8.02x - Lect 9 - Electric Currents, Resistivity, Conductivity, Ohm's Law
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- Опубликовано: 3 окт 2024
- Electric Currents, Resistivity, Conductivity, Ohm's Law, Nice Demos
Assignments Lecture 9, 10 and 11: freepdfhosting....
Solutions Lecture 9, 10 and 11: freepdfhosting....
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
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.
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 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
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 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
:)
😍
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.
Thank you Professor Lewin for all the great lectures and resources you provide! You are a true inspiration!!
:)
Sir, your lectures are a gift to humanity.
:)
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 ?
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
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!!
:)
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 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
Enjoying lockdown by your mind blowing electricity lectures💓
This is the answer to our lack of education problems. Get our most brilliant minds and make their knowledge free and universal.
This man is a legend. Please keep teaching. Thank you Sir!
You are very welcome
Thank you professor lewin for all your efforts!
Love from INDIA 🇮🇳🥰🤗
Thanks for the professor Walter Lewin🙏for making us to love physics ❤
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.
:)
Sir really your teaching skills are amazing , love from Bangladesh :)
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.
Congratulations sir ,for your biggest achievement. That you know.
I find your lectures useful and inspiring. Thank you.
6:34 turtle and rabbit story still valid here wow
Love you and your lectures a lot sir they are making me love physics 💓 💓 💓💓 💓
Huge respect, sir i am really in love with physics because of you. Thanku so much sir ❤ love from india 🇮🇳
The beauty of ur teaching is how easily u convert complex things easy
Sir you are the legend of physics. These lectures are useful. I watched every lecture.
This was really helpful for preparing for my test. Thanks!!!
The graph 📈 is so good
Thank you sir to help poor Indian students in this situation by giving your lecturers on RUclips.
i cant express enough how much i loved this
I'm so glad that my teacher suggested me about ur lectures on youtube sir😊😊😊😊😊😊😊😊
excellent explanation of Ohm's Law extended to encompass the world around us.
Thank you Prof. Lewin
Hey almamater u r just phenomenal Loved ur experiments evry time u watch u learn even more with the dimension of thinkng being enhanced
An excellent lecture. Thanks for uploading it.
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
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?
"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
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.
as a 9th grader i understood everything. u make learning so easy. thank u so much sir
Glad to hear that
he made me fall in love with physics once again.....
So why did you break up?
I am in class 12 but the knowledge provided by him is just great
Sir, ur way of teaching is fabulous 🤓
Awesome experiments, absolutely loved it!
I am Mexican and thank you for this. It is great
:)
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.
I watch this and i am 10 th grade and i love all the lectures i've watched from you!
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
Indian students are spending tons of money in coaching centers without being aware of this masterpiece.
Finally you changed your channel's profile picture!!!!
GLAD TO SEE!!!!☆★☆★☆★☆★
I have been taking many mit courses during the lockdown and there are very few teachers like prof. lewin
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._
This lectures his teaching is awesome also extremely helpful who are class 12
Glad to hear that
Outstanding the superb
:)
i am motivated to learn physics now
Thank you for helping me go through my physics mock.
Sir I Love your lectures and it helped me to learn more abou physics
thank you very much for pointing out that Ohm's law is not much of a law, since resistance changes as a function of current. blessings,
Thank you for your wonderful lectures...
You're most welcome!
@@lecturesbywalterlewin.they9259 ;)
@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
THANKU FOR HELPING ME
LOVE YOUR VIDS
Really appreciable nice method love it
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
Simply amazing lectures!!! 😃
Glad you like them!
Eternally Grateful Sir!
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.
GENERATIONS will learn from him.
Last experiment you showed that by scuffing shoe on a carpet we can produce electric charge on electroscope
As the shoe sole has a greater resistances the electric current don't leak from the bottom of your feet charge on the electroscope and when you remove the shoes the resistance will you fall over great factor and there will be no static charge produce produces it will immediately flow through your feet that's what happened !
Thanks... loved it
I'm watching this in *"480p"* !!
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?
He is right, if forgot to discharge static charge it may follow by the electron dust (Just saw the news about passenger plane hit unknown object, I expecting the light bolt). 🛩⚡
Planes are struck by lightning routinely. It doesn’t hurt them.
loved it absolutely thanks so muchh...
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
Sir you are very energetic 💪👌
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 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!
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
Sir ,Current is lift over in glass
Because, glass is an insulator
And it does not neutralize so easily
T .Hema Bharathi, Andhra Loyola College , Vijayawada, India
I love how he went to the effort of also bringing pepper lolololol
Very good lecture Sir. Regards 🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏
Keep watching
I liked physics but now I love it.
Sir I have a doubt why current flows in the opposite direction of electron flows
I can't understand it from my heart
it's due to the defintion of current (goes back to the days of Benjamin Franklin) - use google
@@lecturesbywalterlewin.they9259 Love u sir.
My lecturer suggest this to watch by all college student in his class
Why can't we store lightning strikes as an source of POWER?
make a proposal and then patent it
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
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
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
Excellent class sir🙏
Sir suppose there is bulb. When it is cold its resistance is low than when it is hot. As we know power of the bulb=i2R. Therefore power of bulb is higher when bulb is at room temperature and its power decreases when it becomes hot. It means power of the bulb decreases as its temperature increases. Sir it means that when we switch on the bulb its power must be high and after some time its power must be less. Sir than why not bulb show much glow when we switch on the bulb and why not its glow decreases after switch on the bulb because temperature is increases hence resistance must be increases but current decrease hence power must be decreases Sir.
Thanks Sir
>>>after switch on the bulb because temperature is increases hence resistance must be increases but current decrease hence power must be decreases>>>
of course, power is IV
Great lesson.
I am from India
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!
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