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You have ten days left for your motor, so that's a nice project for Spring Break.
I'll give you some hints.
Keep the friction of your rotor as low as you can.
You can't use any oil, of course; that's not allowed.
Balance your rotor to the best you can.
And try to avoid that the rotor begins to bounce, begins to vibrate, because when it vibrates it loses contact with the current when it needs it so there's no torque.
How will we test your motors?
We do it with a stroboscope, and I've decided to demonstrate to you how we're going to do that.
That's probably the best thing to do.
We here have a disk, and we're going to rotate the disk at 1000 RPM.
Let's assume that is your motor.
And we're going to strobe it with a strobe light until it stands still.
In this case, I have set the strobe so that it will stand still, roughly, and the strobe is now going at 500 RPM, and the motor is going at 1000 RPM.
So this clearly is not the rotation rate of your motor.
In fact, your motor goes twice around between the blinks.
And we'd have no way of knowing that, so we double the frequency.
I'm trying to double it now, double the frequency of the blinking of the strobe light.
And now it stands still again.
So now we may think that your motor is going 1000 RPM, but we don't know yet.
Maybe it's going 3000 R- 2000 RPM.
Maybe 3000 RPM.
So what are we going to do now, we're going to double the frequency.
And so we go now with the strobe light to 2000 RPM.
And what we see now is we see a double image.
So 2000 RPM is out, and any multiple of 2000 RPM is out.
So 4000 RPM is out, 6000, and 8000 is out.
But what is not yet out is 3000 and 5000 and 7000.
So we would have to test for that.
On the other hand, I told you already that this motor is going 1000 RPM, so there's no sense us testing that now.
But during the actual contest, of course, we will continue all the way until we are convinced that we have the right RPM for your motor.
And so that's the way we will do it.
We will put a little bit of white paint on one side of your rotor, so that's the way it will be done.
Of course, if your motor is highly unstable in terms of rotation rate, it will not be easy to get a right correct number.
I want to talk with you about the heart.
The heart, our heart has four chambers.
Looks sort of like this.
The left atrium and right atrium.
Maybe this is why it's -- this is why it's called the heart.
And here is the left and the ri- and the right ventricle.
And here is the aorta.
The sole purpose of the heart is to pump blood.
About 5 quarts per minute, which is 75 gallons per hour, which is 70 barrels per day, which is about 2 million barrels in 75 years.
And it pumps about 70 times per minute.
If the blood to your brain stops for about 5 seconds, you lose consciousness.
So it's five skips of the heartbeat, and you're down on the floor.
And four minutes later, permanent brain damage.
The way the heart works is absolutely mind-boggling.
Extremely complicated.
Nature had one billion years to design it, but nevertheless it's impressive.
Each heart cell is a mini chemical battery, and it pumps ions in or out as it pleases.
In the normal state, each heart cell is minus 80 millivolts on the inside relative to the outside.
There are some cells which are called pacemaker cells.
They are located in a very small area, about 1 square millimeter, near the atrium, the right atrium, and they change their potential from minus 80 millivolts to plus 20 millivolts.
Now why they do that is a different story, which I will not address.
Once they go to plus 20 millivolts, the neighboring cells follow, and a wave propagates over the heart.
I'll make you a drawing shortly.
So the wave first moves over the atrial chambers and then over the ventricle chambers.
And when the cells are at plus 20 millivolts inside relative to the outside, they contract.
So they form a muscle.
The whole heart is one big muscle.
And after about 2/10 of a second, the cells return to minus 80 millivolts, and this wave goes from below to above.
And then the whole thing waits again for another message from the pacemaker cells.
Takes about one second, and then the whole process starts all over.
Now I want to be more precise.
Here is one heart cell.
So this is about 10 microns in size.
And this cell has 80 millivolts with respect to the outside.
So that means it has repelled positive ions, and so the inside is negative.
And there is no E field here outside, because if you put a Gaussian surface around here, there is no net charge inside.
But there is, of course, a electric field across the walls here, from plus to minus.
Now the depolarization, which is the change to the plus 20 millivolt state starts, and it starts from above.
And I will assume now that it is not plus 20 but 0 millivolts, and it's easier to see.
If we have this cell, and the wave is, say, halfway down, and this is now 0 millivolts, then there is no longer minus charge here and no longer plus charge here, because 0 millivolts relative to the outside world.
So there is no electric field across here anymore.
In other words, what the cell has done, it has moved positive ions back in.
But here the situation is still as it was before, so this is still at your minus 80 millivolts.
And if you look now, you have here a minus layer on top of a positive layer.
Positive here, minus on top.
And that creates an electric field, which has roughly the shape of a electric dipole.
It has this shape.
So as the wave goes through the cells, only then do they create a dipole.
And we call this the depolarization.
A little later in time, when this wave has passed, the whole thing is plus 20 millivolts.
I chose 0 here, but it really goes to plus 20.
This is just easier to explain.
So that means that now the inside is plus, so positive ions are now inside, negative ions are outside, and the E field here is again 0.
Now there is the repolarization wave, which comes from below, when it goes back to minus 80 millivolts.
And I will again do the same trick that I did before; I will just assume the wave is halfway, that it is not minus 80 but that it is 0 millivolts.
So there are no charges here, but the charges here are unchanged.
So what do you have now here?
You have a minus layer on top of a plus layer.
So you have exactly what you had before.
So again you get an electric field, which is an electric dipole field, which has again the same shape.
So what's going to happen is the depolarization wave is going to run down, leaves behind here the cells at plus 20 millivolts, when they are contracted, so this part of the heart has already pumped, and it moves down.
And only the cells where the depolarization occurs, that's only the ones on the ring, contribute to that electric dipole field.
If there is no wave, which is a sizeable fraction of the heart, of the cycle, there is no wave, then there is no electric dipole field.
And when the repolarization goes in the other direction, when the heart relaxes because the cells go back to minus 80 millivolts, then again there is an electric dipole field, but only from the cells through which the repolarization wave moves.
And you can very easily see that the electric dipole fields of all these cells here support each other.
So you get a dipole field from the heart.
And so if I make you look at your heart -- so this is you, this is your body, your legs, and this is your arms, and here is your heart, and there goes this wave.
And so here is your electric field that is generated while the wave is going, either depolarization down or repolarization up.
But if there is an electric field, there's going to be a potential difference between different parts of your body.
You look here at your belly button, and you follow this electric field line at your head, there is an E field.
The integral E dot dL gives you a potential difference.
And so now you see that there're going to be potential differences between the various parts of your body.
And that's the idea behind an electrocardiogram.
Typically there are 12 electrodes attached to arms, legs, head, and chest to get as much information about the heart as we can.
And the maximum potential difference between two electrodes, in general, is not more than about 2 to 3 millivolts.
I'd like to show you a healthy heart cardiogram, of a healthy person.
I have that here.
The time here is about 1 second, and from here to here is about 1 millivolt.
The P wave -- we call this the P wave -- that is observed when the atrium is being depolarized, so when the depolarization wave goes over the atrium.
A little later it goes over the ventricle, and you get a larger potential difference because there is more muscle in the ventricle.
So that's why this R wave is higher.
The T wave is the repolarization, when the wave goes back over the ventricles.
The dipole field is in the same direction, remember.
That's the T wave.
It's not known, at least it wasn't known recent- until recently, what causes the U wave.
I talked to a heart expert about this, Professor Cohen at MIT, and I was surprised to learn that it's not known what the U wave is about.
Not everyone's cardiogram looks as healthy as this one.
There is a terrible disease, which 4000 people die of per year in the United States, which is known as ventricular fibrillation, also known as sudden death.
The ventricles fire without any message from the pace wave, pacemaker wave, and there is random, non-synchronous depolarization.
So the heart doesn't pump anymore, in 5 seconds you lose consciousness, on the floor, and in four minutes you, um, have permanent brain damage.
In hospitals, heart patients are being monitored, and as soon as it's noticed that there is something wrong like this, so severe as the fibrillation, ventricular fibrillation, then they apply electric shock treatment.
So you have to be fast, you only have a few minutes before you get brain damage.
And 3000 volts is applied, 1 amperes, for about a tenth of a second.
Large plates are being used on each side of the chest.
And this, of course, is enough to kill the patient.
But it makes little difference, because the patient would have died anyhow.
Heart patients can also get synchronization problems and then they implant a pacemaker, that's a circuit.
And this pacemaker takes over the role from the pacemaker cells.
When the heartbeat rate falls below a certain rate, the artificial pacemaker takes over.
About 10 milliamperes for half a millisecond, and it does it 60 times per minute.
And so it triggers, then, the depolarization wave.
These pacemakers are susceptible to influences from the outside world, and one person's pacemaker stopped, for instance, every 10 seconds due to a radar sweep from a police car.
It's also possible that you get a built-in defibrillator, in other words, a system that gives you electric shocks when sudden death might otherwise occur.
So it senses that something is wrong, that the ventricle is going into fibrillation, and then it applies, all by itself, 650 volts, about 5.5 milliseconds, up to 5 to 10 amperes.
And that's not enough to kill the patient, and the whole idea is sort of a wake-up call to the heart to get it back into synchronization, to get this depolarization wave being synchronized again.
So clearly I would like to show now a heart cardiogram of a student, and I prefer to have a healthy one to avoid some difficulties.
You feel strong?
You a healthy person?
You don't mind volunteering?
Tight pants, we have to do something about.
OK, why don't you sit down.
[laughter].
Well, there's nothing I -- come in.
We'll, we'll, we will, we'll, we'll find a way.
All right, so we have to attach -- we don't have twelve electrodes, we only use three.
And the first one -- that's why I was worried about your tight pants.
Can you roll them up a little?
OK.
Oh, this one goes here.
Let's hope that it makes good contact.
Now the others go on your arm, and we need very good electrical contact, and therefore we put some conducting grease on there.
It will make it a little -- it will make it a bit of a mess, but we'll give you a chance later to clean up.
So let's first put this one -- you're relaxed, right?
Yes, of course.
So can you roll up your sleeve there?
Very good.
And mayb- oh, oh, and maybe you can put this over your arm, yeah, over your -- yeah, that's good.
High up.
Oh man, boy, you have muscles.
[laughter].
