8.02x - Lect 5 - E= - grad V, Conductors, Electrostatic Shielding (Faraday Cage) 

This website contains all my 94 course lectures (8.01, 8.02 and 8.03) with improved resolution. They also include all my homework problem sets, my exams and the solutions. Also included are lecture notes and 143 short videos in which I discuss basic problems. ENJOY!

thank you sir , i lost interest in physics due to my teacher ( i am in india ) , but suddenly you tube recommended me your videos 94 mit lectures , and now i have to say i have fallen n love with physics , I LOVE YOU SIR !! 😇😇😇😇

These lectures are amazing! I consider them a gift to humanity, no less. Thanks again, Sir!

Professor Lewin, that lecture was shocking, electrifying and simply *wicked cool* !

its so nice to be able to take your lectures despite being thousands of miles away from your classroom, they really have changed my vision of physics, thanks a lot!!

21:39 WOW! Thousands of video lectures on RU-vid but I decided to watch this one on Valentine's day. What are the odds of that? BTW, thank you professor for uploading your lectures online. Awesome stuff.

what a taste you've got for humour! makes learning all the more fun

You're the most amazing and best phy magician and you really made me fall in love with physics ♥️ thnku Walter Lewin sir

After taking multi variable calculus last summer, this lecture paired with the last have to be my favorites so far. The insight into grad makes me me complete!

I took my undergrad physics class back in 1982 and enjoyed it quite a bit. Still, I'm going through these videos and loving it once again. Thanks very much! For what it's worth, I'm an electrical engineer and did some work with Electromagnetic Compatibility testing, where I did testing in a semi-anechoic chamber... which is really a very large (and expensive) Faraday cage that absorbs the radio frequency reflection inside the chamber. In that sort of work, it's quite important to remember all of the basic physics! Thanks again.

49:11 “Since I’ve taken 8.02, I’m not afraid.” Thank you, Sir.

Haha, just resumed the video at 21:50 and first thing he says - “In honor of Valentine’s day...” and today is really the Valentine’s day 😃

Thank you so much for uploading your lectures Prof.Lewin...Im really beginning to love physics.

These lectures are done so much better than my first year physics lectures. They also seem more challenging, (integrating vectors looks scary).. My school has a very poor physics department. Thank you for the lectures!!

The best physics's creation is you sir :-)

This lecture is the best thing I've seen ever especially the experiment with the Faraday's cage. These experiments build the intuition one needs for studying physics. Really I am mesmerized after watching this. Thanks professor❤

Awesome lecture... I wished I too had a teacher like you sir. All I learn in my college days was nothing .... Wish I can meet you once before I die... Lots of love and respect Sir. From India, Nagaland

I never thought this could also be an explanation for charge distribution on conductors U made my concept so crystal clear 😃 thank you so much sir 😄

YOU ARE SAVING MY PHYSICS GRADE. i love the way you teach the concepts

This course is amazing Sir. You really made me love physics.

The master at work! So very enjoyable to watch.

One thousand Thanks to you Mr.Walter Lewin for sharing such beautiful courses with really make every scientific brain loves Physics and Science in General. All Respect and please continue innovating and sharing wiht us and we hope we can do so.

Awesome lectures given by an awesome teacher. Thank you for the excellence!

Probability of me having a notably different career right now if I'd had these lectures circa 2004: .99. Thank you for the re-inspiration, professor!

Thank you so much! These lectures really make me love physics. It's becoming kind of addictive :)

sir nothing to say u are just of god-level ur lectures are just out of the world. love from India.

I wouldn't attend 50 mins class of school... But your lectures, but your lectures will be never missed..

Professor Lewin, I love your lectures. :)

I would like to call your teaching as great innovation in ACADEMIA. Love from india❤

That brooch on the shirt is perfect❗🔥🔥

I am so thankful for these lectures. We need more professors like you!

I don't feel wasted if I watch one of his lectures every day.

Hey professor !!! You are really amazing l am in class 12 and preparing for JEE 2021 and l missed the lecture of electrostatic shielding in the class .. But now l am not tensed ....because your lectures are really amazing 🙃🙃🙃🙃🙃🙃🙃

This is wonderful. This man is spreading knowledge on RU-vid like I spread butter on my morning toast.

Sir,your lectures are very informative and to the point,very comprehensive indeed

Hlo sir iam from india and iam 16 years old and I am very astonished by your great lectures Thanku so much for teaching me building my concepts :)

I wonder where this students are now. They were really fortunate to attend your classes. Love you professor

Thank You Sir! Cheers from Aerospace Engineering in Rome!

Thanks a lot sir,for making them available for anyone around the world! :)

When the metal plate is brought in contact with the rubbed glass which has excess of +ve charges, the lower side of the plate gets excess of -ve charges while the upper side of the plate has excess of +ve charges. You create a path for electrons to flow from the ground onto the upper side of the plate, by touching it thereby rendering the plate with a net -ve charge which is then transferred off to the containter. Am I correct in my explanation?

scene at 30:15 Why metal basket was negatively charged? I think positive charge induced by cat fur flows into metal basket. Was metal plate on the glass negatively charged at the moment Dr.Lewin touched the metal plate?

you are the greatest teacher for me forever.

the force line must be perpendicular to the equipotential then the forming surface eventually forms a sphere?

THANK YOU SIR, I AM AN ENGINEERING STUDENT IN SPAIN AND SINCE I FOUND YOU I DON'T HAVE TO MEMORIZE ANY EQUATIONS, CONGRATULATIONS

The coolest professor on earth.

Sir which book can be used as a reference book for the lectures?

Sir you are the best physics professor that I have ever seen

Excelent Course! I'm from Argentina. I've the following question: How "imperfect" can the cage be? I mean, the electrostic shielding, when yo explained it, and when it's explained in every other course, is with closed hollow conductors. But in practice one uses meshed cages. So, is there a parameter on how big the holes can be? Thank you very much!

Beautiful lectures by sir Lewin ....

You play in another league professor, truly amazing

I am very happy I took 18.02 before this course! Makes it very intuitive.

Is it OK to draw lines with arrows to show potential between the plates?

Ooo god sir u r literally amazing 🔥🔥🔥❤️❤️😘.... your lectures r out of the world ❤️

The coolest professor on earth..

You are the one who made me a physics lover. THANK YOU SO MUCH SIR

Not just educational, but entertaining too.

Sir, what would happen if we place a conductor with a charge of +q inside it's cavity in an external electric field ? Since the electric field inside a conductor is always zero. There will be a charge of -q and +q induced in the inner and outer surface of the conductor because of the charge in cavity. But if the conductor is itself placed in an electric field, wouldn't there be a change in the total number of charges on the outer surface of the conductor? If so, then wouldn't we loose the information about the enclosed charged?

Very conceptual lecture! Makes me love Physics.

Professor Lewin, I have purchased the book you used for 8.02. I have a question on doing problems from the book. What problems does Problem 1.3 Continuous charge distribution. Giancoli 21-49 refer to?

At 42:24 , if we move that inside charge +q at a speed near the speed of light, do you think the charges of the conductor has the time to rearrange themself to keep a uniformly distributite outside charge? I said that because the -q charges at the inner surface will have to rearrange with respect to the position of the inside charge +q, since a charge move there is an electric field. Therefore it will change the configurations of its nearest neighbours charges. Does the outside charges (by a chain reaction) have "the time" to rearange instentenatly also at this speed? Thank you

can someone elaborate the heart problem? I was fine untill then but could not understand the explaination. Also is the heart a 2d i.e surface?

Sir, l am from India.I love all your lectures.They make me love physics.The demonstrations are too tough to do in a 50 minutes class.But you did it very well.Before I heard of your lectire, I wanted to be a scientist.But, now I want to be a teacher like you.Thank you sir, giving these high quality lectures free of cost.

To me its intuitive, as i see the charge arrangement on conducting surface like forces between them, they cannot lie inside, in that case they will feel much greater force than it were outside on the surface, the forces gives intuition. In that heart shaped conductor, there will be more charges on the outer curvature than the curve thats protruding in.

Sir please help me I'm not getting the following... Do the hollow conductors shield the inside cavity from the influence of the outer static electric fields in the case if we have net charges inside the cavity of that conductor i.e. Do those net charges inside the cavity of the conductor remains shielded by the influence of the outer static fields ..?

I’m but confused with the heart conductor example. Is he saying charge it positively and then touch it with your hand, or touch it with something that has a positive to charge to make it positively charged?

Hello Sir what would be the trajectory equation of equipotential surfaces between two identical positive charges separated by an appreciable distance?

Professor Lewin.. 50:00 .What if at the end of the lecture , you put one foot at ground and another in contact with cage..?

Hello Professor. Why were you touching the instrument that you used to transfer the charge to the hollow cylinder from the cat fur rubbed glass slab?

If you hold a metal key while doing electrophorus and use that to ground the metal plate it will be less painful, since the charge travel through all of the skin that is in contact with the key instead of the infinitesimal area on your finger when doing it without a key.

I still can't figure out why you had to touch the electrophorous. Could someone explain please?

Hello Professor good evening , hope you are well and good , I have a question so at NY first attempt to find potential gradient , I first started with vector dot products i.e the work equation ( integral of F.dr , then I got reduced to scalars , now at the end I get E=-dv/dr , now everyone says that -ve indicates the direction of field , and I know in my head that its kinda like slope of tangent which can be -ve or +ve , but all the vector stuff has ended and now this E in the final equation represents magnitude only because directions were gone at the moment I opened my dot product so how this -ve links a magnitude to a vector direction , I am getting crazy at this and trying to solve it for months , but couldn't come up with a satisfying explanation , can you please help me?

reguarding the demonstrationwith the paint can. what happens if you charge the inside surface of the paint can. Will it the charge still only be on the outside of the can?

Sir you are awesome Love from INDIA

I just happened to watch this lecture on Valentine's day!

Hello Dr. Lewin. I am struggling to understand a couple things about electrons. By what process do they (and the charged sub atomic particles) become polarized? #2 From where do free electrons bouncing around in a conductor derive the energy to keep going? Thank you!

Sir what would have happened if you took a charged ping pong ball inside Would you have got the shock by touching the cage from inside

If we have high voltage inside the faraday cage (or maybe we get an electric discharge on the cage's wall) and someone touches the outside of the cage will he get electrocuted by the positive charge on the outside ? If the answer is yes, what if the cage is grounded ?

Sir please explain... We know that if we have a conductor with a cavity and there's no charge inside the cavity then this conductor acts as a protective shield (faraday cage) which shields the inside cavity from the external static electric fields but now if we place a net charge inside the cavity, does this conductor with cavity now acts as as a faraday's cage for the charge that is placed inside the cavity?

You are fabulous. You are amazing teacher.

Hello Sir, i am curious how is that possible you have unenclosed electrostatic shield? I know that the Faraday cage is meshed as long it is smaller than the wavelength of the wave, in this case radio. But in 45:15 , Wouldn't the electrostatic field enter from the unopened part so the net of E inside the shielding is not zero? The hole is quite big though. Also, could we able to calculate the minimum thickness of the shielding (ie. using aluminium foil), i assume that we would use the concept of Gauss ' but i have no further idea about it. Thank you Sir.

Even in a stable electrostatic configuration (no internal E fields in the conductor) the free electrons are still moving randomly about with kinetic energy proportional to the temperature, no? I should probably rewatch all of 8.01 while I'm at it. edit: i suppose it would be the "extra" electrons that are hanging out on the surface while the rest of the free electrons were moving about randomly.

Professor lewin i had a doubt that why did you consider at 6:12, to imagine that we are in space? Why not earth.

can some charges exist inside the conductor but the sum of their values is zero ?

At 24:26 how do you say that if there is a electrostatic field inside a conductor the electric field every where on the surface of the conductor is zero. Can anybody in the comment section answer my question?

should the charge on both sides of a conductor equal each other after induction? and if so why didn't the electroscope at 45:00 move back to the original location

Sir you touched the metal can directly with the electrophorous,which should also make the can positively charged. By induction it would have been negatively charged

Sir, Is there any of yours 8.04 and 8.05 courses to be find? I looked all over the internet and I couldnt find any

Ever wonder how Sir Walter Lewin does the dotted line thingy so fast?

Please let me know if I'm mistaken about the electroforce at 30:00 You first rub the glass plate with the cat fur. It charges the glass positively. By putting the metal plate on top of it, the positive charges distribute themselves on the metal surface as it is a conductor. When you lift it up, it is charged positively. When you touched the metal container, the positively charged metal plate attracts the negative charges. I don't understand why the metal container is negatively charged not why do you have to touch the metal plate (and get a shock) first.

47:54 cannot figure out what he said that was so funny, "A woman who opposes a bombing"? XD Thanks for the lecture, Dr. Lewin! This was one of your best yet!

Interesting and well explained lectures Thanks for giving love for physics❤❤

Does anyone know if the assignments he’s talking about are publically available. The linked problems don’t contain a problem 2.1 that he talks about around 37:00

Professor, will plx to answer to this question is that in x ray why anode focal spot is at angle I need detail answer

Sir, in the case when we put charge inside the cavity...will that charge get stuck on the inner surface if it isn't placed at the centre of the spherical shell..??

I think I know how you charged the paint can when you touch the metal charges flow from glass to metal because of potential difference and when you touch the paint can with the plate the charges go to the paint can. Some charges are lost that go from your body to ground. Please tell if I am correct or not professor lewin. 30:10

Sir , I have a doubt. If u touch that conducting heart, won't the charges flow through your body to earth so that the conducting heart becomes uncharged?

Sir I have a doubt. I made a closed conducting body with a closed steel vessel and attached a wire (having insulation) within it and took it out through the side of its tight cap. Then I attached a knife to this body(just for capturing the spark like lightning arrester) and then passed a small current through it with the help of a gas lighter. I got a slight shock from its outside and what surprised me is I got the same shock from the wire which was taken from inside the conductor. How could this happen? Is it against electrostatic shielding?

Sir, can you please explain why did you jump when exiting the faraday cage? I suppose it has to do something related to potential difference and your safety.

Hello Professor Lewin, if no charge goes in the inside of the Faraday Cage, why is it that the bottom of the inside of the Faraday Cage must be insulated?

Can anyone explain why the radio vol turns off when he goes in??