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  • I'm Walter Lewin. My lectures will in general not

  • be a repeat of your book but they will be complementary to

  • the book. The book will support my

  • lectures. My lectures will support the

  • book. You will not see any tedious

  • derivations in my lectures. For that we have the book.

  • But I will stress the concepts and I will make you see beyond

  • the equations, beyond the concepts.

  • I will show you whether you like it

  • or not that physics is beautiful.

  • And you may even start to like it.

  • I suggest you do not slip up, not even one day,

  • eight oh two is not easy. We have new concepts every week

  • and before you know you may be too far behind.

  • Electricity and magnetism is all around us.

  • We have electric lights. Electric clocks.

  • We have microphones, calculators,

  • televisions, VCRs, radio,

  • computers. Light

  • itself is an electromagnetic phenomenon as radio waves are.

  • The colors of the rainbow in the blue sky are there because

  • of electricity. And I will teach you about that

  • in this course. Cars, planes,

  • trains can only run because of electricity.

  • Horses need electricity because muscle contractions require

  • electricity. Your nerve system is driven by

  • electricity. Atoms, molecule,

  • all chemical reactions exist because of electricity.

  • You could not see without electricity.

  • Your heart would not beat without electricity.

  • And you could not even think without electricity,

  • though I realize that even with electricity some of you may have

  • a problem with that. The modern picture of an atom

  • is a nucleus which is very small compared to the size of the

  • atom. The nucleus has protons which

  • are positively charged and it has neutrons which have no

  • charge. The mass of the proton is

  • approximately the same as the mass of the neutron.

  • It's about six point seven times ten to the minus

  • twenty-seventh kilograms. One point seven.

  • The positive charges here with the nucleons,

  • with the neutrons, and then we have electrons in a

  • cloud around it. And if the atom is neutral the

  • number of electrons and the number of protons is the same.

  • If you take one electron off you get a positive ion.

  • If you add an electron then you get a negative ion.

  • The charge of the electron is the

  • same as the charge of the proton.

  • That's why the number is the same for neutral atoms.

  • The mass of the electron is about eighteen hundred thirty

  • times smaller than the mass of the proton.

  • It's therefore negligibly small in most cases.

  • All the mass of an atom is in the nucleus.

  • If I take six billion atoms lined up touching other,

  • I take six billion because that's about about the number of

  • people on earth. Then you would only have a

  • length of sixty centimeters. Gives you an idea of how small

  • the atoms are. The nucleus has a size of about

  • ten to the minus twelfth centimeters.

  • And the atom itself is about ten thousand times larger.

  • The cloud of electrons. Which is about ten to the minus

  • eight centimeters. And if you line six billion of

  • those up you only get this much. Already in six hundred BC,

  • it was known that if you rub amber that it can attract pieces

  • of dry leaves. And the Greek word for amber is

  • electron. So that's where electricity got

  • its name from. In the sev- sixteenth century

  • there were more substances known to do this.

  • For instance glass and sulfur. And it was also known and

  • written that when people were bored at parties that the women

  • would rub their amber jewelry and would

  • touch frogs which then would start jumping of desperation

  • which people considered to be fun, not understanding what

  • actually was happening to the amber nor what was happening to

  • the frogs. In the eighteenth century it

  • was discovered that there are two types of electricity.

  • One if you rub glass and another if you rub rubber or

  • amber for that matter. Let's call one A and the other

  • B. It was known that A repels A

  • and B repels B but A attracts B. And it was Benjamin Franklin

  • without any knowledge of electrons and protons who

  • introduced the idea that all substances are penetrated with

  • what he called electric fluid, electric fire.

  • And he stated if you get too much of the fire then you're

  • positively charged and if you have a deficiency of that fire

  • then you're negatively charged.

  • He introduced the sign convention and he decided that

  • if you rub glass that that is an excess of fire and he called

  • that therefore positive. You will see later in this

  • course why this choice he had fifty percent chance is

  • extremely unfortunate but we have to live with it.

  • So if you take this fluid according to Benjamin Franklin

  • and bring it from one substance to the other then the one that

  • gets an excess becomes positively charged but

  • automatically as a consequence of that the

  • one from which you take the fluid becomes negatively

  • charged. And so that's the whole idea

  • behind the conservation of charge.

  • You cannot create charge. If you create plus then you

  • automatically create minus. Plus and plus repel each other.

  • Minus and minus repel each other.

  • And plus and minus attract. And Benjamin Franklin who did

  • experiments also noticed that the more fire you have the

  • stronger the forces.

  • The closer these objects are to each other the stronger the

  • forces. And there are some substances

  • that he noticed which conduct this fluid, which conduct this

  • fire, and they are called conductors.

  • If I have a glass rod as I have here and I rub it then it gets

  • this positive charge that we just discussed.

  • So here is this rod and I rub it

  • with some silk and it will get positively charged.

  • What happens now to an object that I bring close to this rod

  • and I will start off with taking a conductor.

  • And the reason why I choose a conductor is that conductors

  • have a small fraction of their electrons which are not bound to

  • atoms but which can freely move around in the conductor.

  • That's characteristic for a conductor, for metals.

  • That's not the case with nonconductors.

  • There the all electrons are fixed to individual atoms.

  • So here we have a certain fraction of electrons that can

  • wander around. What's going to happen that

  • electrons want to be attracted by these positive charges.

  • Plus and minus attract each other.

  • And so some of these electrons which can freely move will move

  • in this direction and so the plus stay behind.

  • This process we call induction. You get sort of a polarization.

  • You get a charge division. It's a very small effect,

  • perhaps only one in ten to the thirteen electrons that was

  • originally here will end up here but that's all it takes.

  • So we get a polarization and we get a little bit more negative

  • charge on the right side than we have on the left side.

  • And so what's going to happen is since the attraction between

  • these two will be stronger than the repelling force between

  • these two because the distance is smaller and Franklin had

  • already noticed the shorter the distance the

  • stronger the force. What will happen is that if

  • this object is free to move it will move towards this rod.

  • And this is the first thing that I would like you to see.

  • I have here a conductor that is a balloon, helium-filled

  • balloon. And I will rub this rod with

  • silk. And as I approach that balloon

  • you will see that the balloon comes to the rod.

  • I will then try to rub with that rod several times on that

  • balloon. It will take a while perhaps

  • because the rod itself is a very good nonconductor.

  • It's not so easy to get charge exchange between the two.

  • But if I do it long enough I can certainly make that balloon

  • positive. Then they're both positive.

  • And then they will repel each other.

  • But first the induction part whereby you will see the balloon

  • come to the glass rod. These experiments work best

  • when it is dry. In the winter.

  • They don't work so well when it is humid so it's a good time to

  • teach eight oh two in the winter.

  • OK there we go this should be positively charged now.

  • And the balloon wants to come to the glass.

  • You see that? Very clearly.

  • Come on baby. OK.

  • So now I will try to get this balloon charged a little so

  • there is a change of electrons that go from the balloon to the

  • glass. And the glass doesn't it's not

  • a conductor itself so it is not always so easy to get charge

  • exchanges. OK let's see whether I have

  • succeeded now in making the balloon positively charged as

  • well as the glass rod. If that's the case then the

  • balloon is not going to like me. The balloon will now be

  • repelled. And you see that very clearly.

  • To show you now that there are indeed two different kinds of

  • electricity if I now rub with cat fur by tradition we do that

  • with cat fur I don't know why by tradition we use silk for the

  • glass. So if we do this with cat fur

  • now then this becomes negatively charged.

  • Remember there were two types of electricity.

  • And since that balloon is positively charged now the

  • balloon will come to me. And there it is.

  • Now it comes to me. So you've seen

  • for the first time now clearly that there are two different

  • kinds of electricity. The positive charge is chosen

  • by Franklin on the glass rod and the negative charge on the

  • rubber. So now you may think that if I

  • approach a nonconducting balloon with a glass rod and I have a

  • nonconducting balloon here you may think now that this balloon

  • will not come to the glass rod because there are no free

  • electrons. So these electrons cannot

  • freely move and so you don't get this polarization.

  • You don't get this induction. But that is not the case.

  • And this is actually quite subtle.

  • You have to look now at the atomic scale.

  • If I take an atom like you have here.

  • You have positive charge and you have the electrons here in a

  • cloud around the positive nucleus.

  • If I bring a glass rod positively charged nearby then

  • these electrons which are stuck to the atoms,

  • they cannot freely move like in conductors, however will spend a

  • little bit more time on the side where the glass rod is because

  • they feel attracted by the glass rod, whereas the nuclei if

  • anything want to go away from the glass rod,

  • so what you're going to see is that

  • in a way if I started off with a spherical atom let's suppose

  • this were a spherical atom or a spherical molecule then what

  • will happen is that you get sort of a shape like this and the

  • electrons spend a little bit more time here than they spend

  • here and that means that I have actually polarized that atom.

  • If the electrons spend more time on this side of the atom

  • than on this side I have also created the phenomenon of

  • induction and I therefore expect that this side

  • becomes more negative than that side.

  • And I can show you that in a nice way with a transparency

  • whereby I have plus and minus signs and I have equal number of

  • plus and minus signs. So they represent neutral

  • atoms. There you see them.

  • Boy. It's a little dirty but maybe

  • see I can clean it a little.

  • OK. OK.

  • So here we go. So notice there are equal

  • amount of pluses and minuses, so think of the plus and the

  • minuses as one neutral atom. Just a representation.

  • Now I'm holding a glass rod on this side which is positively

  • charged. And so each atom the electrons

  • want to go a little bit to this side and so the nucleus stays

  • behind. And if each atom does that this

  • is what's going to happen. And now notice what you end up

  • with. In the middle of the substance

  • plus and minuses cancel each other out again.

  • But on the right side you have created a negatively charged

  • layer and on the left side you have created a positively

  • charged layer. And so in a way you have again

  • induction. So even in the nonconducting

  • objects this side will turn negative and this side will turn

  • positive and therefore if I approach a nonconducting balloon

  • with a glass rod I will also see the balloon

  • come to me. And so I can easily show you

  • that. It doesn't make any difference

  • whether I choose glass or whether I choose rubber.

  • I can do it with both. Nonconducting balloons always

  • have a potential problem. The potential problem is that

  • they can be charged by themselves just like the metal

  • balloons can be charged by themselves.

  • However, if I touch the metal balloon then any charges there

  • will immediately flow through me to the earth we will understand

  • that later. Because this is a conductor.

  • That remember the electric fluid is conducted by a metal

  • but not by a nonconductor. So with this it's more

  • difficult. Even if I kiss it and touch it

  • it's not clear that I can take all the charge off.

  • In fact by doing that I may even make it worse.

  • Let's hope that it is not charged

  • too much and let's approach it with this glass rod and see

  • whether I can convince you that indeed it's coming to the rod

  • not because of the free electrons but because of that

  • process. Oh boy.

  • Ho. And it should also do the same

  • with rubber I hope. If it were negatively it'd go

  • away. Ha it does go away so it is

  • negatively charged you see that. By touching it I actually

  • probably charged it and there's not much I can do about it.

  • Very difficult to get charge off.

  • I already had a suspicion when I approached it with the glass

  • it was too eager to come to the glass.

  • Still negatively charged. That's the way it goes.

  • It's not because the demonstration

  • failed but it's because the balloon is charged and doesn't

  • want to give it up because it's a it is a nonconductor.

  • Friction can cause electric charge and that's exactly what

  • happened when I touched this balloon and tried to discharge

  • it. Through friction I may actually

  • have charged it. If I take these party balloons

  • that all of you may have seen and you just rub them on your

  • shirt on your trousers they stick to my hand.

  • They have charge on them. Whether it's positive or

  • negative I don't know, I don't even remember.

  • It's not important. And so when I bring them to my

  • hand, my hand is not a good conductor but you get induction,

  • this phenomenon that we just discussed and so the two attract

  • each other. The positive and the negative

  • side attract each other. And you can stick them on the

  • ceiling. Or you can stick them on the

  • board. You can decorate your room that

  • way. Very pretty isn't it.

  • All that you can do now because of eight oh two.

  • Now these heavy balloons may be a little bit more difficult.

  • Also I'm wearing cotton. If you wear nylon or polyester

  • it's much better. It's much easier to get oh

  • that's good, that's a nice one, I think we need a blue one.

  • There we go. So you see friction causes

  • electricity. That's of course why the silk

  • when we rubbed the glass and the cat fur we rub the rubber then

  • we create charge on one. Of course if you make the glass

  • positively charged your silk will be automatically negatively

  • charged. When you comb your hair you may

  • have noticed with dry weather that you hear some cracking

  • noise. Cracking noise means sparks.

  • And you will learn all about sparks

  • in this course though not today.

  • But you can hear it if you're very quiet.

  • And as you do that you charge the comb.

  • I can hear the cracking. Interesting.

  • So the comb is now charged. Probably so am I and there it

  • comes. See.

  • It's not as good as the glass but same idea.

  • If you take your shirt off and you make it and you make it dark

  • in your dormitory and you stand in front of a mirror an amazing

  • experience. And I'd be happy to do it for

  • you because but I told you I really wear cotton and it

  • doesn't work with cotton so well.

  • You really have to do it with a nylon shirt.

  • And when you take that nylon shirt

  • off not only do you hear the cracking but you actually see

  • the glow of these teeny weeny little sparks.

  • You actually are like a light bulb.

  • It is an experiment that you cannot miss.

  • And I would suggest you try that this weekend.

  • Do it with a friend. That's even more fun.

  • We'll all perhaps remember when you just walk around.

  • Do your normal things during the day.

  • There are rugs in rooms and you want to leave the room and

  • you touch the doorknob and you get a shock.

  • It's a spark that flies over. It's electricity.

  • Even when you touch a person you sometimes feel this shock.

  • When you cook and you take saran wrap off these rolls the

  • damn stuff just doesn't want to come off because as you roll it

  • off there is friction and it gets charged and it often gets

  • crumpled up and it's very bad, very difficult to handle it.

  • You've all experienced that. Also cellophane around boxes

  • with chocolate the same thing happens.

  • As you take it off you charge it, whether you like it or not.

  • I now want to do an experiment and I need a volunteer.

  • I need a student who actually is wearing preferably not all

  • cotton but I think Simon you have a beautiful wonderful nylon

  • parka. So if you are willing to

  • sacrifice a little bit for the sake of

  • science and come over here and sit down here.

  • Just relax. Make sure that your feet are

  • off the ground. OK.

  • So what I'm going to do now Simon I'm going to beat you with

  • cat fur. And as I beat you with cat fur

  • you will get charged and since I

  • don't want you to be the only person who suffers under this

  • experiment I will also stand on an insulated stool so if you

  • become for instance positively charged I don't know whether

  • it's positive or negative I would get the other amount of

  • charge. So we share in the charge.

  • And as I beat you you will charge up more and more and I

  • will charge up more and more and then we

  • will have to convince the class that that we are both charged.

  • And we will do that in a way that will be hopefully rather

  • convincing. I let me just start beating you

  • a little bit. To make you feel at home.

  • We know each other right. OK.

  • Now of course as I mentioned to you these experiments work well

  • when it is dry and so if you are too wet it won't work.

  • But let's see if you sweat a little bit too much then it

  • doesn't work too well. So we ready?

  • I have here in my hand a neon flash tube.

  • And although we don't know yet what voltage is because we

  • will learn about that in this course, to get a good flash out

  • of these you need about a few thousand volts.

  • And so we will see and we'll make it dark shortly and I will

  • hold the flashlight, the flashlight in one hand,

  • the neon discharge tube, and then Simon will touch it on

  • the other side. And if we've succeeded then you

  • may see some light. So Simon look at me first,

  • don't touch it yet, because we're going to make it

  • all the way dark. You know where it is,

  • it's there, OK, make it darker Marcos.

  • Touch it. Touch it.

  • OK, try it again, touch it again.

  • OK. Thank you.

  • Can we have some light. [applause] Thank you very much.

  • Equal charges repel each other. I've shown that,

  • the demonstration with the balloons.

  • Here we have an instrument which is called the Vandegraaff.

  • It's named after Professor Vandegraaff, who invented it.

  • It was an MIT professor. And this instrument,

  • which I will not discuss in any detail though but you will

  • understand it later on in the course, I'll tell you all about

  • it later. Just think of this instrument

  • as a super amber rod. And although we don't know yet

  • what voltage is, I mentioned already the twenty

  • thousand volts between Simon and me, in this instrument you have

  • to think in terms of several hundred thousand volts.

  • So this instrument is not without danger.

  • But that of course makes it more exciting to work with it.

  • So it's a super amber rod and what I will do first now is to

  • put some confetti on top and when we turn on the Vandegraaff

  • the confetti may at first go to the charged dome,

  • it is already on top of it, and when it picks up some of

  • the charge it will then spread out because it it will repel.

  • So let's get some some light on there which will make it a

  • little bit better to see. Let me put some of this

  • on top. It's just regular confetti,

  • pieces of paper. All right now all I have to

  • remember is how to start the most of the action has already

  • occurred. I will put a little bit more

  • on. [laughter] If you see sparks

  • don't worry yet. [laughter] Put some more on.

  • More and nothing left for the second

  • class. [laughter] Make it perhaps a

  • little darker. Ah that's too dark.

  • [laughter] OK. We'll try it once more give it

  • a zap so look at the confetti on top.

  • And I think it's quite convincing.

  • Some of the confetti will stay there.

  • Well that's the reason that it's not a good conductor and

  • so it get it first sucked in and if it doesn't get charge of the

  • Vandegraaff then it will not spread out.

  • All right. So now let's try for the first

  • time to be a little bit more quantitative.

  • If I take two charges and we use in

  • general we use for charge the symbol Q.

  • So here we have Q one. And here we have Q two.

  • And let's say they're separated by a distance R.

  • And the unit vector in the direction from one to two I call

  • that R roof one-two. The roof stands for unit

  • vector. These charges are equal,

  • both minus or both plus, then they will repel each other

  • and so here there is a force F which I call one-two.

  • It is the force on two due to number one and since action

  • equals minus reaction force here is to one equal in magnitude but

  • a hundred eighty degrees in opposite direction.

  • Coulomb, the French physicist, who did a lot of research on

  • this in the nineteenth eighteenth century actually.

  • Coulomb found the following relationship.

  • That the force is proportional to the product of the two

  • charges. So it's Q one times Q two.

  • Times a constant which nowadays we call Coulomb's constant,

  • K. Divided by the distance between

  • these charges squared. And it is in direction of the

  • unit vector that goes from one to two.

  • This is the force on number two due to one.

  • And notice that this equation is sign sensitive.

  • Because if Q one and Q two are both negative the source is in

  • the the force is in this direction and if they are both

  • positive it's also in this direction as I have it.

  • However if the if one is positive and one is negative you

  • get minus this direction so this force

  • flips over and that one then obviously also flips over.

  • In the SI units in this course we will use for the unit of

  • charge the coulomb named after this great man.

  • One coulomb charge is a horrendous amount of charge.

  • More than you will ever see in your lifetime.

  • We normally work with microcoulombs,

  • sometimes even less than that. The charge of one proton,

  • which is exactly the same as the charge of one electron,

  • is approximately one point six times ten to the minus nineteen

  • coulomb. So one coulomb is something

  • like six times ten to the eighteen protons or electrons if

  • the charge is negative. This constant K in SI units is

  • nine times ten to the ninth.

  • And the unit you can find out because you know that this is

  • newtons, this is coulomb squared and this is square meters.

  • So the unit is newton square meters newtons square meters

  • divided by square coulombs. But that's not so important.

  • No one ever thinks of it that way.

  • For historical reasons which may at

  • times be a pain in the neck for you we write for K one divided

  • by four pi epsilon zero. There is nothing magic about

  • that. It's just a historical reason.

  • And so one divided by four pi epsilon zero is nine times ten

  • to the ninth. That's all that matters.

  • This epsilon zero has a name it's called the permittivity of

  • free space. But you can forget about that.

  • It's not important the name. Notice that there is a clear

  • parallel with gravity. Newton's law of gravity that

  • the force, which in that case is always attracting,

  • gravity never repels, is the product of two masses

  • and then you have here the gravitational constant and again

  • you have the distance squared. So there is an enormous

  • parallel between the two. There's a great beauty that

  • electricity acts in a way that is

  • very parallel to the way that gravity works.

  • If I added a third charge, for instance here,

  • Q three, and if now I want to know what the force is on Q two,

  • then I use the superposition principle which we've used many

  • times in eight oh one, and we say OK the net force on

  • number two is the force due to number one plus the force from

  • number three. If number three if this is

  • positive and this is positive and this were negative then this

  • force would be in this direction, F one,

  • F three two and then the net force on number two would be the

  • vectorial sum of these two. Is it obvious that the

  • superposition principal works? Not at all.

  • It's not at all obvious. Do we believe in it?

  • Yes we do. Why do we believe in it?

  • Because it's consistent with all experiments that we have

  • done. But the superposition principle

  • which is very powerful is really not a matter of course.

  • But it works. We can always use it.

  • And we will. If you compare eight oh one

  • with eight oh two thereby comparing electricity with

  • gravity you will see that electric forces are

  • way more powerful than gravitational forces.

  • And the way I can best show you that is by taking two protons

  • which are a distance D apart. Here is a proton and here is a

  • pro- proton and they are separated by a distance D.

  • They repel each other. And the force by which they

  • repel each other is of course extremely easy to calculate.

  • We know Coulomb's law. That law is called after

  • Coulomb. And so the force,

  • the electric force with which they repel each other,

  • this is just the magnitude now of the force,

  • is the charge of the proton which is one point six times ten

  • to the minus nineteen but I have to square that,

  • I have to multiply it by Coulomb's constant,

  • which is nine times ten to the ninth, and I divide it by D

  • squared. That's the electric force.

  • If I want to know the gravitational force,

  • which is the force with which they attract each other,

  • these are repelling forces, but I just want magnitudes

  • here, then I have to take the mass of the proton,

  • which is one point seven times ten to the minus twenty-seven I

  • have to square that remember M one

  • times M two times the gravitational constant.

  • The gravitational constant in SI units is six point seven

  • times ten to the minus eleven and I divide that by D squared.

  • If now I compare the electric force with the gravitational

  • force, so I divide one by the other, notice that the D

  • cancels. They both have D squared

  • downstairs. And so you will easily be able

  • to show that this ratio is roughly

  • ten to the thirty-six. So the electric force is

  • thirty-six orders of magnitude more potent than the

  • gravitational attraction. This teaches you some respect

  • perhaps for eight oh two. If these were the only forces

  • that acted on the protons and you bring them in the nucleus

  • which has a size of only ten to the minus

  • twelfth centimeters then the acceleration that the proton

  • will experience is the electric force divided by the mass of the

  • proton. F equals MA.

  • Basis of eight oh one. And if you take this electric

  • force when you make D ten to the minus twelfth centimeters which

  • is ten to the minus fourteen meters and you calculate this

  • ratio you will find that it is twenty-six orders of

  • magnitude higher than the gravitational acceleration on

  • earth. Twenty-six orders of magnitude

  • higher. So you wonder what the hell

  • holds the nucleus together. If there is such a tremendous

  • force on these protons. Well, what is holding them

  • together are the nuclear forces, which we do not fully

  • understand, but thank goodness the nuclear forces are not part

  • of eight oh two so I will leave that alone for now.

  • So what holds our world together?

  • Well on the nuclear scale ten to the minus twelve centimeters

  • very important are the nuclear forces.

  • On an atomic scale up to thousands of kilometers,

  • it's really electric forces that hold our world together.

  • But on a much larger scale, planets and stars and the

  • galaxy, it is gravity that holds our world together.

  • And now you may say ah that's very inconsistent with what you

  • just told us because didn't you tell us that D

  • cancels if you compare gravity with electricity.

  • Yes, however, most objects are neutral or

  • very close to neutral and so if you take the earth it is very

  • unlikely even that the earth as a whole would have a charge of

  • more than ten coulombs. That probably is already an

  • exaggeration. So if I take the earth and I

  • take the moon and I put on both a charge of ten

  • coulombs, here's the earth and here's the moon,

  • and I put say just arbitrarily ten coulombs here and that is

  • put on here either minus, minus ten coulombs,

  • so they will attract each other, but given their distance,

  • it's almost nothing. The force is negligibly small.

  • But of course the force of gravity, which is proportional

  • to their masses, wins and in this particular

  • case if you take the earth and the

  • moon the gravitational force wins over the electric force by

  • twenty-five orders of magnitude. So even though our immediate

  • surroundings are dominated by electric forces,

  • including your own body for that matter, the behavior of the

  • universe on a large scale is dictated by gravity.

  • We will use various instruments to measure charge in a

  • quantitative way and one of the instruments that

  • you will see we will use it often in the lectures that are

  • to come, is called an electroscope.

  • It's a very simple instrument. In general it is just a

  • conducting rod. It could be aluminum,

  • metal, and at the end are two pieces of tinsel,

  • two pieces of aluminum foil, and often there is a nice knob

  • here, and if I touch this with a charged object,

  • then because this can conduct electricity, this can conduct

  • the fire, as defined by Benjamin Franklin, if I touch it with an

  • object which is positively charged, then this object will

  • become positively charged. If I touch it with an object

  • which is negatively charged it will become negatively charged.

  • And you see now here these two very light pieces of aluminum

  • foil will repel each other. And so you will see that this

  • shows a certain angle, and the more charge there is

  • the larger that angle. Sort of gives us a way of doing

  • some quantitative measurements. There are other electroscopes

  • which are not too different. There's one central rod and

  • they would have one leaf hanging there and when you charge that

  • one up then this leaf will go out and if

  • the charge is more it will go out even further.

  • I don't have an electroscope now here.

  • But what I want you to see that if I charge myself up and I hold

  • in my hands these Christmas tree tinsels, that in a way if I get

  • enough charge on me, then these tinsels will

  • spread out. It's an idea that immediately

  • follows from the fact that you get a certain amount of charge,

  • whether it's negative charge from me, or whether I'm

  • positively charged, that doesn't make any

  • difference, these tinsels will spread out.

  • And of course the best way I can do that is if I charge

  • myself with the Vandegraaff. And as I said earlier

  • experiments of this nature are not entirely without risk.

  • And so there's always the possibility of course that I

  • don't survive this demonstration.

  • [laughter] But don't worry because in that case there will

  • be someone else who will lecture eight oh two except he is not

  • likely to show this demonstration again.

  • [laughter] So you might as well take a close look because this

  • may be the only time you will ever see it.

  • So I will give you some nice light on the Vandegraaff and

  • it's always a scary moment for me,

  • sleepless nights about the Vandegraaff.

  • Am I going to turn it on, Marcos, or you have the courage

  • to turn it on? You will turn it on?

  • OK, hold it Marcos, this is too close for comfort.

  • You ready? Are you nervous?

  • Feel. [laughter] So look at the

  • tinsels and try not to look at me please.

  • Go ahead. I am now a living electroscope.

  • [laughter] If the if the weather is cooperating today and

  • if I had long hair you might even see that my hair would

  • start to act like an electroscope.

  • We can try that too. Why don't you throw it.

  • [laughter] [applause] Is it working?

  • OK, well, this weekend make sure you take this nylon shirt

  • off in front of the mirror and enjoy your enjoy the experiment

  • at home. Don't try this ever.

  • See you Friday. [applause]

I'm Walter Lewin. My lectures will in general not

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B1 中級

Lec 01: 是什麼把我們的世界連在一起?| 8.02 電和磁,2002年春季 (Walter Lewin) (Lec 01: What holds our world together? | 8.02 Electricity and Magnetism, Spring 2002 (Walter Lewin))

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    Cheng-Hong Liu 發佈於 2021 年 01 月 14 日
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