 4.5. This is how much? 1.7. There is max kinetic energy at the emitter. By the time it reaches collector, how much it will become? 3.7. 1.7 plus 2. That is 3.7 electron volt. So they are not asking the minimum and maximum kinetic energy from the emitter. They are asking by the time it reaches collector what it will be. So I hope you got that point. Horizontal plate. You have to do the first one. Do you understand the question? In a way they are asking you with what velocity the plate should move so that the electrons can be ejected out of it. You can move the plate so fast that electrons will not be able to come out itself. What is that velocity? Suppose there is a plate, this is the plate. Electrons will be coming out like this let us say. Now suppose maximum velocity with which electron comes out is Vmax. And if plate start moving with let's say 2 times of Vmax will electron be able to come out? Everyone are you getting it? Will electron be able to come out or not? No I won't. It won't. It has to move faster than the plate's velocity for it to come out. And that much velocity it cannot have. So we need to basically find out Vmax for this particular scenario. Nothing else. Maximum kinetic energy is 1242 divided by 250 electron volt minus 1.9. This is roughly 5 times or 4.9 minus 1.9. So this is 3 electron volt. What happened? So now 3 electron volt is the kinetic energy. Half 9.1 into 10 is equal to minus 31 half m V square is equal to 3 into 1.6 10 is equal to minus 19. So V square is 4.8 into 2 divided by 9.1 into 10 raise to power this is 31 minus 19. It is how much? 22. No 12. So you can see numerator is close to the denominator only. So take a square root close to 10 raise to power 6 meter per second. One second. Okay. Anyone? Anyone is able to get something? One minute sir. Richard Parek got something. Is it very similar to the question where the potential difference was 2 volts? So it is very similar to that if you can understand that. Okay. Fine. First of all what is the electric field due to a sheet? Do you remember the formula? Sigma by 2 epsilon. Sigma by 2 epsilon mod. How much it is? Sir can you give 20 seconds? Just calculate. Do you calculate? I am doing it from start. By the time I complete you will also be done. E is how much? Have you calculated E? Sir I didn't calculate. I went to the last step. Good. That is the best thing. You can leave the calculation to the very end. That is also good. 1.1 into 10 parts 3. This is what? You will have 18 pi divided by 2. 18 pi Newton per coulomb is your electric field. Richard? Sir you can go. I will keep trying. So what is the potential difference between the sheet and wherever the cesium plate is? How do you get that? 18 pi into? Multiply by the length. Length which is 0.2. So this will become how much? 3.6 pi volts. Now I have to find out what minimum and maximum kinetic energy. Minimum kinetic energy will be 3.6 pi electron volt. Because it is like the previous one where it ejects with zero kinetic energy and undergoes this potential difference. So it will be that. And mind you, you need to be very careful. The same question, the potential. Suppose it is a negatively charged plate. If it is negatively charged plate, then what is the minimum kinetic energy that will reach the plate? If it is negatively charged, what do you think? Sir it won't reach the plate. It won't reach. The answer is zero. Minimum. Maximum will be what? The maximum kinetic energy minus the potential difference. You should not add then if it is a negative potential. And if for the for the maximum also you are getting negative kinetic energy. It means that your potential is more than the stopping potential. So both minimum and maximum is zero. So you need to ensure whether it is possibly charged or negatively charged also. Anyway, so now the maximum kinetic energy that get ejected out is equal to 1242 divided by 400 minus 1.9. And on this you need to add 3.6 pi this much electron volt it will be. Fine. How much it is? Has anyone calculated? So I got 3.2. 3.2? Oh no this is yes 13.2 electron volt. 13.2 electron volt. This is how much? 3.6 pi? 10? 12. 12 electron volt. 12? Yes sir. Yes. Huh? Okay. Fine. Not a big difference. Fine. Let us take couple of radioactivity also. I have couple of them. They are different. There is one. Agricultural experiment. Do the first one. Second one also is good. So what is one QT? One QT, did you forget? 3.7 into 10 to the power 10. One QT is 3.7 into 10 is power 10 becquerel. See I am not looking for the final answer here. You just tell me that you are done. As in the expression you got. That is all. You may require lock table or something like that. Okay. Should I discuss? Chatty, Chatanya what do you think? Yes sir you can discuss. You are done? Yeah. Okay. So we have one mole of radioactive material that was injected in a root of a plant. The plant is allowed 70 hours and then activity was measured. You need to find the percentage of activity that got transmitted from the root to the fruit after 70 hours. So after 70 hours what will be the overall activity? You know dn by dt is equal to n0, sorry dn by dt is lambda into n, right? Lambda into n. After time t you can write n as n0 e to the power minus lambda t. So activity as a function of time is this. Now lambda is 0.693 divided by this is per day, right? Every day has 24 hours, 60 minutes, 60 seconds. This is your lambda n0 is our hydro number 6 into 10 to the power 23 e to the power. Now here you can convert lambda in hours only. That is 0.693 divided by 14.3 into 24. That's all you have to do because you are trying to find out after 70 hours. So if you put t in hours here, lambda you can have per hour because lambda into t is any way dimensionless. So this is dn by dt, overall dn by dt. That's a 14.3 message, let me know. Now the activity on the fruit is 1 micro curie. So that is 3.7 into 10 is power 10 into 10 is power minus 6. Just have to take a ratio. You divide this overall with that. That is the ratio. That's all they want. Calculation wise it will be intensive. But then this is how you have to do. What is this? There was this one, a vessel volume tritium pascal. Tell me how will you do this? You don't need to actually solve it. Tell me, just tell me the idea. So we find out the number of moles by the ideal gas equation and then you can just use the lambda. So you are assuming it is an ideal gas. So you just need to find number, right? So you can get the number by any means. Here you have to assume it is an ideal gas. So like that, it is an ideal gas equation. That is a trick here. These vessels will be there in the mains exam. Do the first question only. This is the last thing for today. Then we are done. Richard get the final answer. You have the calculator, right? Anyways. I don't have one. You know I am saying you are like Sakuntala Devi, you heard of? Yes, I am a mathematician. Yeah, your brain is like that. Get it quickly. Alright, I will quickly solve this now. Is there anyone in the middle of a calculation? So one minute sir. Life is this. One gram has how many potassium nuclei? How do I get that? One divided by what? So 40 into the... You have to divide by the atomic mass of KCl or the molecular mass of KCl. Molecular mass. Sir, molecular mass. Molecular mass, right? Molecular mass. Molecular mass. How much is the molecular mass? Potassium is 39. 25. 35.5. So this is... 74.5. 74.5. That into Avogadro number. This is the number of KCl and number of potassium is same as number of KCl. Okay. Suppose the molecular formula would have been K2Cl2. Then the number of KCl would be half of number of potassium. So what have you got? You have to double it. Your counting number of nucleus ultimately. It doesn't matter what is the chemical combination. Nuclear reaction doesn't care about the chemical combination. It is all about number of nuclei. And nuclear reaction rate doesn't depend on external conditions also. It doesn't depend on temperature, pressure or anything else from outside because very strong force is involved in the radioactivity and any other things related to nucleus. Anyways, this is the number of... Total number of potassium and we need to find fraction of potassium inside it. Now, this is the thing which I have got from this data and radioactive will be only K40. So K40 activity is given dN by dt. Activity is 160. 160 should be equal to lambda times number of K40. So number of K40 will be equal to 160 divided by lambda. This is number of K40 and this is total number. So the fraction will be number of 40 divided by the total number of potassium. Got it? So this is the way we have to solve this question. Alright guys, so I guess that's it from my side today. We will meet next week with a chapter. Which chapter should we take? The first one? Can you suggest? Type it, quick. Electrostatic, AC, electrostatic, electro-electrostatics. Okay, we'll be taking electrostatics. Bye for now. Thank you, sir. Thank you, sir.