8.02x - Lect 9 - Electric Currents, Resistivity, Conductivity, Ohm's Law
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- เผยแพร่เมื่อ 25 ก.ย. 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!
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
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 👏👏👏
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!!
:)
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 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.
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!
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
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 LOVE HIS TEACHING
:)
😍
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.
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.
:)
This man is a legend. Please keep teaching. Thank you Sir!
You are very welcome
Love from INDIA 🇮🇳🥰🤗
Thanks for the professor Walter Lewin🙏for making us to love physics ❤
Congratulations sir ,for your biggest achievement. That you know.
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.
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!
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 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.
Huge respect, sir i am really in love with physics because of you. Thanku so much sir ❤ love from india 🇮🇳
I find your lectures useful and inspiring. Thank you.
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.
Love you and your lectures a lot sir they are making me love physics 💓 💓 💓💓 💓
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!
I'm so glad that my teacher suggested me about ur lectures on youtube sir😊😊😊😊😊😊😊😊
6:34 turtle and rabbit story still valid here wow
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 you are the legend of physics. These lectures are useful. I watched every lecture.
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.
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
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.
This was really helpful for preparing for my test. Thanks!!!
excellent explanation of Ohm's Law extended to encompass the world around us.
Thank you Prof. Lewin
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
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
The graph 📈 is so good
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
i cant express enough how much i loved this
An excellent lecture. Thanks for uploading it.
Hey almamater u r just phenomenal Loved ur experiments evry time u watch u learn even more with the dimension of thinkng being enhanced
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
@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
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?
Finally you changed your channel's profile picture!!!!
GLAD TO SEE!!!!☆★☆★☆★☆★
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
3:40 What the drift velocity somehow follows from the acceleration from the electric field?? I dont follow
watch this lecture - I derive it
The electric field in a connecting wire is zero or non zero?
only zero if no current is going through it
But in Griffith and many other books it is mentioned that electric field in a current carrying wire is negligible i.e. 0 , why so?
@@Noname-kp7rd if the potential difference over the wire (lenght L) is V, then E times L = V. Thus there will then be an E-field in the wire =V/L
All the E-fields cancel except along the wire L, where there is a potential difference, so there is net force on each charge: F=q·E{q}.
Σq·E{X}•dL ="Work done along L
for each X{q}"=w{q}.
Σw{q, X} ="Total work done for all q"=W
along path a→b.
W=(Σq)·(ΣE{X}•dL{X}), assuming each
charge is the same.
Total voltage:
V{a→b}= W/ Σq= Σ Ε{X}•dL{X}= E{net}·L{a→b}
By fundamental theorem of Calculus & fact that E(X) is along the same line as dL, but due to the dot product, at least some directions may be opposite, so might get +E-E+E-2E+2E...for example, thus E{net} may be an average of E components that may include opposite signs.
Indian students are spending tons of money in coaching centers without being aware of this masterpiece.
Sir, ur way of teaching is fabulous 🤓
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
Why doesn't the water electrolyze in the demonstration experiment on decrease in water/electrolyte resistance with the addition of salt?
Why can't we store lightning strikes as an source of POWER?
make a proposal and then patent it
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._
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
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
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
I am Mexican and thank you for this. It is great
:)
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
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!
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 ?
I watch this and i am 10 th grade and i love all the lectures i've watched from you!
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 😊❤️🙏🙏
Say we have a 600 000 Km long conductor connected at some potential V.
If we place 2 ammeters next to the terminals, which ammeter is going to move first and how long until the second one moves? Why?
+lauroflorin Your problem is not well defined. A current will flow through a closed circuit - it is unclear what you mean by "the terminals". In any case I am sure you will be able to do this problem on your own. Keep in mind that the propagation of E-fields in metals goes with the speed of light. Good luck!
Yeah, I meant the long wire would be connected to some power source and thus the ammeters would be next to its positive/negative terminals.
Ok, so as soon as I complete the circuit, the E-field will propagate in the direction + -> -
If its speed is ~C, that means the ammeter next to the positive terminal will start moving and after ~2 seconds the one next to the negative terminal will as well. Is this correct? If so, how do we reach that the E-field in metals goes with the speed of light? Thanks !
+lauroflorin make a circuit with the power source (battery?) in it and a switch
The problem is somewhat similar to waveguides (see my lectures in 8.03)
Hello Professor,please provide the exact definition of series and parallel combination of resistors.
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)
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
Sir charge density(j) =di/ds and not i/s,why?
Sir , if we connect a dc battery across a wire , why the current density is uniform over the cross section of the wire ?
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?
I cannot add to the clarity of this lecture
sir I a simple question from my practical life. sir as the power of the bulb is decreaseing when we switch on our home bulb because current is decreasing. sir than why light by the bulb is not decreasing asw we see in our home bulb
It takes time for the filament to heat up.
Hello sir
If we increas the potential difference across a conductor the current will increase Becaues the number of electrons moving increases or because the velocity of electrons is increased?
The number of free electrons in the conductor will not change if the E-field goes up. Thus a higher E-field means that the drift velocities of these free electrons go up.
@@lecturesbywalterlewin.they9259 thanks sir
Thank you for helping me go through my physics mock.
the drift velocity being a*tau, is a approximation right because between collisions the drift velocity is variable, not constant, right ?
If the drift velocity is almost negligible compared to the velocity due to thermal motion, why does temperature increase with current? Thanks!
www.quora.com/Why-does-a-current-cause-the-temperature-of-a-real-resistor-to-increase-What-effect-does-this-heating-have-on-the-resistance-and-why
Could I find course 6 (of that complex resistor network) in MIT OCW
make an effort and search
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.
Awesome experiments, absolutely loved it!
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,
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
Outstanding the superb
:)
i am motivated to learn physics now
Sir I Love your lectures and it helped me to learn more abou physics
I don't understand, in the last experiment, Although you took off your shoes, which decreases R, but you no current move through your body into the electroscope !!! How come ? Since, I = V/R, i.e. I inversely proportional to R which means it must move many electrons through your body !!!
Ahmad Haitham that's because in order for electrostatic charge to build up on you rather than being grounded, resistance is a must, if there is no insulator present then there is no place for the charge to be induced
Hey Dr.Lewin,
So is current in the sense of circuit analysis just a simplification of a vector for convenience sake?
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