## Senin, 29 November 2010

### PUSTAKA FISIKA (PF)

1. Sebuah Visi Pengumpulan 100.000 Buah Buku yang terkait dg Fisika
2. Pengumpulan Data-Data Kefisikaan sebesar 1 Terra byte

Tempat Pengumpulan dan Pendataan Buku-buku fisika via Internet
1. Photonic Crystal Fibers
2. Photonic Crystal
3. Photonic Crystals Molding the Flow of Light
4. Metamaterials and Plasmonics
5. Silicon Photonics
6. Coarse Wavelength Division Multiplexing
7. Silicon Photonics
8. Integrated Photonics Fundamentals

Sumber:
FISIKA FOREVERMORE
Media Saling Berbagi Ilmu dan Informasi

## Jumat, 26 November 2010

### Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

# 9: Review of Lectures 1 through 5

The exam on Wednesday will cover our first five lectures and the first two homework assignments.

And so I list here the topics the way we discussed them.

Of course, it is not possible to discuss all of them today but I will make a selection.

I recall that we discussed scaling and we used the interesting example of Galileo Galilei--

an animal, and the animal has legs.

And we defined the overall size of the animal as yea big--

we called that "s." And then we said, well, there is here the femur and the femur has length l and thickness d.

It was completely reasonable to say well... that l will have to be proportional to S.

If an animal is ten times larger than another its legs will be typically ten times longer.

Since the mass of the animal must be proportional to its size to the power three it will also be proportional to the length of the femur to the power three, and then came in this key argument--

namely, you don't want the bones to be crushed.

Which is called "yielding" in physics.

If I take a piece of concrete, a block of concrete, and I put too much pressure on it, it starts to crumble.

And that's what Galileo Galilei may have had in mind.

And in order to protect animals who get bigger and bigger and bigger against this crushing, we argued--

and I will not go through that argument now anymore--

that the mass will have to be proportional to d squared, which is the cross-section of the femur.

And so, you see immediately that d squared has to be proportional to l to the third so d must be proportional to the length of the femur to the power one and a half.

So this would mean that if you compare an elephant with a mouse the elephant's overall size is about 100 times larger than a mouse.

You would expect the femur to be about 100 times larger, which is true.

But you would then expect the femur to be about 1,000 times thicker and that turns out to be not true, as we have seen.

In fact, the femur of the elephant is only 100 times thicker, so it scales just as the size.

And the answer lies in the fact that nature doesn't have to protect against crumbling of the bones.

There is a much larger danger, which we call "buckling." And buckling is the phenomenon that the bones do this and if now you put too much pressure on it the bones will break.

And if that's the case, you remember that, in fact, all you have to do is you have to scale d proportional to l, which is not intuitive--

that's not so easy to derive--

but that's the case.

And so the danger, then, that nature protects animals against is this buckling, and when the buckling becomes too much then, I would imagine, the bones, at some point in time--

well, these are tough bones, aren't they?--

[snaps]

will break, and that's what nature tries to prevent.

So that was a scaling argument.

And let's now talk about dot products.

If I look there...

I scan it a little bit in a random way over my topics, so let's now talk about dot products.

I have a vector A...

Ax times x roof, which is the unit vector in the x direction, plus Ay y roof plus Az Z roof.

So these are the three unit vectors in the x, y and z direction.

And these are the x components, y and the z component of the vector A.

I have another vector, B.

B of x, x roof, B of y, y roof, B of z, z roof.

Now, the dot product...

A dot B--

also called the scalar product--

is the same as B dot A and it is defined as Ax Bx plus Ay By plus Az Bz.

And it's a number.

It is a scalar, it is a simple number.

And so this number can be larger than zero--

it can be positive--

it can be equal to zero, it can also be smaller than zero.

They're just dumb numbers.

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

b. Prof. Bernd Surrow, Ph.D.

2. Para Dosen Pendidikan Fisika, FPMIPA, Universitas Pendidikan Indonesia.

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.

## Sabtu, 20 November 2010

### Pusat Penelitian Pendidikan Fisika Indonesia

Indonesian Physics Education Research Group

Visi

Fisika untuk Sekolah

Misi

Perbaikan Terus Menerus dalam bidang Pengajaran Fisika

Program

1. Sains Fisika untuk Anak-anak dan Sekolah Dasar
2. Fisika untuk Sekolah Menengah Pertama
3. Fisika untuk Sekolah Menengah Atas

## Jumat, 19 November 2010

### PUSTAKA FISIKA (PF)

1. Sebuah Visi Pengumpulan 100.000 Buah Buku yang terkait dg Fisika
2. Pengumpulan Data-Data Kefisikaan sebesar 1 Terra byte

Tempat Pengumpulan dan Pendataan Buku-buku fisika via Internet

Fotonik

• Photonic Crystals Molding the Flow of Light 2nd edition
Buku ini terdiri dari 305 halaman yang terbagi dalam 10 bab.(Download Buku)
• Nano Optics and Nano Photonics
• Photonic Waveguides
• Biophotonic
• Electromagnetic Theory and Applications for Photonic Crystals
• Elements of Photonics Volume 1
• Elements of Photonics Volume 2
• Advanced Photonic Structures for Biological and chem
• Electromagnetic Theory and Applications for Photonic

• Sumber:
FISIKA FOREVERMORE
Media Saling Berbagi Ilmu dan Informasi

## Kamis, 18 November 2010

### Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

# 8: Frictional Forces

Today we're going to talk about friction, something...

( students murmuring )

Please, I have a terrible cold.

My voice is down.

Help me to get through this with my voice--

thank you.

We're going to talk about friction which we have never dealt with.

Friction is a tricky thing, not as easy as you may think.

I have an object on a horizontal surface.

The object has a mass, m, gravitational force, mg.

This is the y direction.

This could be the x direction.

There must be a force pushing upwards from the surface to cancel out mg because there's no acceleration in the y direction.

We normally call that the "normal force" because it's normal to this surface and it must be the same as mg.

Otherwise there would be an acceleration in the y direction.

Now I am going to push on this object with a force--

force Walter Lewin.

And we know that the object in the beginning will not start accelerating.

Why is that? That's only possible because there is a frictional force which adjusts itself to exactly counter my force.

I push harder and harder and harder and there comes a time that I win and the object begins to accelerate.

It means that the frictional force--

which is growing all the time as I push harder--

reaches a maximum value which it cannot exceed.

And that maximum value that the friction can achieve--

this is an experimental fact--

is what's called the friction coefficient mu which has no dimension, times this normal force.

We make a distinction between static friction coefficients and kinetic.

This is to break it loose, to get it going.

This is to keep it going when it already has a certain velocity.

The static is always larger than the kinetic for reasons that are quite obvious.

It's a little harder to break it loose.

Once it's going, it's easier to keep it going.

It is fairly easy to measure a friction coefficient by putting an object on an incline and by changing the angle of the incline, increasing it.

This is the angle alpha.

You increase it to the point that the objects start to slide down.

Here is the object.

This is the gravitational force, mg which I will decompose in two forces: one in the y direction--

which I always choose perpendicular to the surface--

and another one in an x direction.

You are free to choose this plus or this plus.

I will now choose this the plus direction.

I am going to decompose them, so I have one component here and this component equals mg times the cosine of alpha.

And I have a component in the x direction which is mg sine alpha.

There is no acceleration in the y direction, so I can be sure that the surface pushes back with a normal force, N and that normal force N must be exactly mg cosine alpha because those are the only two forces in the y direction.

And there is no acceleration in the y direction so this one must be mg cosine alpha.

Now this object wants to slide downhill.

Friction prevents it from doing so so there's going to be a frictional force in this direction.

And as I increase the tilt this frictional force will get larger and larger and larger and then there comes a time that the object will start to slide.

Let us evaluate that very moment that it's just about to break loose.

I'm applying Newton's Second Law.

In this direction, now, the acceleration is still zero but the frictional force has now just reached the maximum value--

because I increase alpha--

so this component will get larger and this component will get larger.

This component will get larger.

This component is still holding its own but then all of a sudden it can't grow any further and it starts to accelerate.

So Newton's Second Law tells me that mg sine alpha minus F f maximum at this point is zero.

And this one is mu static times N, which is mg cosine alpha.

This one is mg sine alpha.

This equals zero.

I lose my mg, and you see that mu of s equals the tangent of alpha.

It's that easy to measure.

So you increase the tilt.

We will do that later until it starts to slip and then at that critical angle that it starts to slip you have a value for mu of s for the static friction coefficient.

It is very nonintuitive that this friction coefficient is completely independent of the mass.

The mass has disappeared.

it's very nonintuitive.

If you double the mass, the angle would be the same given the fact that you have the same kind of object.

The friction coefficient only depends on the materials that you have the materials that are rubbing over each other.

It's also independent of the surface area that is in contact with this incline which is equally nonintuitive.

It's very nonintuitive, but we will see that that's quite accurate within the uncertainties that we can measure it.

If you have a car and you park your car you throw it on the brakes and you put it at an angle and you increase the angle of the slope the friction coefficient for rubber on concrete is about one so the tangent is one, so the angle is about 45 degrees.

So if the road were 45 degrees, the car would start to slide independent of the mass of the car--

no matter whether it's a truck or whether it is a small car--

independent of the width of the tires.

It doesn't enter into it even though you may think it does.

They would both start to slide at the same angle given the fact, of course the same road and the same kind of rubber.

I first want to show you some of this which is at first very qualitative.

I don't want it to become quantitative yet.

The difficulty with these experiments are--

I'm going to use this plank here--

that the moment that my fingers touch this plank or touch the bottom of any of the objects that I'm going to slide, then all hell breaks loose.

A little bit of water on the plank would locally make the friction coefficients larger.

My fingers have chalk on them.

A little bit of chalk on a local place would make the friction coefficient go down.

That's why, at this point, we'll keep it a little qualitative.

The first thing I want to show you is, if I take a rubber puck and I put the rubber puck on this incline and I have a plastic bin--

this is quite smooth, I put it on here--

that it's immediately intuitive that the friction coefficient of this plastic bin will be lower than of the rubber puck.

So when I increase the angle, you expect that first the plastic bin will start to slide and then the rubber puck.

And if I gave you the angles at which that happens you could actually calculate the two values for the friction coefficient--

which I will not do now, but I will do that later.

So all I want you to see--

I hope--

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

b. Prof. Bernd Surrow, Ph.D.

2. Para Dosen Pendidikan Fisika, FPMIPA, Universitas Pendidikan Indonesia.

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.

## Senin, 15 November 2010

### Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

Kelistrikan dan Kemagnetan

Topics covered:

Magnetic field
Lorentz Force
Torques
Electric Motors (DC)
Oscilloscope

Instructor/speaker: Prof. Walter Lewin

### Video

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

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

b. Prof. Bernd Surrow, Ph.D.

#### 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

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)

2. Para Dosen Pendidikan Fisika, FPMIPA, Universitas Pendidikan Indonesia.

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.

## Rabu, 10 November 2010

### Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

Kelistrikan dan Kemagnetan

Topics covered:

Batteries
EMF
Energy Conservation
Power
Kirchhoff's Rules
Circuits
Kelvin Water Dropper

Instructor/speaker: Prof. Walter Lewin

### Video

We have often talked about power supplies, which are devices which maintain a constant potential difference.

Here, we have such a power supply, potential difference V, this being the plus side, and this being the minus side.

I'm going to connect this, I have a resistor here, R, and as a result of this, current will start to flow in this direction, this direction, this direction, so in the power supply, the current flows in this direction.

Through the resistance, the current flows in this direction.

In what direction is the electric field?

The electric field always runs from plus to minus potential.

So right here, in this resistor, the electric field is in this direction, from plus to minus.

But inside the supply it must also go from plus to minus.

And so inside the supply, the electric field is in the direction that opposes the current.

So some kind of a pump mechanism must force the current to go inside the supply, against the electric field.

A boulder does not, all by itself, move up a hill.

And so something is needed to push it.

You remember, with the Van de Graaff, we were spraying charge onto a belt, and then we rotated the belt, and the belt forces the charge into the dome.

It had to overcome the repelling force of the dome.

So work had to be done.

So the energy must come from somewhere.

And in the case of the Van de Graaff, it was clearly the motor that kept the belt running.

In the case of the Windhurst it was I who turned the crank, so I did work.

In the case of common batteries, the ones that you buy in the store, it is chemical energy that provides the energy.

And I will discuss now with you and demonstrate a particular kind of chemical energy, which is one whereby we have a zinc and a copper plate in a solution.

So we have here, H2SO4, and we have here, zinc plate, and we have here a copper plate.

This side will become positive, and this side will become negative.

You will get a potential difference between these two plates.

To understand that really takes quantum mechanics, this goes beyond this course.

But the potential difference that you get is normally something around 1 volt.

The secret, really, is not necessarily in the solution, because if you take two conductors, two different conductors, and you touch them, metal on metal, there will also be a potential difference.

So let's look at his now in some more detail.

We have here a porous barrier that the ions can flow freely from one side to the other.

And we connect them here, with a resistor, and so a current is now flowing.

A current is flowing in this direction, through the resistor, from the plus side of the battery to the minus side, that means inside the battery, the current is flowing like this, and the electric field, here, is in this direction, from plus to minus, but also inside the battery, the electric field must be from plus to minus, so you see again, as we saw here, that the electric field is in the opposite direction of the current.

You will have here SO4 minus ions, and you have copper plus ions in this solution, and here you have zinc plus and you have SO4 minus.

And as current starts to run, SO4 minus ions, which are now the current-carrier inside this battery, is going from the right -- they're going from the right to the left.

Now why would SO4 minus ions travel through an electric field that opposes them?

That opposes their motion?

And they do that because, in doing so, they engage in a chemical reaction which yields more energy than it costs to climb the electric hill.

And while a current is flowing, while the SO4 minus is going from the right to the left, you get fewer SO4 minus ions here, this liquid here remains neutral, so copper plus must disappear.

And it precipitates onto this copper bar.

So it is like copper-plating.

On this side, you get an increase of SO4 minus, therefore you also must get an increase of zinc plus, because, again, this liquid there remains neutral, and that means that some of the zinc is being dissolved, so you get an increase in the concentration of the zinc.

So the charge carriers inside this battery, the SO4 minus ions, travel through this barrier, and they go from here to here, so they travel through an electric field that opposes their motion.

And this happens at the expense of chemical energy.

Now, when the copper solution becomes very dilute, because all the copper has been plated onto the copper, and when this becomes concentrated zinc plus, then the battery stops, and now what you can do, you can run a current in the opposite direction, so you can run a current, now, in this direction, you can force a current to run with another external power supply, and now the chemical reactions will reverse, so now, copper will go back into the solution, so it will dissolve, and now the zinc will be precipitated onto the zinc, and so now, if you do this long enough, you can run the battery again the way it is here.

A car battery is exactly this kind of battery, except that you have lead and lead oxide instead of zinc and copper, but you also have sulfuric acid, like you have here, and a nickel-cadmium battery is well-known, you can charge that, too, those are the ones that are readily available in the stores, you can run your flashlights with these nickel-cadmium batteries.

The symbol for battery that we will be using in our circuits is this, this is the positive side, and this is the negative side, this is a symbol that symbolizes that we are dealing with a -- with a battery.

So let this point be B, and let this point be A, and here, we have a resistor R.

So we have a current going, the current is going in this direction, a current I.

This could be a light bulb, could be your laptop, could be a hair dryer, whatever, that you supply.

If this R is not there, that means that the resistance is infinitely large, that means that the current that is running is 0, then the voltage that we would measure over this battery, which is VB minus VA -- for which I will simply write down, V of the battery -- that voltage we call a curled E, which stands for EMF, which is electromotive force.

I will show you that later.

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

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

b. Prof. Bernd Surrow, Ph.D.

#### 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

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)

2. Para Dosen Pendidikan Fisika, FPMIPA, Universitas Pendidikan Indonesia.

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.

## Selasa, 09 November 2010

### PUSTAKA FISIKA (PF)

1. Sebuah Visi Pengumpulan 100.000 Buah Buku yang terkait dg Fisika
2. Pengumpulan Data-Data Kefisikaan sebesar 1 Terra byte

Tempat Pengumpulan dan Pendataan Buku-buku fisika via Internet

Laser dan Polimer

Polimer

• Fibre- Reinforced Polymer
• The Physical Properties of Organic Monolayers

• Laser

• Prinsip Dasar Laser Polimer Hibrid
• Principles of Lasers and Optics
• Fundamentals of Laser Diode Amplifiers
• Tunable Laser Diodes and Related Optical Sources
• Laser-induced Damage of Optical Materials
• Photonics and Lasers, An Introduction
• Photonics and Lasers, An Introduction
• Principles of Lasers 4th edition
• Ultrashort Laser Pulse Phenomena
• Industrial Applications of Laser
• Tunable Laser Optic

• Sumber:
FISIKA FOREVERMORE
Media Saling Berbagi Ilmu dan Informasi

## Sabtu, 06 November 2010

### Fisika Dasar

FISIKA DASAR

1. Materi Pertemuan ke-1

2. Materi Pertemuan ke-2

3. Materi Pertemuan ke-3

5. Materi Pertemuan ke-5

6. Materi Pertemuan ke-7

7. Materi Pertemuan ke-8

8. Materi Pertemuan ke-9

9. Materi Pertemuan ke-10

10. Materi Pertemuan ke-10 tambahan

11. Materi Pertemuan ke-11

12. Materi Pertemuan ke-12

13. Materi Pertemuan ke-13

SOLUSI TUGAS FISIKA DASAR

1. Solusi Tugas 1

Sumber:
FISIKA FOREVERMORE
Media Saling Berbagi Ilmu dan Informasi

## Senin, 01 November 2010

### Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

Kelistrikan dan Kemagnetan

Topics covered:

Currents
Resistivity
Ohm's Law

Instructor/speaker: Prof. Walter Lewin

### Video

When positive charges move in this direction, then per definition, we say the current goes in this direction.

When negative charges go in this direction, we also say the current goes in that direction, that's just our convention.

If I apply a potential difference over a conductor, then I'm going to create an electric field in that conductor.

And the electrons -- there are free electrons in a conductor -- they can move, but the ions cannot move, because they are frozen into the solid, into the crystal.

And so when a current flows in a conductor, it's always the electrons that are responsible for the current.

The electrons fuel the electric fields, and then the electrons try to make the electric field 0, but they can't succeed, because we keep the potential difference over the conductor.

Often, there is a linear relationship between current and the potential, in which case, we talk about Ohm's Law.

Now, I will try to derive Ohm's Law in a very crude way, a poor man's version, and not really 100 percent kosher, it requires quantum mechanics, which is beyond the course -- beyond this course -- but I will do a job that still gives us some interesting insight into Ohm's Law.

If I start off with a conductor, for instance, copper, at room temperature, 300 degrees Kelvin, the free electrons in copper have a speed, an average speed of about a million meters per second.

So this is the average speed of those free electrons, about a million meters per second.

This in all directions.

It's a chaotic motion.

It's a thermal motion, it's due to the temperature.

The time between collisions -- time between the collisions -- and this is a collision of the free electron with the atoms -- is approximately -- I call it tau -- is about 3 times 10 to the -14 seconds.

No surprise, because the speed is enormously high.

And the number of free electrons in copper per cubic meter, I call that number N, is about 10 to the 29.

There's about one free electron for every atom.

So we get twen- 10 to the 29 free electrons per cubic meter.

So now imagine that I apply a potential difference -- piece of copper -- or any conductor, for that matter -- then the electrons will experience a force which is the charge of the electron, that's my little e times the electric field that I'm creating, because I apply a potential difference.

I realize that the force and the electric field are in opposite directions for electrons, but that's a detail, I'm interested in the magnitudes only.

And so now these electrons will experience an acceleration, which is the force divided by the mass of the electron, and so they will pick up, a speed, between these collisions, which we call the drift velocity, which is A times tau, it's just 8.01.

And so A equals F divided by Me.

F is e E, so we get e times E divided by the mass of the electrons, times tau.

And that is the the drift velocity.

When the electric field goes up, the drift velocity goes up, so the electrons move faster in the direction opposite to the current.

If the time between collisions gets larger, they -- the acceleration lasts longer, so also, they pick up a larger speed, so that's intuitively pleasing.

If we take a specific case, and I take, for instance, copper, and I apply over the -- over a wire -- let's say the wire has a length of 10 meters -- I apply a potential difference I call delta V, but I could have said just V -- I apply there a potential difference of 10 volts, then the electric field -- inside the conductor, now -- is about 1 volt per meter.

And so I can calculate, now, for that specific case, I can calculate what the drift velocity would be.

So the drift velocity of those free electrons would be the charge of the electron, which is 1.6 times 10 to the -19 Coulombs.

The E field is 1, so I can forget about that.

Tau is 3 times 10 to the -14, as long as I'm room temperature, and the mass of the electron is about 10 to the -30 kilograms.

And so, if I didn't slip up, I found that this is 5 times 10 to the -3 meters per second, which is half a centimeter per second.

So imagine, due to the thermal motion, these free electrons move with a million meters per second.

But due to this electric field, they only advance along the wire slowly, like a snail, with a speed on average of half a centimeter per second.

And that goes very much against your and my own intuition, but this is the way it is.

I mean, a turtle would go faster than these electrons.

To go along a 10-meter wire would take half hour.

Something that you never thought of.

That it would take a half hour for these electrons to go along the wire if you apply potential difference of 10 volts, copper 10 meters long.

Now, I want to massage this further, and see whether we can somehow squeeze out Ohm's Law, which is the linear relation between the potential and the current.

So let me start off with a wire which has a cross-section A, and it has a length L, and I put a potential difference over the wire, plus here, and minus there, potential V, so I would get a current in this direction, that's our definition of current, going from plus to minus.

The electrons, of course, are moving in this direction, with the drift velocity.

And so the electric field in here, which is in this direction, that electric field is approximately V divided by L, potential difference divided by distance.

In 1 second, these free electrons will move from left to right over a distance Vd meters.

So if I make any cross-section through this wire, anywhere, I can calculate how many electrons pass through that cross-section in 1 second.

In 1 second, the volume that passes through here, the volume is Vd times A but the number of free electrons per cubic meter is called N, so this is now the number of free electrons that passes, per second, through any cross-section.

And each electron has a charge E, and so this is the current that will flow.

The current, of course, is in this direction, but that's a detail.

If I now substitute the drift velocity, which we have here, I substitute that in there, but then I find that the current -- I get a e squared, the charge squared, I get N, I get tau, I get downstairs, the mass of the electron, and then I get A times the electric field E.

Because I have here, is electric field E.

When you look at this here, that really depends only on the properties of my substance, for a given temperature.

And we give that a name.

We call this sigma, which is called conductivity.

Conductivity.

If I calculate, for copper, the conductivity, at room temperature, that's very easy, because I've given you what N is, on the blackboard there, 10 to the 29, you know what tau is at room temperature, 3 times 10 to the -14, so for copper, at room temperature, you will find about 10 to the 8.

You will see more values for sigma later on during this course.

This is in SI units.

I can massage this a little further, because E is V divided by L, and so I can write now that the current is that sigma times A times V divided by L.

I can write it down a little bit differently, I can say V, therefore, equals L divided by sigma A, times I.

And now, you're staring at Ohm's Law, whether you like it or not, because this is what we call the resistance, capital R.

We often write down rho for 1 over sigma, and rho is called the resistivity.

So either one will do.

So you can also write down -- you can write down V equals I R, and this R, then, is either L divided by sigma A, or L times rho -- let me make it a nicer rho -- divided by A.

That's the same thing.

The units for resistance R is volts per ampere, but we call that ohm.

And so the unit for R is ohm.

And so if you want to know what the unit for rho and sigma is, that follows immediately from the equations.

The unit for rho is then ohm-meters.

So we have derived the resistance here in terms of the dimensions -- namely, the length and the cross-section -- but also in terms of the physics on an atomic scale, which, all by itself, is interesting.

If you look at the resistance, you see it is proportional with the length of your wire through which you drive a current.

Think of this as water trying to go through a pipe.

If you make the pipe longer, the resistance goes up, so that's very intuitively pleasing.

Notice that you have A downstairs.

That means if the pipe is wider, larger cross-section, it's also easier for the current to flow, it's easier for the water to flow.

So that's also quite pleasing.

Ohm's Law, also, often holds for insulators, which are not conductors, even though I have derived it here for conductors, which have these free electrons.

And so now, I want to make a comparison between very good conductors, and very good insulators.

So I'll start off with a -- a chunk of material , cross-sectional area A -- let's take it 1 millimeter by 1 millimeter -- so A is 10 to the -6 square meters.

So here I have a chunk of material, and the length of that material, L, is 1 meter.

Put a potential difference over there, plus here, and minus here.

Current will start to flow in this direction, electrons will flow in this direction.

The question now is, what is the resistance of this chunk of material?

Well, very easy.

You take these equations, you know L and A, so if I tell you what sigma is, then you can immediately calculate what the resistance is.

So let's take, first, a good conductor.

Silver and gold and copper are very good conductors.

They would have values for sigma, 10 to the 8, we just calculated for copper, you've seen in front of your own eyes.

So that means rho would be 10 to the minus 8, it's 1 over sigma.

And so in this particular case, since A is 10 to the -6 the resistance R is simply 10 to the 6th times rho.

Because L is 1 meter.

So it's very easy -- resistance here, R, is 10 to the -2 ohms.

1/100 of an ohm.

For this material if it were copper.

Let's now take a very good insulator.

Glass is an example.

Quartz, porcelain, very good insulators.

Now, sigma, the conductivity, is extremely low.

They vary somewhere from 10 to the -12 through 10 to the -16.

So rho, now, the resistivity, is something like 10 to the 12 to 12 to the +16, and if I take 10 to the 14, just I grab -- I have to grab a number -- then you'll find that R, now, is 10 to the 20 ohms.

A 1 with 20 zeros.

That's an enormous resistance.

So you see the difference -- 22 orders of magnitude difference between a good conductor and a good insulator.

And if I make this potential difference over the wire, if I make that 1 volt, and if I apply Ohm's Law, V equals I R, then I can also calculate the current that is going to flow.

If I R is 1, then the current here is 100 amperes, and the current here is 10 to the -20 amperes, an insignificant current, 10 to the -20 amperes.

I first want to demonstrate to you that Ohm's Law sometimes holds, I will do a demonstration, whereby you have a voltage supply -- put a V in here -- and we change the voltage in a matter of a few seconds from 0 to 4 volts.

This is the plus side, this is the minus side, I have connected it here to a resistor which is 50 ohms -- we use this symbol for a resistor -- and here is a current meter.

And the current meter has negligible resistance, so you can ignore that.

And I'm going to show you on an oscilloscope -- we've never discussed an oscilloscope, but maybe we will in the future -- I'm going to show you, they are projected -- the voltage is go from 0 to 4, versus the current.

And so it will start here, and by the time we reach 4 volts, then we would have reached a current of 4 divided by 50, according to Ohm's Law, I will write down just 4 divided by 50 amperes, which is 0.08 amperes.

And if Ohm's Law holds, then you would find a straight line.

That's the whole idea about Ohm's Law, that the potential difference, linearly proportional to the current.

You double the potential difference, your current doubles.

So let's do that, let's take a look at that, you're going to see that there -- and I have to change my lights so that you get a good shot at it -- oh, it's already going.

So you see, horizontally, we have the current, and vertically, we have the voltage.

And so it takes about a second to go from 0 to 4 -- so this goes from 0 to 4 volts -- and you'll see that the current is beautifully linear.

Yes, I'm blocking it -- oh, no, it's my reflection, that's interesting.

Ohm's Law doesn't allow for that.

So you see how beautifully linear it is.

So now, you may have great confidence in Ohm's Law.

Don't have any confidence in Ohm's Law.

The conductivity sigma is a strong function of the temperature.

If you increase the temperature, then the time tau between collisions goes down, because the speed of these free electrons goes up.

It's a very strong function of temperature.

And so if tau goes down, then clearly, what will happen is that the conductivity will go down.

And that means rho will go up.

And so you get more resistance.

And so when you heat up a substance, the resistance goes up.

A higher temperature, higher resistance.

So the moment that the resistance R becomes a function of the temperature, I call that a total breakdown of V equals I R, a total breakdown of Ohm's Law.

If you look in your book, they say, "Oh, no, no, no, that's not a breakdown.

You just have to adjust the re- the resistance for a different temperature." Well, yes, that's an incredible poor man's way of saving a law that is a very bad law.

Because the temperature itself is a function of current, the higher the current the higher the temperature.

And so now, you get a ratio, V divided by I, which is no longer constant.

It becomes a function of the current.

That's the end of Ohm's Law.

And so I want to show you that if I do the same experiment that I did here, but if I replace this by a light bulb of 50 ohms -- it's a very small light bulb, resistance when it is hot is 50 ohms, when it is cold, it is 7 ohms.

So R cold of the light bulb is roughly 7 ohms, I believe, but I know that when it is hot, it's very close to the 50 ohms.

Think it's a little lower.

What do you expect now?

Well, you expect now, that when the resistance is low in the beginning, you get this, and then when the resistance goes up, you're going to get this.

I may end up a little higher current, because I think the resistance is a little lower than 50 ohms.

And if you see a curve like this, that's not linear anymore.

So that's the end of Ohm's Law.

And that's what I want to show you now.

So, all I do is, here I have this little light bulb -- for those of you who sit close, they can actually see that light bulb start glowing, but that's not important, I really want you to see that V versus I is no longer linear, there you go.

And you see, every time you see this light bulb go on, it heats up, and during the heating up, it, um, the resistance increases.

And it's the end of Ohm's Law, for this light bulb, at least.

It was fine for the other resistor, but it was not fine for this light bulb.

There is another way that I can show you that Ohm's Law is not always doing so well.

I have a 125 volt power supply, so V is 125 volts -- this is the potential difference -- and I have a light bulb, you see it here, that's the light bulb -- the resistance of the light bulb, cold, I believe, is 25 ohms, and hot, is about 250 ohms.

A huge difference.

So if the resistance -- if I take the cold resistance, then I would get 5 amperes, but by the time that the bulb is hot, I would only get half an ampere.

It's a huge difference.

And what I want to show you, again with the oscilloscope, is the current as a function of time.

When you switch on a light bulb, you would expect, if Ohm's Law holds, that when you switch on the current -- or switch on the voltage, I should say -- that you see this.

This is then your 5 amperes.

And that it would stay there.

That's the whole idea.

Namely, that the voltage divided by the current remains a constant.

However, what you're going to see is like this.

Current goes up, but then the resistance goes down, then the resistance goes up, when the current goes up, the resistance goes up, and then therefore the current will go down, and will level off at a level which is substantially below this.

So you're looking there -- you're staring at the breakdown of Ohm's Law.

And so that's what I want to show you now.

So, here we need 125 volts -- and there is the light bulb, and when I throw this switch, you will see the pattern of the current versus time -- you will only see it once, and then we freeze it with the oscilloscope -- turn this off -- so look closely, now.

There it is.

Forget these little ripples that you see on it, it has to do with the way that we produce the 125 volts.

And so you see here, horizontally, time, the time between two adjacent vertical lines is 20 milliseconds.

And so, indeed, very early on, the current surged toward -- to a very high value, and then the filament heats up, and so the resistance goes up, the light bulb, and the current just goes back again.

From the far left to the far right on the screen is about 200 milliseconds.

That's about 2/10 of a second.

And here you get a current level which is way lower than what you get there.

That's a breakdown of Ohm's Law.

It is actually very nice that resistances go up with light bulbs when the temperature goes up.

Because, suppose it were the other way around.

Suppose you turn on a light bulb, and the resistance would go down.

Light bulb got hot, resistance goes down, that means the current goes up.

Instead of down, the current goes up.

That means it gets hotter.

That means the resistance goes even further down.

That means the current goes even further up.

And so what it would mean is that every time you turn on a light bulb, it would, right in front of your eyes, destruct itself.

That's not happening.

It's the other way around.

So, in a way, it's fortunate that the resistance goes up when the light bulbs get hot.

All right.

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

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

b. Prof. Bernd Surrow, Ph.D.

#### 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

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)

2. Para Dosen Pendidikan Fisika, FPMIPA, Universitas Pendidikan Indonesia.

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.