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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