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Yesterday we had 225 motors, and six of those motors went faster than 2000 RPM, which is a reasonable accomplishment.
And the elite is here.
These are the elite, the six highest.
The winner is, um, Jung Eun Lee, I talked to her on the phone last night.
If all goes well, she is here.
Are you here?
Where are you?
There you are.
Why don't you come up so that I can con- congratulate you in person.
I thought about the, the prize for a while, and I decided to give you something that is not particularly high tech, but come up here, give me a European kiss, and another one -- in Europe, we go three.
OK.
Um, the prize that I have for you is a thermometer which goes back to the days of Galileo Galilei -- come here.
Uh, it was designed in the early part of the, um, seventeenth century.
Uh, it doesn't, uh, require any knowledge of 8.02 to explain how it works.
If anything, you need 8.01.
It's not a digital thermometer.
But it's accurate to about 1 degree centigrade, and if you come here, you can tell, you look at these floaters, and the highest floater indicates the temperature.
It's now 72 degrees here.
And I suggest that you brush up on your knowledge of 8.01 so that perhaps next week you can explain to me how it works.
[laughter].
And of course tell your grandchildren about it.
You may want to leave it here.
It's very fragile.
Uh, there is also some package material here, so that you can take it home without breaking it.
So congratulations once more [applause] and of course -- [applause].
Terrific.
And you will join us for dinner on the thirteenth of April with the other five winners.
Thank you very much.
There are two other people who are very special who I want to mention.
And one is a person who is not enrolled in, uh, 8.02, but he did extremely well, and he was very generous.
He was not competing.
His name is Daniel Wendel.
His motor went 4900 RPM.
And then there was Tim Lo.
Is Tim Lo in the audience?
I hope he's going to be there at eleven o'clock.
Tim made a motor -- when I looked at it, I said to myself, it'll never run, but it's so beautiful.
It was so artistic that we introduced a new prize, a second prize, for the most artistic motor, and Tim Lo definitely is the one, by far the best, the most beautiful, the most terrific artistic design.
And so for him I bought a book on modern art -- what else can it be for someone who built such a beautiful motor?
It is here for those of you who want to see it later.
It's very hard to display it on television because it's so delicate.
It's like a birdcage that he built instead of having just -- looks like that it's a birdcage.
It's very nice.
The winning motor I have here, and I'm going to show you the winning motor, and I also want to teach you some, some physics by demonstrating the winning motor to you in a way that you may never have thought of.
So this is the winning motor.
And when we start this motor, the ohmic resistance of the current loop is extremely low.
So the moment that you connect it with your power supply, a very high current will run.
But the moment that the motor starts to rotate, you have a continuous magnetic flux change in these loops, and so now the system will fight itself, and it will immediately kill the current, which is another striking example of Faraday's Law.
I will show you the current of this motor when I block the rotor so that it cannot rotate.
It's about 1.6 amperes.
And you will see the moment that I run the motor that that current plunges by a huge amount.
Striking example of Faraday's Law.
So I now have to first show you this current, so here you see the 1.5 volts, and on the right side you see the current.
There is no current flowing now because the loop is hanging in such a way that the, that it makes no contact with the battery.
And I'm going to try to make it -- there it is.
Do you see the 1.6 amperes on the right?
The current is so high that due to the internal resistance of the power supply, the voltage also plunges.
But you saw the 1.6, right?
Now I'm going to run the motor.
See, the motor is running now, and now look at the current.
Current now, forty milliamperes, thirty milliamperes, fifty milliamperes.
It's forty times lower than when I blocked the rotor.
And so this is one of the reasons why when you have a, a motor, whichever motor it is, it could be just a drill, you try not to block it all of a sudden, because an enormous current will run, and it can actually damage the motors.
So you see here how the current goes down by a factor of forty between running and not running.
All right.
Electric fields can induce electric dipoles in materials, and in case that the, the molecules or the atoms themselves are permanent electric dipoles, an external electric field will make an attempt to align them.
We've discussed that in great detail before when we discussed dielectrics.
And the degree of success depends entirely on how strong the external electric field is and on the temperature.
If the temperature is low, you have very little thermal agitation, then it is easier to align those dipoles.
We have a similar situation with magnetic fields.
If I have an external magnetic field, this can induce in material magnetic dipoles.
And it, uh, induces magnetic dipoles at the atomic scale.
Now in case that the atoms or the molecules themselves have a permanent magnetic dipole moment, then this external field will make an attempt to align these dipoles, and the degree of success depends on the strength of the external field, and again on the temperature.
The lower the temperature, the easier it is to align them.
So the material modifies the external field.
This external field, today I will often call it the vacuum field.
So when you bring material into a vacuum field, the field changes.
The field inside is different from the external field, from the vacuum field.
I first want to remind you of our definition of a magnetic dipole moment.
It's actually very simple how it is defined.
If I have a current -- a loop, could be a rectangle, it doesn't have to be a circle -- and if the current is running in this direction, seen from below clockwise, and if this area is A, then the magnetic dipole moment is simply the current times the area A.
But we define A according to the, the vector A, according to the right-hand corkscrew rule.
If I come from below clockwise, then the vector A is perpendicular to the surface and is then pointing upwards.
And so the magnetic dipole moment, for which we normally write mu, is then also pointing upwards.
And so this is a vector A, which is this normal according to the right-hand corkscrew.
And if I have N of these loops, then the magnetic dipole moment will be N times larger.
Then they will support each other if they're all in the same direction.
I first want to discuss with you diamagnetism.
Diamagnetism.
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