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