a) x =165 MeV b)Control rods (made of materials like boron or cadmium) absorb excess neutrons, preventing them from causing additional fissions. By adjusting the position of these rods, operators can control the rate of the reaction and keep it stable. c) In an uncontrolled reaction, such as in a nuclear bomb, there are no control rods to absorb the neutrons, so the reaction proceeds very rapidly, resulting in a massive explosion.
a) 2,052.4 MeV b) Since each fission reaction releases 3 more neutrons, they will go on to initiate another fission, hence the cycle goes on. In a controlled reactor (I did a bit of research) some of these neutrons are absorbed, hence controlling the reaction. c) Since there was no control mechanism, the fission occurred rapidly, releasing neutrons that went on to initiate more fission. Hence an uncontrollably large amount of energy was released, causing the detonation of the bomb. I really enjoy your questions sir. If my answer is incorrect, please let me know, as I'll try it again. (TH-cam isn't properly showing my comment. I commented hours ago, but it isn't showing up in another account.)
I did part (a) by looking up the mass of U236 and the masses of the products. Assuming I read the right sources correctly, U236 = 236.045568 amu; products = 235.866578 amu. Difference = 0.178990 amu = ~ 1.67e8 electron volts. A sustained chain reaction can be obtained if on average one of the three product neutrons initiates another fission. If on average more than one of the neutrons initiates another fission, the process runs away, and if quickly enough, an explosion. I recall learning that the neutrons released are too energetic for efficient capture by U235 and require a "moderator" substance to slow them down. In the first atomic pile, the moderator was the pile of graphite blocks in which the fuel was embedded.
I try to remember what I learned some decades ago. It's not much. When comparing E=mc^2 before and after the fission I get 236.04 units before and 235.84 units after the fission. So the mass is reduced by 0.20 units which is roughly 3e-11J or 190MeV. To control the chain reaction, a neutron absorber is necessary. You want an average of 1 neutron to cause another fission. What is also needed: A medium which slows down the free neutron, since fission can only occur when neutrons hit the target with a certain amount of energy. The chain reaction gets uncontrolled if the average number of free neutrons stays above one. That happens, if a large enough chunk of U-235 is formed. A free neutron exists all the times due to cosmic radiation. Then, the 3 neutrons after the first fission of an U-235 don't just leave the chunk of U-235 but split other U-235 nuclei (after having bounced off other U-235 nuclei several times to reduce their kinetic energy). So for a bomb you need a critical mass of U-235. That's what I remember from my nuclear physics lecture some 20y ago, combined with your mini-lecture.
a) 138MeV b and c) because of the 3 released neutrons. When a barrier disallows them to meet other nuclei then the reaction is controlled; otherwise the reaction is uncontrolled and leads to an explosion.
a. About 173 MeV. b. Released neutrons trigger additional fission. Low-enriched Uranium is used in nuclear power plants, and control rods, which absorb excess neutrons, help control reaction rates. c. Highly enriched Uranium and no control mechanisms enable exponential fission in an extremely short time, that is, explosive chain reaction.
Professor Lewin, actually I did solve 214. I had to look up some data such as conversion factors and I used more significant figures only rounding at the end. Before rounding I found 20,145 tons of TNT compared with your 19k tons but agreeing with the 20k tons figure in the solver’s solution. We’re out here even if we seldom comment.
a) 177 MeV b) The fast neutrons are slowed down to thermal neutrons by a moderator,e.g. water. c ) Nuclear weapons employ highly enriched fuel exceeding the critical size and geometry (critical mass) necessary in order to obtain an explosive chain reaction.
a) 179.7 MeV. That's just the binding energy delta + the kenetic energy of escaping neutrons (3*2 )+ 7 for the gamma ray photons released. ( no daubt wrong ) b) This causes a chain reaction in a power plant when the reactor in this powerplant becomes critical, which is the case if exactly one of ( 3 in the example case ) escaping neutrons cause another Fission reaction and the others are absorbed without any effect. This absorption can be controlled in a pressured water reactor by control rods for rapid or emergency changes and the addition of borium to the main coolant fluid for normal operation. The borium is used instead of control rods whenever possible because if only control rods were used the fuel rods would be used up in the bottom part while the top would remain unused, which is of course highly unpractical. Note that a sufficient concentration of U235 is needed in order to reach the critical mass required to become critical. Also note the process has a negative temperature coefficient thus stabilizing the reaction naturally. c) A weapon chain reaction happens when the U core of such a weapon becomes supercritical. This happens when more than 1 in 3 escaping neutrons cause another Fission reaction. Way more for a practical weapon. For this a high concentration of U235 is needed called weapon grade uranium. The critical mass is postponed before detonation by physically seperating pieces of U235. When detonation is required these pieces are joined and the critical mass ( or maybe supercritical mass I suppose ) is reached and the weapon detonates. I believe there is no material causing absorption of neutron but not sure. This is due to the fact that if kinetic energy for a neutron is to high it has less chance of causing another fission. So there might be some materials or techniques present to optimize the number of fission reactions in that respect.
@@lecturesbywalterlewin.they9259 Ha, that's a lot of credit for something I understand so poorly. For answer b and c I got some head start. Although the physics is not always that clear to me, some parameters are tightly controlled by the electronics I maintain. Boric acid in the primary system is one. Pressure in virtually all systems is another. Steamgenerator parameters are important. These produce steam for the turbine but are also the main method of removing heat from the reactor. ( many emergency systems in place if these happen to fail. ) Even after shutdown, each element in the reactor has it's own half time and significant power is produced after control rods fall in, for maybe 15 hours or so. I once spoke to one employee there and he explained it as exponential decay for each fission product adding up. The grand total is not exponential decay as half life for each product is different but the graph of power produced after shutdown looks somewhat simular. Thanks for this problem series, I might not always understand perfectly but I sure have improved on my understanding of nuclear physics.
Sir i have a question..if u become a doctor or engineer in any field you are not that famous but honest towards your work on the other side people not having good graduation they are becoming TH-camrs or influencer and getting name and fame instead who should have been the real heroes for us they are ignored...what is the motivation to move ahead and to achieve the goals... please answer it❤u are my inspiration
Sir my question was those who are studying a lot and becoming doctor or engineer they are not that famous than a influencer, TikToker they are getting more fame and people like them so much instead they should praise our real heroes... I hope you get the question
Obviously the question was not directed toward me, but here’s my two cents anyway. It takes skill and dedication to become a professional, and it usually takes skill and dedication to become famous. The “real heroes” you describe do get recognized, but why covet fame in the first place? Doctors and engineers already make plenty of money. I strongly believe that your motivation should be intrinsic regardless, but difficult careers like being a doctor get tons of recognition and appreciation. Likewise, most popular influencers also put a lot of time and effort into their craft.
Can you create a video that examines the controlled fission of U235 in a medium such that the products react with additional substrates to produce controlled mass creation?
In der Natur gibt es wahrscheinlich Bewegungen mit so großer Geschwindigkeit, dass sie sich in einer Milliardstelsekunde mit so großer Geschwindigkeit bewegen, dass die gesamte Existenz existiert. Menschen und Pflanzen im Universum sind alle extrem schnell. Was halten Sie von diesem wissenschaftlichen Eindruck?
Sir , Kindly clear my query .... Considering two massive shells placed concentrically. Due to the outer shell , the net gravitational field is zero inside , so is the force on the shell . But due to the smaller inside shell there is a field outside, so should a force . But how can in a system of two bodies only one experiences a force while the other does not. How does Newton's 3rd law hold here ... ? Thank you .
The inner shell experiences a force, equal and opposite in all directions (radially outwards), thus the forces cancel out, resulting in zero net force and zero gravitational field (as you mentioned). Same is the case for the outer shell, which experiences an equal and opposite force due to inner shell in each direction (radially inwards) and thus it also does not experience any net force. Therefore, due to both the spheres, the net force on each other is zero. If you want details, if you consider individual particle, one from each sphere, they are exerting a gravitational force on each other, but since they are bound to their respective shells(which has the tension of adjacent particles), they cannot move. This applies to all the particles, and so they cancel out each other's forces. Let me know if you understood it.
Hi sir I have completed all physics course of class 11 and 12 in nepal but while I am going to solve mcqs I can't solve all of them? Why can you suggest me a right path to memorize physics in good way
Thank you, sir, if it is not too much, can I respectfully ask you to send me OHANIAN's Physics PDF and its Solutions? I can not find it anywhere and I am in a place that I can not access to the textbook.
I have a question the nuclear forces are there to hold the protons together in a nucleus against the coulomb forces..why do the nuclear forces only occur among the protons why not also between the electrons ?
they are called nuclear forces for a reason - the neutrons and protons are held togther by a strong force ONLY if they are very very close to each other - of order 10^-15 m
@@lecturesbywalterlewin.they9259 Then what holds the electrons in their orbits together?why dont they go away from each other (because of the repulsive force between them)
@@YuvikaChaudhary-1803 electrons are not in orbits - that's a highschool misunderstanding. Take MIT's QM1 course (called 8.04). It's on my channel. QM2 (8.05) is also on my channlel
@@lecturesbywalterlewin.they9259 in my school the teachers are more money centered than physcis centered and If I were to ask for advice they would ridicule me . I am very determined and I can study with no sleep about physics
@@lecturesbywalterlewin.they9259 Noted Mr. Lewin. The fundamental laws of physics do not apply universally, The conditions in space can lead to phenomena that are very different from what we experience on Earth. Physics in space can seem to turn things upside down.
a) 2,052.4 MeV b) Since each fission reaction releases 3 more neutrons, they will go on to initiate another fission, hence the cycle goes on. In a controlled reactor (I did a bit of research) some of these neutrons are absorbed, hence controlling the reaction. c) Since there was no control mechanism, the fission occurred rapidly, releasing neutrons that went on to initiate more fission. Hence an uncontrollably large amount of energy was released, causing the detonation of the bomb. I really enjoy your questions sir. If my answer is incorrect, please let me know, as I'll try it again. (TH-cam isn't properly showing my comment. I commented hours ago, but it isn't showing up in another account.)
We are lucky we got a teacher that are teaching us for free🎉
Thankyou sir! This is the power of social media.
You are great sir 🙏because your way of teachings is very nice ❤
Thanks.
@@lecturesbywalterlewin.they9259 thanks a lot sir🙏you are the king of physics in entire world
a) x =165 MeV
b)Control rods (made of materials like boron or cadmium) absorb excess neutrons, preventing them from causing additional fissions. By adjusting the position of these rods, operators can control the rate of the reaction and keep it stable.
c) In an uncontrolled reaction, such as in a nuclear bomb, there are no control rods to absorb the neutrons, so the reaction proceeds very rapidly, resulting in a massive explosion.
Thank you so much for your lectures sir ! They truly made me love physics when I felt like I knew nothing about it.
a) 2,052.4 MeV
b) Since each fission reaction releases 3 more neutrons, they will go on to initiate another fission, hence the cycle goes on. In a controlled reactor (I did a bit of research) some of these neutrons are absorbed, hence controlling the reaction.
c) Since there was no control mechanism, the fission occurred rapidly, releasing neutrons that went on to initiate more fission. Hence an uncontrollably large amount of energy was released, causing the detonation of the bomb.
I really enjoy your questions sir. If my answer is incorrect, please let me know, as I'll try it again.
(TH-cam isn't properly showing my comment. I commented hours ago, but it isn't showing up in another account.)
Thanks for the opportunity to think again about concepts that I took for granted.
I did part (a) by looking up the mass of U236 and the masses of the products. Assuming I read the right sources correctly,
U236 = 236.045568 amu; products = 235.866578 amu. Difference = 0.178990 amu
= ~ 1.67e8 electron volts.
A sustained chain reaction can be obtained if on average one of the three product neutrons initiates another fission. If on average more than one of the neutrons initiates another fission, the process runs away, and if quickly enough, an explosion.
I recall learning that the neutrons released are too energetic for efficient capture by U235 and require a "moderator" substance to slow them down. In the first atomic pile, the moderator was the pile of graphite blocks in which the fuel was embedded.
I try to remember what I learned some decades ago. It's not much.
When comparing E=mc^2 before and after the fission I get 236.04 units before and 235.84 units after the fission. So the mass is reduced by 0.20 units which is roughly 3e-11J or 190MeV.
To control the chain reaction, a neutron absorber is necessary. You want an average of 1 neutron to cause another fission. What is also needed: A medium which slows down the free neutron, since fission can only occur when neutrons hit the target with a certain amount of energy.
The chain reaction gets uncontrolled if the average number of free neutrons stays above one. That happens, if a large enough chunk of U-235 is formed. A free neutron exists all the times due to cosmic radiation. Then, the 3 neutrons after the first fission of an U-235 don't just leave the chunk of U-235 but split other U-235 nuclei (after having bounced off other U-235 nuclei several times to reduce their kinetic energy). So for a bomb you need a critical mass of U-235.
That's what I remember from my nuclear physics lecture some 20y ago, combined with your mini-lecture.
a) 138MeV
b and c) because of the 3 released neutrons. When a barrier disallows them to meet other nuclei then the reaction is controlled; otherwise the reaction is uncontrolled and leads to an explosion.
a. About 173 MeV.
b. Released neutrons trigger additional fission. Low-enriched Uranium is used in nuclear power plants, and control rods, which absorb excess neutrons, help control reaction rates.
c. Highly enriched Uranium and no control mechanisms enable exponential fission in an extremely short time, that is, explosive chain reaction.
Professor Lewin, actually I did solve 214. I had to look up some data such as conversion factors and I used more significant figures only rounding at the end. Before rounding I found 20,145 tons of TNT compared with your 19k tons but agreeing with the 20k tons figure in the solver’s solution. We’re out here even if we seldom comment.
I believe I mentioned you all conversion factors in the problem
@ I had to look up MeV to Joules, maybe others, I don’t recall atm.
a) 177 MeV
b) The fast neutrons are slowed down to thermal neutrons by a moderator,e.g. water.
c ) Nuclear weapons employ highly enriched fuel exceeding the critical size and geometry (critical mass) necessary in order to obtain an explosive chain reaction.
Way they are so poweful ?, this is my cuestión sir, thanks for the clues,
You are welcome
a) 179.7 MeV. That's just the binding energy delta + the kenetic energy of escaping neutrons (3*2 )+ 7 for the gamma ray photons released. ( no daubt wrong )
b) This causes a chain reaction in a power plant when the reactor in this powerplant becomes critical, which is the case if exactly one of ( 3 in the example case ) escaping neutrons cause another Fission reaction and the others are absorbed without any effect. This absorption can be controlled in a pressured water reactor by control rods for rapid or emergency changes and the addition of borium to the main coolant fluid for normal operation. The borium is used instead of control rods whenever possible because if only control rods were used the fuel rods would be used up in the bottom part while the top would remain unused, which is of course highly unpractical. Note that a sufficient concentration of U235 is needed in order to reach the critical mass required to become critical. Also note the process has a negative temperature coefficient thus stabilizing the reaction naturally.
c) A weapon chain reaction happens when the U core of such a weapon becomes supercritical. This happens when more than 1 in 3 escaping neutrons cause another Fission reaction. Way more for a practical weapon. For this a high concentration of U235 is needed called weapon grade uranium. The critical mass is postponed before detonation by physically seperating pieces of U235. When detonation is required these pieces are joined and the critical mass ( or maybe supercritical mass I suppose ) is reached and the weapon detonates. I believe there is no material causing absorption of neutron but not sure. This is due to the fact that if kinetic energy for a neutron is to high it has less chance of causing another fission. So there might be some materials or techniques present to optimize the number of fission reactions in that respect.
I couned you among the 5 winners
@lecturesbywalterlewin.they9259
• 0 seconds ago
I counted you among the 5 winners
@@lecturesbywalterlewin.they9259 Ha, that's a lot of credit for something I understand so poorly. For answer b and c I got some head start. Although the physics is not always that clear to me, some parameters are tightly controlled by the electronics I maintain. Boric acid in the primary system is one. Pressure in virtually all systems is another. Steamgenerator parameters are important. These produce steam for the turbine but are also the main method of removing heat from the reactor. ( many emergency systems in place if these happen to fail. ) Even after shutdown, each element in the reactor has it's own half time and significant power is produced after control rods fall in, for maybe 15 hours or so. I once spoke to one employee there and he explained it as exponential decay for each fission product adding up. The grand total is not exponential decay as half life for each product is different but the graph of power produced after shutdown looks somewhat simular. Thanks for this problem series, I might not always understand perfectly but I sure have improved on my understanding of nuclear physics.
Sir is really awesome
Just subscribed to this great man
Love you sir, I hope to meet you even though I am a 15 year old kid.
thanks for all lectures sir
you're welcome
Sir i have a question..if u become a doctor or engineer in any field you are not that famous but honest towards your work on the other side people not having good graduation they are becoming TH-camrs or influencer and getting name and fame instead who should have been the real heroes for us they are ignored...what is the motivation to move ahead and to achieve the goals... please answer it❤u are my inspiration
question unclear
Sir my question was those who are studying a lot and becoming doctor or engineer they are not that famous than a influencer, TikToker they are getting more fame and people like them so much instead they should praise our real heroes...
I hope you get the question
Obviously the question was not directed toward me, but here’s my two cents anyway. It takes skill and dedication to become a professional, and it usually takes skill and dedication to become famous. The “real heroes” you describe do get recognized, but why covet fame in the first place? Doctors and engineers already make plenty of money. I strongly believe that your motivation should be intrinsic regardless, but difficult careers like being a doctor get tons of recognition and appreciation. Likewise, most popular influencers also put a lot of time and effort into their craft.
@@somedude4832 thank you so much for such a wonderful ans and giving me time 😊😊
Thanks for this lecture sir 😊😊
Great sir 💞💞
Can i get 1 reply from one of the greatest phy teacher.😊
I have jumped in way too late, I will go back to a few lectures back. I will be back!
good idea
Can you create a video that examines the controlled fission of U235 in a medium such that the products react with additional substrates to produce controlled mass creation?
ask Quora
Sir in your pov which SCIENCETIEST you like more
Dear teacher please explain
What is difference between Theory & Law🙏🙏🙏
use a dictionary
Sir can we generate electricity by walking on stairs please make video on thi I request you to sir ❤
yes we can - to make a video abnout it would be silly
In der Natur gibt es wahrscheinlich Bewegungen mit so großer Geschwindigkeit, dass sie sich in einer Milliardstelsekunde mit so großer Geschwindigkeit bewegen, dass die gesamte Existenz existiert. Menschen und Pflanzen im Universum sind alle extrem schnell. Was halten Sie von diesem wissenschaftlichen Eindruck?
Hey sir i am jee aspirant from india class 9 and i have completed all physics syllabus so i should do now.
Sir , Kindly clear my query .... Considering two massive shells placed concentrically. Due to the outer shell , the net gravitational field is zero inside , so is the force on the shell . But due to the smaller inside shell there is a field outside, so should a force . But how can in a system of two bodies only one experiences a force while the other does not. How does Newton's 3rd law hold here ... ? Thank you .
I do not solve problems for viewers, I teach physics
The inner shell experiences a force, equal and opposite in all directions (radially outwards), thus the forces cancel out, resulting in zero net force and zero gravitational field (as you mentioned). Same is the case for the outer shell, which experiences an equal and opposite force due to inner shell in each direction (radially inwards) and thus it also does not experience any net force. Therefore, due to both the spheres, the net force on each other is zero.
If you want details, if you consider individual particle, one from each sphere, they are exerting a gravitational force on each other, but since they are bound to their respective shells(which has the tension of adjacent particles), they cannot move. This applies to all the particles, and so they cancel out each other's forces.
Let me know if you understood it.
Hi sir I have completed all physics course of class 11 and 12 in nepal but while I am going to solve mcqs I can't solve all of them? Why can you suggest me a right path to memorize physics in good way
Thank you, sir, if it is not too much, can I respectfully ask you to send me OHANIAN's Physics PDF and its Solutions? I can not find it anywhere and I am in a place that I can not access to the textbook.
I do not have that pdf and I don't think it is available online
Thanks Sir 🙏🙏🙏🙏
I have a question
the nuclear forces are there to hold the protons together in a nucleus against the coulomb forces..why do the nuclear forces only occur among the protons why not also between the electrons ?
they are called nuclear forces for a reason - the neutrons and protons are held togther by a strong force ONLY if they are very very close to each other - of order 10^-15 m
@@lecturesbywalterlewin.they9259 Then what holds the electrons in their orbits together?why dont they go away from each other (because of the repulsive force between them)
@@YuvikaChaudhary-1803 electrons are not in orbits - that's a highschool misunderstanding. Take MIT's QM1 course (called 8.04). It's on my channel. QM2 (8.05) is also on my channlel
@@lecturesbywalterlewin.they9259 ok thank you sir:)
@@lecturesbywalterlewin.they9259 ok thank you sir
Sir what kinds of tips would u reccoembd for inpho aspirant?
ask your teachers who know you
@@lecturesbywalterlewin.they9259 in my school the teachers are more money centered than physcis centered and If I were to ask for advice they would ridicule me . I am very determined and I can study with no sleep about physics
Ur the best teacher I ever knew so I figured I could ask you
I believe my message got discarded accidently
Our physcix teacher are more money centered and don't even know how to teach physics if I were to ask them for advice they would ridicule me
Sir how newton first law valid in bomb explosion.(Student of bsc)
Newwtonian mechanics is unrelated to Einstein's E=mc^2
@@lecturesbywalterlewin.they9259 f(R) disagrees
Sir I'm IIT jee aspirant please give some advice
eat yogurt every day but *never on Fridays* That also worked for Einstein and for me
@@lecturesbywalterlewin.they9259 Thanks sir
i wonder how old are you sir, many of them develop and loose interest in their subjects in time you're teaching physics
I am stone age
@@lecturesbywalterlewin.they9259 Thats pretty hard.
I am pwiam😊
That's pwian
hello
isn't it a good idea to retire after age of 70 brain cells are not functioning well in this age.
I retired from MIT when I was 73 yr old after I had been a Professor of Physics there for 43 years.
@@lecturesbywalterlewin.they9259 Noted Mr. Lewin. The fundamental laws of physics do not apply universally, The conditions in space can lead to phenomena that are very different from what we experience on Earth. Physics in space can seem to turn things upside down.
@@PnS_2023 there is no evidence for this at all. On the contrary we believe that the laws of Physics apply universally
a) 2,052.4 MeV
b) Since each fission reaction releases 3 more neutrons, they will go on to initiate another fission, hence the cycle goes on. In a controlled reactor (I did a bit of research) some of these neutrons are absorbed, hence controlling the reaction.
c) Since there was no control mechanism, the fission occurred rapidly, releasing neutrons that went on to initiate more fission. Hence an uncontrollably large amount of energy was released, causing the detonation of the bomb.
I really enjoy your questions sir. If my answer is incorrect, please let me know, as I'll try it again.
(TH-cam isn't properly showing my comment. I commented hours ago, but it isn't showing up in another account.)