Solutions to Problem 218

[@Aspectscinnabar]

Your voluntary contributions to teach, even till now is appreciable . Thanks

[@lecturesbywalterlewin.they9259]

I am glad you think so!

[@luchomolinari]

you are the most cool human i ever meet..... super cool man.... i felt i was on a star war movie, receiving the instructions to how to blow out the dead star, seating on a class room with the helmet in my hands like the rest of the mates...

[@ThatsPeeks]

Your videos never disappoints! I really wish to have my book "For the Love of Physics" signed by you.

[@lecturesbywalterlewin.they9259]

Thank you! I appreciate the kind words.

[@wingsofmine2567]

Lovely being here ❤ Appreciate u n ur videos 💓✨️

[@carloosays]

I got it right Sir YOU are the Best teacher I love your way of teaching

[@lecturesbywalterlewin.they9259]

I'm glad you think so!

[@hopesalive3417]

I've learned alot from your videos sir and now I am a college graduate I just wanted to check up on you and I hope you are healthy and doing well Sir. thank you for your contribution in spreading the joy of PHYSICS ❤

[@lecturesbywalterlewin.they9259]

you are very welcome

[@ld238]

Don't be too disappointed. I remember you once said in a youtube video something like: "...wrong or right answer, remember: We will always be friends". Isn't it? Calculating the field lines can be done with "image charges" if I remember properly. I can't remember if this has been part of your 8.02 or 8.03 lectures.

[@copernicofelinis]

The inner layer of surface charge is shielding the bulk of the material from the electric field inside the cavity. So the outer layer of charge, that is spread on the external surface can only feel its own repulsive force and will redistribute itself according to the curvature of the outer surface. The conductor in the middle is like a dark chasm to both layers of charge. In a dynamic setting, I wonder how, classically, the charge would actually move, microscopically, if we were able to materialize the charge inside the cavity at t=0. Would the entire sea of electron be 'pushed outside' in an asymmetric way, and then the outer layer would redistribute according to curvature (uniform distribution on a sphere), or what?

[@lecturesbywalterlewin.they9259]

E-fields travel with the speed of light

[@copernicofelinis]

@@lecturesbywalterlewin.they9259 well yes, my question is about dynamics at that time scale. The inner layer is asymmetrical, the outer layer is not. If we start from a uniformly neutral body, the appearance of the negative charge inside the cavity will repel the conductor's electron in an asymmetric way. Some information needs to reach the outer surface to 'tell' the electrons they need to 'move outwards' so to speak, and it will travel at the speed of light. But exactly how will the charge density evolve in the d/c time interval? Will there ever be an asymmetric configuration of the outer charge? My guts say no, because if it was the case I could put a radio inside a Faraday's cage and somewhat get a signal out. I guess there will be a fraction of the thickness of the conductor where there will be a transient asymmetric perturbation. The electromagnetic wave due to the (unphysical) 'appearance' of charge will be exponentially attenuated inside the conductor. (A more physical situation would be wiggling the charge inside).

[@lecturesbywalterlewin.they9259]

@@copernicofelinis the dynamics all happens with the speed of light - thus on a time scale of nano seconds

[@copernicofelinis]

@@lecturesbywalterlewin.they9259 I had written a reply but it did not show up, probably still in a waiting list or lost by YT. So, I am now thinking that the perturbation due to the charge inside will die off exponentially inside the conductor, thus losing any information about the asymmetrical configuration on the inner surface. And of course the perturbation will be very fast, moving at the speed of light and would take us outside of electrostatics.

[@copernicofelinis]

@@lecturesbywalterlewin.they9259 both my replies disappeared, so there's no point in repeating them. Just to say that what I was talking about IS the sub-nanosecond transient.

[@sanjaychauhan5915]

Great Teacher

[@Entropy_Bros]

Great Human Being

[@mananagarwal7002]

Ordered your book sir waiting for it eagerly and thanks for your amazing lectures and explanations helps a lot in NEET preperation

[@lecturesbywalterlewin.they9259]

where did you ordfer it?

[@mananagarwal7002]

@lecturesbywalterlewin.they9259 Google forms. The signed book

[@mananagarwal7002]

@lecturesbywalterlewin.they9259 google forms

[@gabrieldanho5828]

Hi I work as physics teacher in the high school in Sweden but you gave me opportunity to discover the charm of Maxwells Equations in unique method specially with your application because I follow your lectures from MIT and you are really great physicist so I want please to write me the name of the textbook you use in those lectures because I want also to read your book besides your lectures and thanks ❤🙏

[@lecturesbywalterlewin.they9259]

8.01 Physics Hans C. Ohanian Physics Volume 1 2nd edition W.W. Norton & Company ISBN 0-393-95748-9 8.02 Physics for Scientists & Engineers by Douglas C. Giancoli. Prentice Hall Third Edition ISBN 0-13-021517-18 8.03 Vibrations and Waves by Anthony French CRC Press ISBN 9780748744473 8.03 Electromagnetic Vibrations, Waves and Radiation by Bekefi and Barrett. The MIT Press ISBN 0-262-52047-8

[@gabrieldanho5828]

@ Thanks 🙏

[@محمدالراهب-ض2خ]

Teacher lm from iraq.. I would ask you from the Electric potential difference what is Electric potential difference I can't imagine it scientifically and What is its relationship to the capacitance of the condenser scientifically in terms of reality is not a mathematical relationship

[@lecturesbywalterlewin.they9259]

watch my 8.02 lectures your answers are all there

[@محمدالراهب-ض2خ]

That you ​@@lecturesbywalterlewin.they9259

[@محمدالراهب-ض2خ]

​@lecturesbywalterlewin.they9259I found a lot of clips. Give me a title for the video, please

[@HackerBot-ez3ot]

❤fan of your sir😊❤

[@adityaprakash6149]

Sir I am preparing for international physics olympiad, can you please give me some tips. Also can you tell me what material should I use, that would be sufficient for my preparation.

[@lecturesbywalterlewin.they9259]

I cannot advice you. Ask your teachers. However, Watch all my 94 MIT course lectures. Start with 8.01, then 8.02, then 8.03. Do all the homework and take all my exams. *I guarantee you that you will then do very well on the Physics portion of any freshman college or JEE exam* You will find all information you need on this channel in three playlists "Homework, Exam, Solutions & Lecture Notes". 8.01 & 8.02 will each take about 200 hours, 8.03 about 250 hours.

[@adityaprakash6149]

Sir you have been my idol, I just have one unusual request and that is you have to be around until I go to International Physics Olympiad,win it, and tell the world that you are my physics teacher, the person who made me love physics, and make you proud.

[@hanslepoeter5167]

Sorry I let you down for the last 2 problems. I had little time but must admit probably too hard for me as well. Maybe the next one.

[@lecturesbywalterlewin.they9259]

Hans, you can afford to let me done once in a while

[@ThatsBlackidoZ]

Sir Walter levin, whenever i post my doubts they keep on disappearing in result that is not reaching to you what could i do to this

[@lecturesbywalterlewin.they9259]

I don't know why. try again - always send me msgs from my last video

[@ThatsBlackidoZ]

@@lecturesbywalterlewin.they9259 sir for now i am posting my questions in this. Could you please take screenshot of them and clear them? In case that they disappear agian. these doubts are from the videos lect 3 on kinematics and lect 4 on kinematics. --- 2) sir you have somewhere mentioned that x-axis will not change just keep moving borely under y-axis and the ball will drop in your hands. but sir when you do that golf ball catching experminet and money experiment you are indeed not doing them in vaccum there is air resistance and all present, then how they are so perfectly predicted by you at 49:15 in lect 3 3.) in the 8.01x - lect 3 video at 43:52 "yt = (vosinaplha)t .. " is the object's direction in y-axis and what does this equations means? 4.) Sir, what things do i need to think if i want to calculate when the projectile going to hit the object in any other situation such as in my home or any different kind of question. I was pouring water from a bottle to my water bottle question came to my mind is that "The water must be having same kind of projectile" is this true sir that the water can also be in projectile motion. 5.) Sir at 45:00 , the times are equal in both cases but i didnt understand how they are really equal 6.) overall sir im being confused that there are two or three "time" formulas are used to find some relations, also how do i can know which equation of motion has to be used to find specifically height max, range, or time ( do we have to calculate time everytime for vertical motion, horizontal motion, for range 'OS' ) 7.) Sir i have also a doubt from magnetism chapter out of this video. In a current carrying hanging conductor on a stand and having horse-shoe magnet aside, when i pass current thru the conductor it will defelect in opposite to magnet's direction and if i reverse direction of current it will experience a force towards the magnet. my question is that "Is the newton's third law valid here if yes then how sir???" also if we perform the experiment in explained above on earth and note the time and do in space will the time be a factor that will change?

[@lecturesbywalterlewin.they9259]

answer to 2) yes there is air drag but on the scale of the lecture hall that can be ignored. answer to 3) - it takes me too much timw to go back to another lecture - sorry answer to 4) practice more - I do several problems + solutions for you - go back to them 5) watch the video again and pay more attention to it 6) you have to practice more. Also do all my homework + homework solutions 7) yes Newton's 3rd is valid

[@Tokito62257]

that happened to me too. Mostly they'll appear after a few hours or so. I couldn't see my comments from any other account in beginning, that was annoying, probably a youtube thing.

[@ThatsBlackidoZ]

@Tokito62257 fr

[@huyuwgygyu]

from where can we get these question ? and where to send the solution ?

[@lecturesbywalterlewin.they9259]

look at my playlists

[@huyuwgygyu]

@@lecturesbywalterlewin.they9259 okay sir thanks you so much for replying love from INDIA i will definitely submit my answers

[@harshidapatel4711]

Respected Sir , I am Swet Patel Studying in 12th standard. Sir I want to become a physics professor so please guide me how did you started teaching and become professor of physics. Please sir I want a guidance.

[@lecturesbywalterlewin.they9259]

eat yogurt every day but *never on Fridays* that also worked for Einstein and for me

[@harshidapatel4711]

Sir I didn't get it !

[@Uzigod-go2hx]

​@@harshidapatel4711 Blud ur grammar is really bad ,i see no scope !!!

[@dwinantosaputra6679]

@@harshidapatel4711 The knee brother, rest your knee after doing the Silas.

[@5lands706]

Wow it’s nice

[@Uminater]

How many hours one should study on order to pass IIT in only 2 months

[@lecturesbywalterlewin.they9259]

roughly 700 hours

[@arasgoshayeshi9156]

Sir with all due respect since the spheres arent connected to anywhere there is no way for the charge to go and the total charge on both spheres mush be zero. There is no reason for the field to be zero in the area between spheres the only requirement is that the conductors are equipotential surfaces. If you take all that into consideration there would be a sigma on the inner surface so that the whole effect of smaller sphere and the point charge would be a -q at the center. And the outer sphere has really nothing to di

[@lecturesbywalterlewin.they9259]

with all due respecyt *you are wrong* - do your homework and watch my 8.02 lectures

[@abhranildebnath]

Sir, how can I become an astrophysicist, please guide me?

[@lecturesbywalterlewin.they9259]

go to the very best college that will accept you - get your Bachelor's degree. Then go to the very best University that will accept you in graduate school to get a PhD. Publish a handful of papers in refereed journals which will have an inpact on the field. Take it from there.

[@adityachaudhary4948]

Sir what shall I do to become a mathematician

[@lecturesbywalterlewin.they9259]

take math courses

[@Trulymind-ack]

Sir please teach sound ,

[@lecturesbywalterlewin.they9259]

watch my 8.01 lectures.

[@the_nuwarrior]

Gracias

[@Noname1234-u9g]

❤

[@malvikasharma6610]

Great sir pls pin it

[@shoaibahmedsayed9296]

Hello

[@ceresfauna4649]

Firsttt