Kelistrikan dan Kemagnetan

**Topics covered:**

Magnetic field

Lorentz Force

Torques

Electric Motors (DC)

Oscilloscope

**Instructor/speaker:** Prof. Walter Lewin

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So far, we have only discussed in this course, electricity.

Calm down.

But this course is about electricity and magnetism.

Today, I'm going to talk about magnetism.

In the fifth century B.C, the Greeks already knew that there are some rocks that attract bits of iron.

And they are very plentiful in the district of Magnesia, and so that's where the name "magnet" and "magnetism" comes from.

The rocks contain iron oxide, which we will call, uh, magnetite.

In 1100 A.D., the Chinese used these needles of magnetite to make compasses, and in the thirteenth century, it was discovered that magnetites have two places of maximum attraction, which we call poles.

So if you take one piece of magnetite, it always has two poles.

Let's call one pole A, and the other B.

A and A repel each other, B and B repel each other, but A and B attract each other.

There is a huge difference between electricity and magnetism.

With electricity, you also have two polarities, but you are free to choose a plus or a minus pole.

With magnetism, you don't have that choice.

The poles always come in pairs.

Isolated magnetic poles do not exist -- or, as a physicist would say, magnetic monopoles do not exist, as far as we know.

If anyone finds a magnetic monopole -- and don't think that people are not looking -- that would certainly be worth a Nobel Prize.

In principle, they could exist, but as far as we know, they don't exist, they have never been seen.

Electric monopoles do exist.

If you have a plus charge, that's an electric monopole.

You have a minus charge, electric charge, that is an electric monopole.

If you have a plus and a minus of equal strength, that is an electric dipole.

Whenever you have a magnet, you always have a magnetic dipole.

There is no such thing as a magnetic monopole.

In the sixteenth century, Gilbert discovered that the Earth is really a giant magnet, and he experimented with compasses, and he was, effectively, the first person to map out the elec- the magnetic field of the Earth.

And if you take one of those magnetite needles, and the needle is pointing in this direction, which is the direction of Northern Canada, then, by convention, we call this side of the needle plus -- uh, not plus -- north, and we call this side of the needle south.

Since A repels A, and B repels B, but A and B attract each other, in north Canada is the magnetic South Pole of the Earth, not the magnetic North Pole.

That's a detail, now, of course.

So this is the way that we define the direction, north and south, of these magnetite needles.

A crucial discovery was made in 1819 by the Danish physicist Orsted.

And he discovered that a magnetic needle responds to a current in a wire.

And this linked magnetism with electricity.

And this is arguably, perhaps, the most important experiment ever done.

Orsted concluded that the current in the wire produces a magnetic field, and that the magnetic needle moves in response to that magnetic field which is produced by the wire.

And this magnificent discovery caused an explosion of activity in the nineteenth century -- notably by Ampere, by Faraday, and by Henry -- and it culminated into the brilliant work of the Scottish theoretician Maxwell.

Maxwell composed a universal field theory, which connects electricity with magnetism.

And that is at the heart of this course.

Maxwell's equations.

You will see them, all fier -- all four, by the end of this course.

If I have a current, a wire, let's say the wire is perpendicular to the blackboard, and the current goes into the blackboard, I put a cross in there.

If the current comes out of the blackboard, I put a dot there.

And there is a historical reason for that.

You're always talked about vectors, in 18.01, and in other courses, but you're never seen a vector.

And I'm going to show you a vector.

This is a vector.

And this is where it comes to you.

That's when you see a dot.

And this is where it goes away from you.

That's when you see a cross.

So this current, when it's going into the blackboard, I can put these magnetite needles in its vicinity, and they will then do this.

And when I put it here, it will go like this.

And they follow the tangents of a circle, and this is the way that we define magnetic fields, and the direction of the magnetic field, namely, that the magnetic field -- for which we always write the symbol B, magnetic fields -- is now in the clockwise direction.

By convention, current goes into the blackboard.

And, if you ever forget that, use what we call the right-hand corkscrew rule.

If you take a corkscrew, and you turn it clockwise, the corkscrew goes in the board.

That connects the B with the current.

If you take a corkscrew and you rotate it counterclockwise, then the corkscrew would come to you, comes out of the cork.

And that's how you find the magnetic field going around current wires.

It's just a convention.

I want to show you how a magnetic needle responds to a current.

I have here a wire through which I'm going to run a fabulous amount of current, something like 300 amperes, and you're going to see that wire there -- I'm going to get my lights right, see how I want it to go, this is the way I want it to go, get you optimum light there.

When I draw a current -- here, you see the the magnetite, the -- we call it a compass, nowadays -- and it's lined up in the direction of the magnetic fields of the Earth.

We're going to run 300 amperes through here, and it will change the direction, it will change the direction which is -- there's going to be a magnetic field around the wire, like this.

So it will go like this.

The current that I run is so high that things begin to smell and smoke within seconds.

The battery is not going to like it when I draw such a high current.

I can, therefore, do it only for a few seconds.

So this compass will swing in this direction, and it starts to oscillate, I can't keep the current so long that it stops the oscillation.

So I will stop it by hand, and convince you that that's really the equilibrium position.

So if you're ready for that -- so we get, now, connection, watch it three, two, one, zero.

There it goes.

Now I'll stop it -- the current is still going.

You see, that's the -- that is the equilibrium position.

And I will stop the current.

And now I will reverse the current, in the opposite direction, now you will see that it swings backwards.

It -- 180 degrees in a different direction.

Three, two, one, zero.

There it goes, I will stop it, [sniffs], few seconds, that's the equilibrium position, and I'll let it go.

So you've seen that, indeed, the magnetic needle responded to the magnetic field that was produced by the wire, this was the great discovery by Erstadt, the discovery -- this demonstration, all by itself, may not be very spectacular for you, but, historically, it is of enormous importance.

I would argue, perhaps, the most important demonstration, the most important research ever done in physics, because it connects electricity with magnetism.

It was the foundation of the creation of the whole concept of a field theory.

Actually, it was magnet's reaction, and that means that if a wire that runs a current has a force on a magnet, then, of course, the magnet must also exert a force on the wire.

And I'm going to demonstrate that to you, too, but now, I have a much more potent magnet, for which I will use this one, and the magnet will not move, it's so heavy that it can't move -- so now you will only see the wire move.

And the basic idea is then as follows, here is that magnet.

Pengembangan Perkuliahan

1. Buatlah sebuah Esai mengenai materi perkuliahan ini

2. Buatlah sebuah kelompok berjumlah 5 orang untuk menganalisis materi perkuliahan ini

3. Lakukan Penelitian Sederhana dengan kelompok tersebut

4. Hasilkan sebuah produk yang dapat digunakan oleh masyarakat

5. Kembangkan produk tersebut dengan senantiasa meningkatkan kualitasnya#### Staff

Visualizations:

Prof. John Belcher

Instructors:

Dr. Peter Dourmashkin

Prof. Bruce Knuteson

Prof. Gunther Roland

Prof. Bolek Wyslouch

Dr. Brian Wecht

Prof. Eric Katsavounidis

Prof. Robert Simcoe

Prof. Joseph Formaggio

Course Co-Administrators:

Dr. Peter Dourmashkin

Prof. Robert Redwine

Technical Instructors:

Andy Neely

Matthew Strafuss

Course Material:

Dr. Peter Dourmashkin

Prof. Eric Hudson

Dr. Sen-Ben Liao

#### Acknowledgements

The TEAL project is supported by The Alex and Brit d'Arbeloff Fund for Excellence in MIT Education, MIT iCampus, the Davis Educational Foundation, the National Science Foundation, the Class of 1960 Endowment for Innovation in Education, the Class of 1951 Fund for Excellence in Education, the Class of 1955 Fund for Excellence in Teaching, and the Helena Foundation. Many people have contributed to the development of the course materials. (PDF)

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.

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