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!
This really belongs to the positive sides of internet, I can attend classes from MIT from my home in Sweden for free. Let's forget about all the downsides of internet, at least for now.
I start getting why the MIT is so prestigeous. Professors like you are capable of perfectly explaining something to the students without confusing them. I am currently studying electrical engineering at the Technical University in Vienna and here are the students the ones, that have to understand stuff out of books instead of listening of such great lectures. i guess everyone has it a bit different and their way of learning may vary, but the finishing pount is the same for everyone. thank you for sharing your knowledge in your unique way.
Not necessarily the case. First, books will always teach you extra stuff, you can definitely graduate from pretty much any University if you understand the lectures, but those students who read always end up knowing much more, this is a truth that is often omitted, the main factor that determines the level of education is the student. The reason MIT students are so good is because it is damn hard to get in, thats pretty much the only thing prestige does to a university. Keep in mind that prestige comes in great part from research and there are great researchers that are bad teachers, in pretty much every university you will always find some good, some bad, and if you are lucky some excellent teachers. As long as you have capabilities and will to always go beyond what is taught in class, you can always get an MIT level education, its just that if you actually went to MIT the world will recognise you more.
I was practicing for my Sat physics subject test, which in taking because it's an MIT entrance requirement. I didn't know an answer about emf and current, but started thinking and heard Walter's voice talking me through it, and figured it out. These lectures did make me
Sir, at first I was very afraid about #Physics but now a days after watching your lectures , I can feel that physics is a theoretical and interesting subject. Thank you Sir. Love from # India... 🇮🇳🇮🇳🇮🇳
I am a retired EE and CS engineer, having gotten my EE degree in the 60s. That being said, I am learning more from your videos, some of which I forgot, some I wasn't taught and some I never really fully understood. You obviously love your job and do a great job teaching. Right now, I am watching your videos selectively to get a better grasp of EM wave propagation, transmission lines and antennas. Thank you for posting these classes Professor Lewin! By the way, you probably should use less salt on your eggs. I know I have to. :-) I wonder what would happen if you poured some Gatorade into the distilled water. I think the answer is easy. I have to avoid the stuff because of the salt content.
Woooow mind blowing , man I love the way you teach in, where you connect everything together to help us understand feel the physics not memorize it !!! Thank you 👏👏👏
Walter Lewin saves! This is a core course in my bachelor's in electrical engineering degree and I started to hate it for the way it was taught. I started following Walter Lewin's lectures more than the ones in my class and well he achieved the impossible. He made me love the course itself and I've been solving problems and deriving equations with so much and enthusiasm! THANK YOU SO MUCH FOR THIS! THANK YOU SIR WALTER LEWIN! LOTS OF LOVE AND BLESSINGS FROM PAKISTAN!
Remember guys you are not gonna use this physics in your real life, and they are just of no use when you will be 40 or 50 years old, unless you choose to become a professor. I personally used to hate physics but now I love it. Thanks to teachers like walter lewin sir and alakh pandey sir. But when you love somebody too much, it becomes a distraction. Be it a girl, or a subject.
@@godson200 it depends on what you wanna do, if you want to be a doctor than physics is distraction but if you really love something you must persue it, if someone loves physics then they must complete a p.h.d and go to research field, here even at the age of 90 you will be using physics 14 hours/day in your life, from teaching to research and writting papers, a subject is not a distraction, everything is a distraction if you don't want it.
Sir you are the hero of all students who wants to learn and love physics may be in life gives me the fortune to touch your feet (it is the way to show respect to our elders) you are my well wisher and a loving teacher of all time
Dear Dr. Lewin: I have greatly enjoyed your lectures. My father was a professor of electrical engineering at VJTI in Mumbai. I have a Ph.D. in Mechanical engineering. Today at the age of 70, I still enjoy clarifying my fundamentals. Your down to earth and experiment based teaching is lucid and profound. My recent calibrations: Light travels 30 cm or 1 ft in 1 nanosecond. So a 1 GHz computer cannot have its primary memory more than 6 inches away!! 1 coulomb charges 1 m apart have a Force of 1 million metric tons!! That is 100m sided cube of water or 1 cubic football field of water!!
hello sir, I've been attending your classes since last week and I've realized that there was so much in physics which i was not aware of , your classes made me realize that physics is very interesting and thought provoking.
Wow......you sir are AMAZING. I mostly had no idea what you were talking and writing about but as a 17 yr employed automotive technician that uses an oscilloscope literally daily, u were directly describing ptc and ntc style thermistors. A negative temperature coefficient thermistor or variable resistor is the most commonly style engine temperature sensor used in automobiles. Temperature up, resistance down. The lab scope is also used for issues pertaining to variable relector, hall effect...whatever sensors controlled by a transistor driver ( fuel injectors, ignitions coils, output solenoids, etc.....) Again....have no clue what you are saying but it is addicting and badass. I could listen to you all day.
Hello sir! At 6:17 you derived the speed of electrons in that conductor. Then how come current flows within just the flick of a switch? Thank you, sir.
Thank you so much for your lectures dear Prof. Walter Lewin. They are very inspiring. Please let me show some doubts about the last demonstration of the lecture: 1) The charge detected by the electroscope is due to a bulk conduction process through the soles or a surface conduction process over the soles, the shoes,... (or both processes)? 2) The charge detected by the electroscope could be an induced one by the charge created is the low surface of the soles during the scuffing? I know that a opposite charge is generated in the pad by the scuffing, but perhaps it can easily go to earth and have a net charge in the soles that could cause an induced charge in your body. 3) The electroscope null response when you stop the scuffing is due to the fact that the charge goes to earth through your body with the shoes. Do you think that can play a role the charge recombination in the interface between the sole and the pad? 4) It could be interesting and informative to repeat the experiment but placing bellow the pad a thick plate of teflon or fused silica to prevent the charge leakage to earth. Perhaps the electroscope do not discharge even when you stop the scuffing. Or may be the electroscope is discharged due to the recombination mentioned in 3) even when the leakage to earth is not posible. Thank you so much.
3:52 Drift velocity v{d} MUST be like ∆v, as ∆v=a∆t, and ∆t here means practically the same thing as τ. One caveat: the m in F=ma here is called EFFECTIVE MASS (according to wiki). _Drift velocity_ v{d} _is the average velocity attained by charged particles in a material due to an electric field._ _The drift velocity of an electron for a unit electric field is called the mobility of the electron._ _It is the average velocity acquired by a charged particle (like an electron or proton) in the body due to an electric field._ _Relaxation time_ (𝛕) _is the time gap between two successive electron collisions in a conductor._ ∆v _is the velocity attained by an object due to an acceleration._ ∆t _is the time gap between two successive changes in velocity._ _I think it's logical to assume these are kinda the same, but if you've got other ideas, plz share._
@19:40 As temperature increases... the chaotic random velocity of all the electrons in the system increases NOT the drift velocity !! I got so confused ... misunderstanding the word "velocity" for drift velocity
showing the (non-linear) time regime impuls of resistivity is one thing, stating "not to trust Ohm's law" is a bit reductive (not to say blunt) ... we realized quite a lot trusting it (as we did with Newton's imperfect laws for instance) ... oh and though it's fortunate that resistance goes up with temperature, it's also necessary from a physical standpoint no?
Q - 1:00 as the electric field moves the electron from lower potential to higher potential it's should have a huge significant value of electric field ?
@@lecturesbywalterlewin.they9259 As we have applied potential approach the conductor the electron flows and tries to cancel out the effect of the electric field within the conductor and it always flow because we maintain the potential difference across the conductor that's why the motion of electron takes place and if we see the second scenario in which there is no potential across the conductor then also the electrons move in randomly why? Should be some factor which is responsible for their randomness GG
45:53 isn’t it that even without the shoes, when one touches the Van de Graaf, he won’t be electrocuted, because even though it has high voltage, it still has a litttle net charge on the surface, so the high current (lets say 300000V/1000 ohms = 300 Amps) will take only a split of second and shoud be harmles? And after the charge is drained out, if you keep touching the VDG, it won’t have the 300000 voltage anymore since the VDG is actually a constant current source generator, and the amount of charge delivered is so small that is, again, harmless, am I right or not?
And hypothetically, when the resistivity of the shoes is about 2M ohms as mentioned in the video, it “should” be harmless to touch the outlet with 230V, right? (110 V in US)
3:25 professor I have a question that why you have used F=ma for acceleration of e As I heard that mass of e changes by the relation m=m°/√1-(v²/c²) I have this confusion about it. Please elaborate 🙏
Professor, I am having a tough time visualising the E field inside the wire. At 4:49 Why is the E field inside the whole conductor a constant (1 V/m). Since the length of the conductor is 10m, shouldn't the magnitude of the E field inside the wire at different distances (from the origin of the E field) be different? Where is the origin of this E field actually?
Lectures by Walter Lewin. They will make you ♥ Physics. So the battery is producing an E field that that is constant everywhere in space, and no matter how far away I extend the wire from the battery the E field in it will always remain same?
Lectures by Walter Lewin. They will make you ♥ Physics. I've googled about it, but no success. Does this field have any special name or something? Can I find a field line sketch for it or somewhere? Could you give me a hint sir, about the nature of this E field. This thing is bothering me a lot.
Hello, thanks for the free lecture! I have a question: Why doesen't 𝛕 (tau the time between collisions) go down when the Electric field goes up? Why does it stay constant even though we applied a force to the electrons and they now are accelerated? Thanks!
I suppose it's because as was shown previously the effect of the electric field is almost entirely negligible when compared to that of temperature, and either way the time an electron takes to crash against a particle is probably the average one, so if the electric field pushes some of the electrons towards crashing maybe (and I mean maybe) it will then prevent other electrons from crashing somewhere else, meaning that its effect on 𝛕 ends up being cancelled
Professor I have one question. Superconductors have zero resistance. Applying ohms law we get I=V/R Since R is zero Current should be infinite But what is infinite current? And if there is no infinite current how much current flows in a superconductor?
Sir at 3:33 you gave the formula for drift velocity as acceleration*tau but by definition drift velocity is the average velocity between two successive collisions whereas acceleration*tau is instantaneous velocity just before the collision. Average velocity would be half of instantaneous.
So professor, at 4:45 we are assuming that the E field is same throughout the copper wire, right? Otherwise we couldn't be saying V=EL (where L is the total length of the wire and V is the voltage difference between the two ends of the wire), right?
at around 7:28, why does the E-field in side the conducting wire equal the potential difference across that wire divided by the length of wire? does it come from E=- del V (in cylindrical coordinate)?
The integral of E dot dl from one end of the wire to the other is + or -- the potential difference between the ends depending on whether E and dl are in the same or in the opposite direction. You can always choose which direction to go.
E=j/sigma if the resistor is uniform, thus sigma's everywhere the same and diameter is everywhere the same, than E is the same throughout the entire resistor.
Now where can anyone find a physics professor who measures the resistance of his footwear and scuff and produce charge! It just made my day. One Question:- Can we say that if the V-I graph does not pass through the origin then is a non-ohmic resistor?
+Anery Patel ha ha ha, I am sure I am not the only physics Professor who is a bit crazy. If the V-I curve is NOT a straight line then Ohm's law is not very useful. If it is a straight line but if it does not go through zero, Ohm's law is also useless. Ohm's law requires that when V = 0, I is also 0. However, a very interesting case is a resistor made of superconductive material. Thus R=0. There cannot be any potential difference over the R as that would give an infinite current. Thus 0=I*0 (consistent with Ohm's law). I can be anything. I can be hundreds of A. That's being used with maglev trains.
+Lectures by Walter Lewin. They will make you ♥ Physics. Okay. I see it now. Basically, for Ohm's law to hold good, V=IR must hold true, whatever the case may be(temperature being constant). And talking about the superconductors, how much ever voltage we apply the potential drop will be zero because resistance of the material is zero and so current will be infinite. I hope I understood it correctly. And I am sure that you are the craziest of all, boring into our skulls and fitting in the subject tight.
+Anery Patel We cannot apply to a superconductor any voltage as it would lead to an infinite current. In any circuit that we build to "push" current through a superconductor, the potential difference over it will be zero. >>>And I am sure that you are the craziest of all, boring into our skulls and fitting in the subject tight. It's my way of making all of you ♥ Physics. ! ! !
Lectures by Walter Lewin. They will make you ♥ Physics. But as far as I know to "push" the current through any material, a voltage or potential difference across its end is required. And if I don't apply any voltage or potential difference across the ends of superconductor, how can the current pass through it? And it is only then when we realize that the potential drop across the material is zero and we come to a conclusion that it has infinite conductance.
+Anery Patel There are various ways to create a current in a superconductor. One way that comes immediately to mind is that you start the current when the material is not yet a superconductor. The temperate is too high. You apply a voltage and you create a current. Now you lower the temperature and the material becomes superconductive. The current will keep going and the voltage over the super conductor becomes zero. I suggest you search the web to find other ways how very high currents can be created through superconductors in the LHC (at Cern) and in maglev trains.
Why is the drift velocity = a * Tau ( at 3:37) ? If it is the average speed of the electron, shouldn't it be 1/2 * a * Tau ? I mean, a * Tau should be the maximal velocity, just befor another colision, not the average... Where is the problem in my reasoning?
I don't think you explained it during the lecture, but here I found an explenation that satisfies me: www.quora.com/What-is-the-relation-between-drift-velocity-and-relaxation-time-of-charge-carriers-in-a-conductor
𝑚𝑎=𝑞𝐸 After a time 𝑡 with no collisions, a charge carrier has the velocity 𝑣=𝑞𝐸𝑡/𝑚. If 𝜏 is the average time between collisions, then 1/𝜏 is the average rate of collisions (for one charge carrier). *they agree with my lecture*
Sir , why is Drift velocity = EeT/m not EeT/2m since it is the average velocity of electrons? Another question: Why T(tao) remains same when temperature and material remains same? The T is the average time of Collison of electrons and since the drift velocity is higher in higher E(electric field) the collison time would get shorter. But this is not shown in you derivation at starting calculating the speed of electrons and I Current
Hello Sir , At 1:12 you say that the derivation of Ohm's law you show in the lecture is a very 'crude derivation' & we need to go into quantum mechanics for a more accurate one ? But why is the derivation not perfect as I think the math is completely fine . Is it because we don't assume the temperature dependence of Resistivity while deriving Ohm's Law by this method ?
@@lecturesbywalterlewin.they9259 May you suggest the good lectures on quantum mechanics , nuclear and particle physics ; and condensed matter physics on youtube or any other way . Please . Any of them you know .
Dear Professor Walter Lewin. At 01:44 you said that the free electrons in copper have a speed, an average speed of about a million meters per second. Can you please share with me how do we know that? In addition, not only me but also my family thank you very much for your amazing lectures.
as we increase the potential difference the electric field increases , as the electric field increases the drift velocity increases ,now this means that the current should reduce as the electrons always oppose the current . but this not so because on increasing the potential difference the current increases .
>>>>at about 0.07 u said that when positive charges go in some direction then the current is in that direction>>>> I NEVER SAID THAT. watch again. EVERYTHING I said is correct.
confused !! not able to judge any direction of current , u said that by convention we take the direction of current same as the direction of positive charges but positive charges cannot move through solid hence the electrons flow from lower potential towards the higher potential and hence constituting the current which is in the direction opposite to that of the convention . and now through battery in which there is liquid the positive charges should move and hence the current is in the opposite direction to that of the the current in the circuit
Professor I have a question...what would those SPECIAL shoes( worn by thief in the end) look like in order to make a working static gun from his finger?
@@lecturesbywalterlewin.they9259 you said the thief would have to wear a special kind of boots to be able to make a static gun by just rubbing his feet on the ground... because normal boots will lose charge as soon as you stop rubbing your feet on the ground...i was asking what would the special boots(the ones that won't lose charge) look like? Would they just be thicker souled than usual or something else would be changed too in them?
@UCiEHVhv0SBMpP75JbzJShqw Thank you teacher but why is there a little increase of current and after that, in the next term, R remains constant and I is not changing? Can you tell me what happen with the resistor?
Hello Pr Lewin. First I would like to thank you for the lectures, they are all amazing and I honestly think they are a gift to the world. I was recently graduated as an engineer and I have already learned some of the subject you discussed, but you treat them way better than anyone :) Here is me question : at about 36:00 you say that the electrons can go to the earth because the air next to the electroscope is hotter because of the candle. How can the electrons go to the earth if you just heat up the air around the electroscope and not everywhere between the electroscope and the earth? You clearly increase the air conductivity next to the electroscope but you don't change this conductivity for the rest of the circuit (between the hot air zone and the ground) so how can the charges flow to the earth? I hope that my message is clear, I'm from Belgium so as you know, english is not my mother tongue ;)
You maybe didn't see it because my first message is too long. You have to click on something like "Read more" at the end of my first message to see the hidden part of the text (my youtube page is in french so I'm not sure that it's "read more" but it should sound like this).
When scuffing the shoes: Does the charge on the sole INDUCE a charge separation in the body, which is then registered by the electroscope? Or is charge actually flowing THROUGH the sole up into the body and the electroscope? Thank you so much for these lectures!
Why is it that the resistance of each of the two shoes is 2 ohms instead of 4 ohms? I thought that if one shoe had a resistance of 4 ohms, if you bring another shoe in you get another 4 ohms. @44:58
Sir, this is Aryan. A huge fan of yours. If I would live near you, I would enjoy the whole day with you sir. Don't feel lone. I am with you sir, you will always be there in my heart. By the way sir, I am a high school student. I do love watching your lectures. I have just started with 8.02 and finding it very interesting but I have to skip some of them sir as I have not been a past learner of Calculus. So, according to you, what else could I do to rock my school with my physics knowledge ?
Dude just wait till class 11. In first month, you will learn all the calculus you need. Otherwise you can obviously go on watching these just for the love of it
Hello, I cannot understand why I would be constant throughout the circuit when considering the series of resistors. I mean, after going through the resistor, the current should increase or decrease, not stay the same. Right ? Is it because of charge density of the material, and that therefore if localy a change in speed takes place, it will spread to junctions ?
For the light to lit on, you have to have free charges to move from one plate to another. There are ridiculously few of them in the clean water, so the light don't lit. When we add salt, it creates Cl-ions which can transfer the charge, so, the light lit up. If you heat up the water, say, with electricity, it may produce some ions. But it will produce ions which are lighter then the water molecule, so they will fly up in the air, and there will not be any charge transfer. But if the potential difference between the plates is so enormously large, then maybe you could see some light. Correct me, if I'm wrong. I'm not a physicist.
Hi Professor Walter, thanks for the fantastic lectures. I have a question: What would happen if you would touch de van de graaf while wearing your shoes? The charge of the VDG (the one you commonly used during your lectures) is only 10uC, so I can't figure out what would happen. I could come up with the follow (probably wrong) hypothesis: I assumed that you would experience a current from the VDG to ground, which would be limited by the VDG charge. The "tricky" part is that since the current flow is I = dQ/dt and Ohm's law gives us 100mA, it means the VDG can't provide enough current to keep that up unless it would get charged almost instantly. My (probably incorrect) conclusion is that therefore you would just end up feeling a initial shock, and if you could hold on the sphere after the shock, you would end up acting as a path for the current between the VDG and ground. Since the moving belt inside the VDG would start inducing more charge on the sphere, you would experience a (very low) current flow by the induced charge. All of this is assuming that the initial current could cause some muscle contraction but wouldn't harm you any further. Where did I go wrong? Once again thanks for the fantastic lectures, they are really unmatched. They make me love physics again and have been an excellent entertainment source every single day since I found out about them. Thank you!
What does scuffing your feet do exactly? Does it increase the resistivity of your feet, slowing the current, and allowing the charge to stay on the thing? If so, why does the resistivity increase? Is the temperature of your soles changing?
Sir, I have two questions, First, you said that resistivity increases with increase in temperature but when you heated up the air the electroscope showed more deviation which means resistivity of air decreased and both these statements contradict. Second, can you please explain the part that why the 2 billion resistance of the shoe is very low for the experiments you carry.
Question about problem 3.1(b) - you say that since charge cannot flow into or out of the circled part of the circuit, that the charge must be symmetrically distributed. But doesn't that also mean that there is no current through that part of the circuit? If so, how does the current "resume" to the right of this portion of the circuit? There must be sparks going between the plates as they reach saturation, at which point there _is_ a current going through the circled portion... No?
hi,thanks your lesson.May I ask that the voltage of the resistor is larger than that of the wire macroscopically, because the number of electrons remains unchanged, and the electrons need to pass through the thin tube at a faster speed to make the current equal, so a larger electric field is needed?
@@lecturesbywalterlewin.they9259 okay but I am confused about how this is universally true without knowing the sign of the value. I=dq/dt so if you point your arrow to the right and electrons move right then the rate of change of charge to the right would be a negative value, which is equivalent to a positive value of the same magnitude going to the left. So isn't it specifically currents that are positve that are opposite electrons
Hi Dr. Lewin. In terms of the drift velocity is it understood that this is concurrent with the electron motion ? They are still bouncing all over and hitting atoms while also meandering in the direction the voltage field is pushing them?
Hello sir... Sir here you have discussed about two velocities of electron (1) velocity due to tenperature ≈ 10^6 m/s and (2) drift speed ≈ 0.5 mm/s... Now my question is when we provide a potential difference then electrons moves in fixed direction with which velocity (1) or (2) and if they move with drift speed then what happens to their veocilty due to temperature? Also how can this drift velocity being so small have such significant effect that we see current flowing? Also I came across some assumptions (velocity becomes zero after every collision between electrons and then they regain the velocity) about drift speed, so are they true or we just accept them as assumptions?
I can perhaps provide my understanding of the lecture. The electrons do move with the drift speed in response to a potential difference, and that does not affect the speed due to temperature (so that value of tau is purely temperature dependent). This small speed does indeed provide the significant effect of what we see as current just because there are so many charges moving and summed up they indeed have a lot of energy, even at such small speed. With that said, he does mention that a more accurate derivation of Ohm's law would require quantum mechanics, and indeed when it comes to questions of electrons and matter at the atomic scale in terms of how they behave, this classical view that is assumed in all these lectures will not work anymore, and we must go to quantum mechanics to give us the more accurate insight. I came across very similar questions in my head when I studied electronics and we talked about semiconductors, and again there the answer lies in quantum mechanics, especially in subjects such as solid state physics.
Can't Ohm's Law be effectively saved if we were able to define temperature for a substance as a function of current? Would that relation be reducible to some constant coefficient multiplied by a function? What would a temperature function of the current look like? What else would it be a function of?
yes that can be done. Every resistor that you then buy will have to come with a written equation which is a function of I. Two resistors both 100 Ohm would have a different equation if they are not made of the same material or if their dimensions differ.
Thanks for sharing your lessons on TH-cam. After studying electricity for so many years, I have learned all kinds of calculations, but I don't even know the principle of electricity. Thank you for making me understand. I have a question, according to U=El, the voltage across the resistor is greater than that of the wire, microscopically because the electrons need to go faster, which means more force through the resistor, yes? According to i=QnAv, previously I thought E would cause more electrons to drift, so the voltage microscopic effect of the resistor is more electrons moving faster, meaning larger n with larger v. Later, it was found that the electrons moved faster, and the number of electrons participating in the movement did not change, that is, n did not change, only v changed. is that so?
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!
Lectures by Walter Lewin. They will make you ♥ Physics. what about the slightly higher version of these courses ,sir ?
Thanks sir
ma man
Lectures by Walter Lewin. They will make you ♥ Physics. thanks sir
Thank u sir
I wait eagerly for him to say ,'So first I want to demonstrate to you....'
"So first I want to demonstrate to you.."
@@lecturesbywalterlewin.they9259 😂😂😂😂😂😂😂😂😂
I now egarly wait for you to start the demonstration 😂😂😜😜😜
@@lecturesbywalterlewin.they9259I trust physics Haha thank you from east Africa Somalia
This really belongs to the positive sides of internet, I can attend classes from MIT from my home in Sweden for free.
Let's forget about all the downsides of internet, at least for now.
Of course sometimes it becomes a blessing too 👍
I start getting why the MIT is so prestigeous. Professors like you are capable of perfectly explaining something to the students without confusing them. I am currently studying electrical engineering at the Technical University in Vienna and here are the students the ones, that have to understand stuff out of books instead of listening of such great lectures. i guess everyone has it a bit different and their way of learning may vary, but the finishing pount is the same for everyone. thank you for sharing your knowledge in your unique way.
That's great
Are u a professor now?
Not necessarily the case.
First, books will always teach you extra stuff, you can definitely graduate from pretty much any University if you understand the lectures, but those students who read always end up knowing much more, this is a truth that is often omitted, the main factor that determines the level of education is the student. The reason MIT students are so good is because it is damn hard to get in, thats pretty much the only thing prestige does to a university.
Keep in mind that prestige comes in great part from research and there are great researchers that are bad teachers, in pretty much every university you will always find some good, some bad, and if you are lucky some excellent teachers.
As long as you have capabilities and will to always go beyond what is taught in class, you can always get an MIT level education, its just that if you actually went to MIT the world will recognise you more.
I was practicing for my Sat physics subject test, which in taking because it's an MIT entrance requirement. I didn't know an answer about emf and current, but started thinking and heard Walter's voice talking me through it, and figured it out. These lectures did make me
You are very welcome!
Sir, at first I was very afraid about #Physics but now a days after watching your lectures , I can feel that physics is a theoretical and interesting subject. Thank you Sir. Love from # India... 🇮🇳🇮🇳🇮🇳
All the best
I am a retired EE and CS engineer, having gotten my EE degree in the 60s. That being said, I am learning more from your videos, some of which I forgot, some I wasn't taught and some I never really fully understood. You obviously love your job and do a great job teaching. Right now, I am watching your videos selectively to get a better grasp of EM wave propagation, transmission lines and antennas. Thank you for posting these classes Professor Lewin! By the way, you probably should use less salt on your eggs. I know I have to. :-)
I wonder what would happen if you poured some Gatorade into the distilled water. I think the answer is easy. I have to avoid the stuff because of the salt content.
thanks for your kind words
I LOVE HIS TEACHING
:)
😍
Woooow mind blowing , man I love the way you teach in, where you connect everything together to help us understand feel the physics not memorize it !!! Thank you 👏👏👏
i discovered his videos now and they are helping me a lot in preparing for my medical entrance exam called neet in India. Thanks a lot lewin sir.
are you in college brother ?
Walter Lewin saves! This is a core course in my bachelor's in electrical engineering degree and I started to hate it for the way it was taught. I started following Walter Lewin's lectures more than the ones in my class and well he achieved the impossible. He made me love the course itself and I've been solving problems and deriving equations with so much and enthusiasm! THANK YOU SO MUCH FOR THIS! THANK YOU SIR WALTER LEWIN! LOTS OF LOVE AND BLESSINGS FROM PAKISTAN!
Sir, your lectures are a gift to humanity.
:)
Thank you Professor Lewin for all the great lectures and resources you provide! You are a true inspiration!!
:)
I started studying physics to clear IITJEE but ended up falling in love with it
I started studying physics for my love for it but ended up in the IITJEE rat race...
Remember guys you are not gonna use this physics in your real life, and they are just of no use when you will be 40 or 50 years old, unless you choose to become a professor. I personally used to hate physics but now I love it. Thanks to teachers like walter lewin sir and alakh pandey sir. But when you love somebody too much, it becomes a distraction. Be it a girl, or a subject.
@@godson200 it depends on what you wanna do, if you want to be a doctor than physics is distraction but if you really love something you must persue it, if someone loves physics then they must complete a p.h.d and go to research field, here even at the age of 90 you will be using physics 14 hours/day in your life, from teaching to research and writting papers, a subject is not a distraction, everything is a distraction if you don't want it.
Sir you are the hero of all students who wants to learn and love physics may be in life gives me the fortune to touch your feet (it is the way to show respect to our elders) you are my well wisher and a loving teacher of all time
I'm legit crying at how much better this is than my *current* professor. lol
Okay I see what you did there
I guess your professor has a low potential then
@@andredelacerdasantos4439low potential difference
The potential difference between your professor and Walter Lewin is so enormous you have no choice but to surge here
Enjoying lockdown by your mind blowing electricity lectures💓
Dear Dr. Lewin: I have greatly enjoyed your lectures. My father was a professor of electrical engineering at VJTI in Mumbai. I have a Ph.D. in Mechanical engineering. Today at the age of 70, I still enjoy clarifying my fundamentals. Your down to earth and experiment based teaching is lucid and profound.
My recent calibrations: Light travels 30 cm or 1 ft in 1 nanosecond. So a 1 GHz computer cannot have its primary memory more than 6 inches away!!
1 coulomb charges 1 m apart have a Force of 1 million metric tons!! That is 100m sided cube of water or 1 cubic football field of water!!
:)
This is the answer to our lack of education problems. Get our most brilliant minds and make their knowledge free and universal.
Sir really your teaching skills are amazing , love from Bangladesh :)
This man is a legend. Please keep teaching. Thank you Sir!
You are very welcome
hello sir,
I've been attending your classes since last week and I've realized that there was so much in physics which i was not aware of , your classes made me realize that physics is very interesting and thought provoking.
Wow......you sir are AMAZING. I mostly had no idea what you were talking and writing about but as a 17 yr employed automotive technician that uses an oscilloscope literally daily, u were directly describing ptc and ntc style thermistors. A negative temperature coefficient thermistor or variable resistor is the most commonly style engine temperature sensor used in automobiles. Temperature up, resistance down. The lab scope is also used for issues pertaining to variable relector, hall effect...whatever sensors controlled by a transistor driver ( fuel injectors, ignitions coils, output solenoids, etc.....) Again....have no clue what you are saying but it is addicting and badass. I could listen to you all day.
:)
Congratulations sir ,for your biggest achievement. That you know.
The beauty of ur teaching is how easily u convert complex things easy
Thank you sir to help poor Indian students in this situation by giving your lecturers on TH-cam.
Thank you professor lewin for all your efforts!
Huge respect, sir i am really in love with physics because of you. Thanku so much sir ❤ love from india 🇮🇳
Hello sir!
At 6:17 you derived the speed of electrons in that conductor. Then how come current flows within just the flick of a switch?
Thank you, sir.
because the E-field propagates into the conductor with the speed of light of the conductor.
@@lecturesbywalterlewin.they9259 thank you sir!
Sir you are the legend of physics. These lectures are useful. I watched every lecture.
I find your lectures useful and inspiring. Thank you.
6:34 turtle and rabbit story still valid here wow
Thank you so much for your lectures dear Prof. Walter Lewin. They are very inspiring. Please let me show some doubts about the last demonstration of the lecture:
1) The charge detected by the electroscope is due to a bulk conduction process through the soles or a surface conduction process over the soles, the shoes,... (or both processes)?
2) The charge detected by the electroscope could be an induced one by the charge created is the low surface of the soles during the scuffing? I know that a opposite charge is generated in the pad by the scuffing, but perhaps it can easily go to earth and have a net charge in the soles that could cause an induced charge in your body.
3) The electroscope null response when you stop the scuffing is due to the fact that the charge goes to earth through your body with the shoes. Do you think that can play a role the charge recombination in the interface between the sole and the pad?
4) It could be interesting and informative to repeat the experiment but placing bellow the pad a thick plate of teflon or fused silica to prevent the charge leakage to earth. Perhaps the electroscope do not discharge even when you stop the scuffing. Or may be the electroscope is discharged due to the recombination mentioned in 3) even when the leakage to earth is not posible.
Thank you so much.
3:52 Drift velocity v{d} MUST be like ∆v,
as ∆v=a∆t, and ∆t here means practically
the same thing as τ.
One caveat: the m in F=ma here is called EFFECTIVE MASS (according to wiki).
_Drift velocity_ v{d} _is the average velocity attained by charged particles in a material due to an electric field._
_The drift velocity of an electron for a unit electric field is called the mobility of the electron._
_It is the average velocity acquired by a charged particle (like an electron or proton) in the body due to an electric field._
_Relaxation time_ (𝛕) _is the time gap between two successive electron collisions in a conductor._
∆v _is the velocity attained by an object due to an acceleration._
∆t _is the time gap between two successive changes in velocity._
_I think it's logical to assume these are kinda the same, but if you've got other ideas, plz share._
@19:40 As temperature increases... the chaotic random velocity of all the electrons in the system increases NOT the drift velocity !! I got so confused ... misunderstanding the word "velocity" for drift velocity
excellent explanation of Ohm's Law extended to encompass the world around us.
Thank you Prof. Lewin
Love from INDIA 🇮🇳🥰🤗
Thanks for the professor Walter Lewin🙏for making us to love physics ❤
showing the (non-linear) time regime impuls of resistivity is one thing, stating "not to trust Ohm's law" is a bit reductive (not to say blunt) ... we realized quite a lot trusting it (as we did with Newton's imperfect laws for instance) ... oh and though it's fortunate that resistance goes up with temperature, it's also necessary from a physical standpoint no?
Hey almamater u r just phenomenal Loved ur experiments evry time u watch u learn even more with the dimension of thinkng being enhanced
Love you and your lectures a lot sir they are making me love physics 💓 💓 💓💓 💓
This was really helpful for preparing for my test. Thanks!!!
I'm so glad that my teacher suggested me about ur lectures on youtube sir😊😊😊😊😊😊😊😊
Would it be possible theoretically to give an electric shock to a person on the other side of the earth, if we had high enough voltage?
THANK YOU SO MUCH SIR PROVIDING VIDEO, ITS REALLY VERY HELPFUL FOR US. I LOVE THE WAY YOU TEACH. LOVE YOU SIR W.L
You are most welcome
Q - 1:00 as the electric field moves the electron from lower potential to higher potential it's should have a huge significant value of electric field ?
If the EMF of the battery does not change then the E-field in the conductor does not change
@@lecturesbywalterlewin.they9259 As we have applied potential approach the conductor the electron flows and tries to cancel out the effect of the electric field within the conductor and it always flow because we maintain the potential difference across the conductor that's why the motion of electron takes place and if we see the second scenario in which there is no potential across the conductor then also the electrons move in randomly why? Should be some factor which is responsible for their randomness
GG
i cant express enough how much i loved this
45:53 isn’t it that even without the shoes, when one touches the Van de Graaf, he won’t be electrocuted, because even though it has high voltage, it still has a litttle net charge on the surface, so the high current (lets say 300000V/1000 ohms = 300 Amps) will take only a split of second and shoud be harmles? And after the charge is drained out, if you keep touching the VDG, it won’t have the 300000 voltage anymore since the VDG is actually a constant current source generator, and the amount of charge delivered is so small that is, again, harmless, am I right or not?
And hypothetically, when the resistivity of the shoes is about 2M ohms as mentioned in the video, it “should” be harmless to touch the outlet with 230V, right? (110 V in US)
An excellent lecture. Thanks for uploading it.
as a 9th grader i understood everything. u make learning so easy. thank u so much sir
Glad to hear that
3:40 What the drift velocity somehow follows from the acceleration from the electric field?? I dont follow
watch this lecture - I derive it
3:25 professor I have a question that why you have used F=ma for acceleration of e As I heard that mass of e changes by the relation m=m°/√1-(v²/c²) I have this confusion about it. Please elaborate 🙏
rest mass, m, never changes. E= gamma*m^2. gamma changes not m. Some books call m*gamma the relativistic mass. That's confusing.
Thank you very much professor 😊❤️🙏🙏
Professor, I am having a tough time visualising the E field inside the wire. At 4:49 Why is the E field inside the whole conductor a constant (1 V/m). Since the length of the conductor is 10m, shouldn't the magnitude of the E field inside the wire at different distances (from the origin of the E field) be different? Where is the origin of this E field actually?
>>>Where is the origin of this E field actually?>>>
battery
Lectures by Walter Lewin. They will make you ♥ Physics. So the battery is producing an E field that that is constant everywhere in space, and no matter how far away I extend the wire from the battery the E field in it will always remain same?
>>>So the battery is producing an E field that that is constant everywhere in space, >>>
NO that is incorrect - Watch my lectures and use google.
Lectures by Walter Lewin. They will make you ♥ Physics. I've googled about it, but no success. Does this field have any special name or something? Can I find a field line sketch for it or somewhere? Could you give me a hint sir, about the nature of this E field. This thing is bothering me a lot.
Is it really a uniform electric field? So does that mean the electrones are accelerating under a uniform electric field?
Hello, thanks for the free lecture! I have a question: Why doesen't 𝛕 (tau the time between collisions) go down when the Electric field goes up? Why does it stay constant even though we applied a force to the electrons and they now are accelerated? Thanks!
I suppose it's because as was shown previously the effect of the electric field is almost entirely negligible when compared to that of temperature, and either way the time an electron takes to crash against a particle is probably the average one, so if the electric field pushes some of the electrons towards crashing maybe (and I mean maybe) it will then prevent other electrons from crashing somewhere else, meaning that its effect on 𝛕 ends up being cancelled
Professor I have one question.
Superconductors have zero resistance.
Applying ohms law we get
I=V/R
Since R is zero
Current should be infinite
But what is infinite current?
And if there is no infinite current how much current flows in a superconductor?
V=IR L is length of the Superconductor, E is the field in the superconductor E*L=V E=0 thus V=0. 0=I*0 Thus I can have any value.
@@lecturesbywalterlewin.they9259 Thank you very much for this kind favour.
The graph 📈 is so good
he made me fall in love with physics once again.....
So why did you break up?
Finally you changed your channel's profile picture!!!!
GLAD TO SEE!!!!☆★☆★☆★☆★
Sir at 3:33 you gave the formula for drift velocity as acceleration*tau but by definition drift velocity is the average velocity between two successive collisions whereas acceleration*tau is instantaneous velocity just before the collision. Average velocity would be half of instantaneous.
I watched 3:00 - 4:00 everything I said and wrote is correct
"Teachers who make physics meaningless are criminals"
I can't remember it exactly ,
my level in physics is improving rapidly thanks alot Dr/walter
The comic got me, what a good way to verify our understanding in a delightful fashion. Thank you Prof Walter Lewinn for these great series on physics.
You're very welcome
So professor, at 4:45 we are assuming that the E field is same throughout the copper wire, right? Otherwise we couldn't be saying V=EL (where L is the total length of the wire and V is the voltage difference between the two ends of the wire), right?
yes, since the copper wires we use are uniform and have the same diameter E is constant in the wire.
at around 7:28, why does the E-field in side the conducting wire equal the potential difference across that wire divided by the length of wire? does it come from E=- del V (in cylindrical coordinate)?
The integral of E dot dl from one end of the wire to the other is + or -- the potential difference between the ends depending on whether E and dl are in the same or in the opposite direction. You can always choose which direction to go.
oh, i got it.
is there any reason for assuming E is constant in a conductor in this electrodynamics case?
E=j/sigma if the resistor is uniform, thus sigma's everywhere the same and diameter is everywhere the same, than E is the same throughout the entire resistor.
+Lectures by Walter Lewin. They will make you ♥ Physics.
I got it. Thank you 🙏🏻
I am Mexican and thank you for this. It is great
:)
Sir, ur way of teaching is fabulous 🤓
Indian students are spending tons of money in coaching centers without being aware of this masterpiece.
I watch this and i am 10 th grade and i love all the lectures i've watched from you!
Now where can anyone find a physics professor who measures the resistance of his footwear and scuff and produce charge! It just made my day.
One Question:- Can we say that if the V-I graph does not pass through the origin then is a non-ohmic resistor?
+Anery Patel ha ha ha, I am sure I am not the only physics Professor who is a bit crazy. If the V-I curve is NOT a straight line then Ohm's law is not very useful. If it is a straight line but if it does not go through zero, Ohm's law is also useless. Ohm's law requires that when V = 0, I is also 0. However, a very interesting case is a resistor made of superconductive material. Thus R=0. There cannot be any potential difference over the R as that would give an infinite current. Thus 0=I*0 (consistent with Ohm's law). I can be anything. I can be hundreds of A. That's being used with maglev trains.
+Lectures by Walter Lewin. They will make you ♥ Physics.
Okay. I see it now. Basically, for Ohm's law to hold good, V=IR must hold true, whatever the case may be(temperature being constant). And talking about the superconductors, how much ever voltage we apply the potential drop will be zero because resistance of the material is zero and so current will be infinite. I hope I understood it correctly.
And I am sure that you are the craziest of all, boring into our skulls and fitting in the subject tight.
+Anery Patel We cannot apply to a superconductor any voltage as it would lead to an infinite current. In any circuit that we build to "push" current through a superconductor, the potential difference over it will be zero.
>>>And I am sure that you are the craziest of all, boring into our skulls and fitting in the subject tight.
It's my way of making all of you ♥ Physics. ! ! !
Lectures by Walter Lewin. They will make you ♥ Physics.
But as far as I know to "push" the current through any material, a voltage or potential difference across its end is required. And if I don't apply any voltage or potential difference across the ends of superconductor, how can the current pass through it? And it is only then when we realize that the potential drop across the material is zero and we come to a conclusion that it has infinite conductance.
+Anery Patel There are various ways to create a current in a superconductor. One way that comes immediately to mind is that you start the current when the material is not yet a superconductor. The temperate is too high. You apply a voltage and you create a current. Now you lower the temperature and the material becomes superconductive. The current will keep going and the voltage over the super conductor becomes zero. I suggest you search the web to find other ways how very high currents can be created through superconductors in the LHC (at Cern) and in maglev trains.
Why is the drift velocity = a * Tau ( at 3:37) ?
If it is the average speed of the electron, shouldn't it be 1/2 * a * Tau ?
I mean, a * Tau should be the maximal velocity, just befor another colision, not the average...
Where is the problem in my reasoning?
what I have is correct; watch it again.
I don't think you explained it during the lecture, but here I found an explenation that satisfies me: www.quora.com/What-is-the-relation-between-drift-velocity-and-relaxation-time-of-charge-carriers-in-a-conductor
𝑚𝑎=𝑞𝐸 After a time 𝑡 with no collisions, a charge carrier has the velocity 𝑣=𝑞𝐸𝑡/𝑚. If 𝜏 is the average time between collisions, then 1/𝜏 is the average rate of collisions (for one charge carrier). *they agree with my lecture*
Sir , why is Drift velocity = EeT/m not EeT/2m since it is the average velocity of electrons?
Another question:
Why T(tao) remains same when temperature and material remains same? The T is the average time of Collison of electrons and since the drift velocity is higher in higher E(electric field) the collison time would get shorter. But this is not shown in you derivation at starting calculating the speed of electrons and I Current
I came to make same comment. Appears like we should use average drift velocity. FFor uniform acceleration
Hello Sir , At 1:12 you say that the derivation of Ohm's law you show in the lecture is a very 'crude derivation' & we need to go into quantum mechanics for a more accurate one ? But why is the derivation not perfect as I think the math is completely fine . Is it because we don't assume the temperature dependence of Resistivity while deriving Ohm's Law by this method ?
we treat electrons here as if they behaved in a Newtonian manner. That's not kosher. Yet my derivation gives a useful picture.
@@lecturesbywalterlewin.they9259 May you suggest the good lectures on quantum mechanics , nuclear and particle physics ; and condensed matter physics on youtube or any other way . Please . Any of them you know .
I have been taking many mit courses during the lockdown and there are very few teachers like prof. lewin
I am in class 12 but the knowledge provided by him is just great
Dear Professor Walter Lewin. At 01:44 you said that the free electrons in copper have a speed, an average speed of about a million meters per second. Can you please share with me how do we know that? In addition, not only me but also my family thank you very much for your amazing lectures.
use google
as we increase the potential difference the electric field increases , as the electric field increases the drift velocity increases ,now this means that the current should reduce as the electrons always oppose the current . but this not so because on increasing the potential difference the current increases .
postive charges (protons) cannot move in a solid. Only the electrons can they go AGAINST the E-field as the force on them is q*E and q is negative.
at about 0.07 u said that when positive charges go in some direction then the current is in that direction
>>>>at about 0.07 u said that when positive charges go in some direction then the current is in that direction>>>>
I NEVER SAID THAT. watch again. EVERYTHING I said is correct.
oh i am soory proff. u are great and can never be wrong
confused !! not able to judge any direction of current , u said that by convention we take the direction of current same as the direction of positive charges but positive charges cannot move through solid hence the electrons flow from lower potential towards the higher potential and hence constituting the current which is in the direction opposite to that of the convention . and now through battery in which there is liquid the positive charges should move and hence the current is in the opposite direction to that of the the current in the circuit
Awesome experiments, absolutely loved it!
Professor I have a question...what would those SPECIAL shoes( worn by thief in the end) look like in order to make a working static gun from his finger?
question unclear
@@lecturesbywalterlewin.they9259 you said the thief would have to wear a special kind of boots to be able to make a static gun by just rubbing his feet on the ground... because normal boots will lose charge as soon as you stop rubbing your feet on the ground...i was asking what would the special boots(the ones that won't lose charge) look like? Would they just be thicker souled than usual or something else would be changed too in them?
thicker & higher resistivity soles
@@lecturesbywalterlewin.they9259 okay...my doubt is cleared professor...thank you
Teacher, can you tell me the reason why (25:25) at the beginning the current surged towards to a very high value?
Thanks you a lot!!!
The E-field propagates through the wire with the speed of light. Thus switching on will "instantaneoulsy" make the current 5 A.
@UCiEHVhv0SBMpP75JbzJShqw Thank you teacher but why is there a little increase of current and after that, in the next term, R remains constant and I is not changing?
Can you tell me what happen with the resistor?
This lectures his teaching is awesome also extremely helpful who are class 12
Glad to hear that
At 47:25, what are you apologizing for?
Sir I Love your lectures and it helped me to learn more abou physics
Hello Pr Lewin.
First I would like to thank you for the lectures, they are all amazing and I honestly think they are a gift to the world. I was recently graduated as an engineer and I have already learned some of the subject you discussed, but you treat them way better than anyone :)
Here is me question : at about 36:00 you say that the electrons can go to the earth because the air next to the electroscope is hotter because of the candle. How can the electrons go to the earth if you just heat up the air around the electroscope and not everywhere between the electroscope and the earth? You clearly increase the air conductivity next to the electroscope but you don't change this conductivity for the rest of the circuit (between the hot air zone and the ground) so how can the charges flow to the earth?
I hope that my message is clear, I'm from Belgium so as you know, english is not my mother tongue ;)
Thanks for your kind words
You're welcome! Do you have the answer to my question?
there was no question
You maybe didn't see it because my first message is too long. You have to click on something like "Read more" at the end of my first message to see the hidden part of the text (my youtube page is in french so I'm not sure that it's "read more" but it should sound like this).
The heat of the candle ionizes the air near the electroscope. That produces free electrons which will neutralize the charge on the electroscope.
When scuffing the shoes: Does the charge on the sole INDUCE a charge separation in the body, which is then registered by the electroscope?
Or is charge actually flowing THROUGH the sole up into the body and the electroscope?
Thank you so much for these lectures!
body will get charged too through the sole
Why is it that the resistance of each of the two shoes is 2 ohms instead of 4 ohms? I thought that if one shoe had a resistance of 4 ohms, if you bring another shoe in you get another 4 ohms. @44:58
two 4 Ohm resistors parallel have a combined resistance of 2 Ohm.
two 4 Ohm resistors in series have a combined resistance of 8 ohms.
Oh yeah that's right. Forgot about that. Thanks!
Sir, this is Aryan.
A huge fan of yours.
If I would live near you, I would enjoy the whole day with you sir.
Don't feel lone. I am with you sir, you will always be there in my heart.
By the way sir, I am a high school student. I do love watching your lectures.
I have just started with 8.02 and finding it very interesting but I have to skip some of them sir as I have not been a past learner of Calculus.
So, according to you, what else could I do to rock my school with my physics knowledge ?
Dude just wait till class 11. In first month, you will learn all the calculus you need. Otherwise you can obviously go on watching these just for the love of it
Thank you for helping me go through my physics mock.
Hello,
I cannot understand why I would be constant throughout the circuit when considering the series of resistors.
I mean, after going through the resistor, the current should increase or decrease, not stay the same. Right ?
Is it because of charge density of the material, and that therefore if localy a change in speed takes place, it will spread to junctions ?
Why doesn't the water electrolyze in the demonstration experiment on decrease in water/electrolyte resistance with the addition of salt?
40:19 will the light lit on if the water is heated up and without adding salt?
For the light to lit on, you have to have free charges to move from one plate to another. There are ridiculously few of them in the clean water, so the light don't lit. When we add salt, it creates Cl-ions which can transfer the charge, so, the light lit up. If you heat up the water, say, with electricity, it may produce some ions. But it will produce ions which are lighter then the water molecule, so they will fly up in the air, and there will not be any charge transfer. But if the potential difference between the plates is so enormously large, then maybe you could see some light.
Correct me, if I'm wrong. I'm not a physicist.
Hi Professor Walter, thanks for the fantastic lectures.
I have a question: What would happen if you would touch de van de graaf while wearing your shoes?
The charge of the VDG (the one you commonly used during your lectures) is only 10uC, so I can't figure out what would happen. I could come up with the follow (probably wrong) hypothesis:
I assumed that you would experience a current from the VDG to ground, which would be limited by the VDG charge. The "tricky" part is that since the current flow is I = dQ/dt and Ohm's law gives us 100mA, it means the VDG can't provide enough current to keep that up unless it would get charged almost instantly. My (probably incorrect) conclusion is that therefore you would just end up feeling a initial shock, and if you could hold on the sphere after the shock, you would end up acting as a path for the current between the VDG and ground. Since the moving belt inside the VDG would start inducing more charge on the sphere, you would experience a (very low) current flow by the induced charge. All of this is assuming that the initial current could cause some muscle contraction but wouldn't harm you any further.
Where did I go wrong?
Once again thanks for the fantastic lectures, they are really unmatched. They make me love physics again and have been an excellent entertainment source every single day since I found out about them.
Thank you!
At 29:38, you mention that the voltage V2=I*R2. Why is it not, V2=I*(R1+R2)?
V= V1+V2=I(R1 + R2) V1=IR1 V2=IR2
Thank You, Professor!
What does scuffing your feet do exactly? Does it increase the resistivity of your feet, slowing the current, and allowing the charge to stay on the thing? If so, why does the resistivity increase? Is the temperature of your soles changing?
they charge me up - use google
Sir, I have two questions,
First, you said that resistivity increases with increase in temperature but when you heated up the air the electroscope showed more deviation which means resistivity of air decreased and both these statements contradict.
Second, can you please explain the part that why the 2 billion resistance of the shoe is very low for the experiments you carry.
resistivity of solids increases with temperature. When you heat air you create ions and that lowers the resistivity
Question about problem 3.1(b) - you say that since charge cannot flow into or out of the circled part of the circuit, that the charge must be symmetrically distributed. But doesn't that also mean that there is no current through that part of the circuit? If so, how does the current "resume" to the right of this portion of the circuit? There must be sparks going between the plates as they reach saturation, at which point there _is_ a current going through the circled portion... No?
I looked at the solutions of 3.1b and I agree with it. If you need more help, use google
hi,thanks your lesson.May I ask that the voltage of the resistor is larger than that of the wire macroscopically, because the number of electrons remains unchanged, and the electrons need to pass through the thin tube at a faster speed to make the current equal, so a larger electric field is needed?
question unclear - ill defined
Dr. Lewin, could we not also say that a negative current (i
current, I, is defined as charge per sec. The direction is defined the way positive charges would move in an E-field. End of strory
@@lecturesbywalterlewin.they9259 okay but I am confused about how this is universally true without knowing the sign of the value.
I=dq/dt so if you point your arrow to the right and electrons move right then the rate of change of charge to the right would be a negative value, which is equivalent to a positive value of the same magnitude going to the left.
So isn't it specifically currents that are positve that are opposite electrons
Hi Dr. Lewin. In terms of the drift velocity is it understood that this is concurrent with the electron motion ? They are still bouncing all over and hitting atoms while also meandering in the direction the voltage field is pushing them?
yes
Hello sir...
Sir here you have discussed about two velocities of electron (1) velocity due to tenperature ≈ 10^6 m/s and (2) drift speed ≈ 0.5 mm/s... Now my question is when we provide a potential difference then electrons moves in fixed direction with which velocity (1) or (2) and if they move with drift speed then what happens to their veocilty due to temperature? Also how can this drift velocity being so small have such significant effect that we see current flowing? Also I came across some assumptions (velocity becomes zero after every collision between electrons and then they regain the velocity) about drift speed, so are they true or we just accept them as assumptions?
potential differences do not require motion of electrons or other charges.
Sorry sir but I can't understand what you have said... Sorry sir...
Will you please elaborate...
Thankyou sir...
I can perhaps provide my understanding of the lecture. The electrons do move with the drift speed in response to a potential difference, and that does not affect the speed due to temperature (so that value of tau is purely temperature dependent). This small speed does indeed provide the significant effect of what we see as current just because there are so many charges moving and summed up they indeed have a lot of energy, even at such small speed. With that said, he does mention that a more accurate derivation of Ohm's law would require quantum mechanics, and indeed when it comes to questions of electrons and matter at the atomic scale in terms of how they behave, this classical view that is assumed in all these lectures will not work anymore, and we must go to quantum mechanics to give us the more accurate insight. I came across very similar questions in my head when I studied electronics and we talked about semiconductors, and again there the answer lies in quantum mechanics, especially in subjects such as solid state physics.
Can't Ohm's Law be effectively saved if we were able to define temperature for a substance as a function of current? Would that relation be reducible to some constant coefficient multiplied by a function? What would a temperature function of the current look like? What else would it be a function of?
yes that can be done. Every resistor that you then buy will have to come with a written equation which is a function of I. Two resistors both 100 Ohm would have a different equation if they are not made of the same material or if their dimensions differ.
the drift velocity being a*tau, is a approximation right because between collisions the drift velocity is variable, not constant, right ?
Thanks for sharing your lessons on TH-cam. After studying electricity for so many years, I have learned all kinds of calculations, but I don't even know the principle of electricity. Thank you for making me understand. I have a question, according to U=El, the voltage across the resistor is greater than that of the wire, microscopically because the electrons need to go faster, which means more force through the resistor, yes? According to i=QnAv, previously I thought E would cause more electrons to drift, so the voltage microscopic effect of the resistor is more electrons moving faster, meaning larger n with larger v. Later, it was found that the electrons moved faster, and the number of electrons participating in the movement did not change, that is, n did not change, only v changed. is that so?
I cannot add to the clarity of this lecture