Lec 25 | MIT 8.02 Electricity and Magnetism, Spring 2002

[bribribri56]

Very enjoyable thank you

[prchecker]

some sweet info here

[thejameskan]

really informative and interesting

[chingusaranghae]

is the girl sleeping @44:43?? LOL

[OKMUNWURX]

No, she is looking down on her note pad

[ripriprippity]

well, just substitute e^mt= Q, m being a complex or real constant , getting L*m^2*e^mt+ R*m*e^mt +1/C * e^mt = 0, sovlign for the homogeneous form first, getting e^mt( L*m^2+ R*m +1/C) = 0, i.e. L*m^2+ R*m +1/C = 0, put that into the quadratic equation, getting m = (-R + or - sqrt( -R^2 -4L/C) all over 2L which should give a complex value i.e. m = r+iq and you'll have e^mt = e^(r+iq)t = e^rt*e^(iq)t which by euler's formula gives e^rt * (cos (qt) + isin(qt))

giving u the homogenous solution.

[OKMUNWURX]

@ripriprippity

short cut for resonance F = 159.155/(sqr(L*C))

F= Hz C= uf L = Hy

F= Mhz C=pf L = uh

[spleichenstein]

@ripriprippity when are you guys going to put this to use and figure out stanley meyers patents? Something is missing. I am thinking it is 2 coils stacked. The 1st at infinite impedance with resonant frequency pulses, transferring magnetic energy to the secondary coil while drawing almost zero amps and the voltage jumps to virtual infinity. Your thoughts?

[MrSma89]

his explanation of resonance is supreme

[MrSma89]

his explanation of resonance is supreme

[oshmunnies]

Does anyone know what lecture Dr. Lewin talks about inductance and magnetic field energy??

[reil0080]

You may have noticed a small error.

If the input is V =Vo cos(wt) one gets tan(phi) = R/(1/wC-wL)

If the input is V = Vo sin(wt) one gets tan(phi) = (wL - 1/wC)/R

... slips of the pen as he's fond of saying. Anyways if you worked through it this should help you.

[thatguy928]

@reil0080 good catch!

[recordsnething]

The omnipotent voice of correction around 45:00 is hilarious.

[Kettenfahrzeugfahrer]

@recordsnething LMAO! The Omnipotent Voice of Rectification is always hilarious.

[desertdick]

There is a contradiction in this lecture..at 01:48 he states clearly that there is NO resistance inside a self - inductor no matter what texts books tell you but then later at 31:50 he says ''well, maybe L has a little bit of resistance but we can ignore that for now.'' Which one is right? I'm sure L always has a bit of resistance....

[princeroyal2008]

Well when he said there is no resistance , he was referring to an ideal Inductor , whose coil does not have any resistance , but in real time there is no suck thing as IDEAL hence the wires of the inductor will in fact have a negligible resistance

[AbsintheDragon1]

@desertdick

Right: unless your inductor is made of some superconductor, it has some resistance. However, since the inductor is probably made of a conducting metal, its resistance is negligible. At 01:48 he's talking about the tendency of some texts to use Kirchoff's Rules when there is an inductor, which is done by treating L*dI/dt = dPhib/dt as a voltage drop over the inductor. From Faraday's Law, you see that this application of the inductance is incorrect.

[Vd12A]

Well he says that the emf is always zero, no matter what the book want u to believe

[1337ASM]

You can make assumptions about everything, you can calculate the current flowing through a circuit (e.g battery, blub, resistor)and have a good deal of accuracy without considering the internal resistance of the battery.

You can ignore certain things, you learn about simplifications first, then move onto more accurate stuff..

e.g, newtons 2nd law, f=ma is wrong when it comes to the earths gravity, it should be F=GgM/r^2.. not a great deal of difference for low-level apps

[kristiankroflin]

steady state solution. transient solution. this means that here no general solution´s found

up to this day which contains those specific moments.

[materiasacra]

You mean Q(t) expressed in arbitrary driving voltage V0(t)? This is generally impossible in closed form.

But you can Fourier-transform your V0(t). For each frequency you get the steady state solution presented in the lecture. The final solution then is the Fourier synthesis (frequency integral) of this. Sometimes the synthesis can be done in closed form. But even if it can't, numerical F-transform is easy. So we can study any 'transient' behavior using steady state solution as building block.

[kristiankroflin]

You´re right that it can be decomposd to a bunch of steady states it consists of, but you have to do that. You don´t have a more general equation where you can avoid doing this instad of simply putting in some values in a more general equation. Is it faster to use addition or multiplication? Of course one can first define all elements and then add one by one. It´s a matter of how fast something´s done. You tell how to get to a more general equation (it´s the way to get it).

[Naam19]

All i got to say is RESPECT for this guy.

[amrosik]

RESPECT.

[goodluckpeace44]

he sure is an interestine person with lot of facinating tghings leaarned *respect*

[dsmith1427]

I graduated College in 1984 and these Videos reminded me of the fun in learning. I would love to attend his classes!

[yousadja]

omg, i would have loved that he belongs to my family

[goodluckpeace44]

perhaps your husbond?

[drallig80]

Love this guy! good way to refresh your mind an bring out new innovations

[dupreee27]

kirchoff's rule does NOT apply in case of 'inductors' grouped with resistors/capacitors...YAY books give *bullshitt*

[fingerboy18]

He seems to complicate a lot of things. That's probably what MIT is noted for.

Impedance is simply a vector with the resistance as a real part and reactance as an imaginary part. This can be expressed in polar or rectangular form. 5Ohm @ 90deg or 0+j5. -j is capacitive, +j is inductive. The resonant frequency is just the frequency when inductive reactance and capacitive reactance cancel each other out. They are 180deg out of phase with the same magnitude.

[sarthakbatra2009]

Um... that theory just helps to explain LR, RC , LRC circuits easily. Actually, there are no vectors and stuff, imagining vectors just HELPS, that is not what actually happens. So while you are right, there is an easier way to understand ,the method you refer to is not something that actually takes place. Cz inductance and capacitance are no vectors and neither is voltage/potential...But i agree things are made much easier via that method.

[bboymarcel]

please I can influence electric sreet light by my presence and I can use telepathy..I think that we are all 100% electriicty...can you please help me find ...why I influecnce electric things??and street lamps?

[persianlolita2002]

why all the physic's professors have that Einstein hairstyle?! lol

P.S. thank for the video

[landin048]

I think they think so hard their brains shoot out so many electrical signals it shoots up their hair folicle and produces static. Not really, just kidding. They're just so busy thinking they don't pay attention to their hair. I love this instructor.

[ryanman51]

all the great ones dont have time to take care of their hair.because they are busy doing their physics stuff

[ShinmaRyuu]

Einstein had it right when he said the whole thing about 7 sets of clothing and who cares what your hair looks like. Big trivial waste of time :D

[180mishue]

This professer is really smart..not many people understand this concept in science. Lol i am only in the 8th grade..so that is why i said Wow.

[180mishue]

wow

[littleoaf]

Wow at what?