Physics 10 Lecture 9

Today we begin our study of electricity. And one can argue that electricity is the key that is, it's a high-tech. When you think that electricity is not just electricity, but well, electricity includes magnetism, watch this magnet here. Rather, the mysterious, first I have a magnet under the table, and I'm moving that around. But amazing. I mean, before class I was doing a few simple magic tricks, like the French drop, which makes things disappear. You know, whoo magic, actually not the single person in this class did that. I heard a little bit of sarcastic, woo magic. No, woo magic. But look at this. I mean, this is magic magic. It's going on here. Hey, I got a motor. I'll not yet, really. If you haven't played with magnets, magnets are basically an aspect of electricity. And that was known for a long time. And after the first great unification of the forces of physics, well, not the first, the second. The first people talk about the unification of the forces of physics, and it sounds very mysterious. But it used to be that there were two forces that people thought had nothing to do with each other. One was gravity. It pulls me to the ground. And the other is the motion of the moon and the planets, which up in the heavens. And they weren't realized that they were the same thing. The moon is actually moving around the earth, held in place by gravity. That wasn't known. It was Newton. We unified those two. And realized this is the same force. The thing that holds the moon in place is the same thing that keeps me coming in. It's the fact that it's moving in a circle. So it's constantly falling. But it's both gravity. Well, electricity and magnetism are the same thing. In the sense that I'll show you some electricity and there's some magnetism. And they really are. We now know two aspects of the same thing. I'll be talking more about that and how that works. Electricity. This morning, we had an electric power failure. As results, some of these demos aren't going to work. As we turn on switched power to emergency generators. But you know what happens when power goes out, the city basically comes to a halt. You can still get around in mechanical things. You're gasoline automobile runs. But the headlights require electricity. The radio. In fact, automobiles don't really run without internal electricity anymore to make the sparks that make the explosions of the gasoline. But electricity is that the heart of computers. What are computers? They're microscopic little things with little channels where electrons flow. And electric lights and electric everything. And this is only under a year's old. And we only began using electric lights under a year's ago. Let me see like a lot to someone who's only 18 years old to someone who's 62, under a year's ago doesn't seem like that much. But that was the beginning of high tech. Our most convenient source of energy is electricity. When we send a radio signal, we are actually using electricity because the electricity in the radio transmitter, the moving electrons in the radio transmitter create an electric wave. And that electric wave is how we transmit information from one place to another. Almost exclusively using electricity. Electric wave, micro wave is an electric wave. Radio wave is an electric wave light. We now know is an electric wave, just a higher frequency electric wave. So to understand the high tech world, there's probably nothing more important to understand than electricity. Most of what we use in electric motor, our iPods. All this is based on electricity. So let's talk about what it is. It came out historically in a peculiar way. When Faraday was looking at electricity, it was basically a parlor tricks. I mean, you have things like this and you rub things together. Now, sometimes you rub things together when you're walking on a rug. And then you walk and you take a door not and you get a little spark. And what's happening there is what was called static electricity. This was the initial type of electricity. It was the only kind that was known. Let me make this off. Rub this on this, this, by the way, is donation from a little kitten that left it in its will or the good of science. Electricity is really turning off today, isn't it? Let me try this. When this sort of electric experiment is done, this was an example of what was called static electricity and there it's repelling. If I could stop it, look at that, I can stop it. They were perpally each other. Wow, just like magnets, it's not a strong. So what's the big deal? When Faraday was doing these kind of experiments and trying to understand this, what would call an electric force, it never looked like it would have a practical application. I mean, so I rub this with some, I think it's actually a rabbit. Rabbit fur, and I rub this with rabbit fur, and something happens to both of them, and who knows what it is. But when I do that, they repell each other, big deal. It's a curiosity. Other tracking each other. Now they do nothing. This is, we have some glass rods here. We don't seem to have any glass rods. Now they're attracting each other. Sometimes electric forces attract, sometimes they repel. What's going on? Only a scientist would be interested in that. What do you do with this? Faraday, I think a powerful story about Faraday who showed this to a prime minister of England, and the prime minister said, okay, but what good is it? And according to the story, which is probably not true, Faraday said, Sir, I assure you that someday you'll be able to tax it. I love that story. I've got the prime minister's interest. But it was a few hundred years before it could be taxed, before it really became practical. It started around 1900 when, a little before that, when better ways of creating electricity were invented. And the first real thing was electric light bulb. Actually prior to that electricity was used for arc lamps. They had, they found that if you made a spark, you can get light from a spark. If you made a spark across two materials, one of which was lime, the lime would glow and you get a very bright light out of it. And so this was used first by the military, of course. They made these lime lights, and it could be used to shine on the enemy. Or if you're a seeding a fortification of some sort, and at night you want to know where you blew all of it. Soon after that it was adapted to use in the theater, where people would come in like this, and they'd be a performer. And dark, and they didn't have this light. So what they had was called the lime light, which was one of these things with a reflector, it had the spark, and the spark would be continuous. And you'd get that bright light as the lime glowed, and they would shine it on the performer, and it gave rise to our expression being in the lime light. I haven't used lime lights for a while, although these are still some of the brightest lights that you can make. Then Edison came up with a way of making it practical, so you could take the lime light, instead of having an in the theater with cable adjustments in a spot, you could have it in your home. The idea was to take electricity and run it through a wire, and it would make the wire hot. And as the hot wire glows for reasons that we will be talking about this week in next, as that wire glows it gives off light, and this was his idea for a practical invention that could be used in the home. Describe it in terms like that. It seems almost ridiculous that this would be a useful thing. But this was the tungsten filament bulb. In the tungsten filament bulb, what you do is you have electricity going to wire. You surround the wire with a glass bulb, because it turns out if you don't, that the hot wire will react with the oxygen in the air, and will basically oxidize and crumble. So you put in here either a vacuum, or you put it a gas, such an inert gas, maybe nitrogen, or argon gas, some gas that doesn't react with a wire. Then you send electricity through it. And what is sending electricity? Well, they didn't know. When you send electricity through it, where do you get electricity? Get from a battery. Batteries are expensive. But it was the only source of electricity. Well, this, this kind of electricity. But the invention of the battery made electricity something that you could carry around. And send electricity through this, it gets hot. Get hot and it goes off light. The whole idea that this will be practical seems amazing to me today. But we still get most of our life from this. Now we have a different kind of light. Well, the fluorescent light. And it's typical fluorescent light also sends electricity in. A fluorescent light basically works as a big, elongated spark on the inside. Now fluorescent lights are a little bit more subtle than that. Because they put a coating. The spark in turns out emits an invisible kind of light known as ultraviolet light. That's absorbed on the surface. And it makes the surface glow. So the ultraviolet light gets here. Makes this glow and then this emits visible light. Like basically, it's a long spark. Like the line light was. We're using electricity for most of our lighting. Not all. You can buy a lantern where you use gasoline to make light. But almost everywhere. Even little head lamps. When I was a teenager, I used to go cave exploring. And I had a little chemical lamp on my head, which would burn acetone. What was it was? Was the gas? Acetylene. Acetylene. Thank you. You burn a settling. Someone here uses these things besides me. What do you use it for? Same thing. Okay. And these days, everybody just uses electricity. Electricity is so much more convenient. So what is electricity? Okay. Well, we know what it is. In almost, we know that atoms are made out of protons and electrons. We know there's a great force between them. That's why the electron stays in orbit. Just like the planet stay in orbit. Because of the gravitational force, the electron is saying orbit because of this attraction force. Unlike gravity though, if you have two electrons, they were a pellet each other. They were electron and a proton. They attract each other. If you two protons, they were a pellet each other. So someone came up with a simple kind of math to describe this. They said, let's assume that the electron has something called the electric charge. I don't know what's the electron, but they knew that that you could make things that would repel. Like things would repel. They decided they'd be somewhat a positive charge and a negative charge. And the properties are that two negative charge things will repel each other. And two positive charge things will repel each other, but a positive and a negative will attract each other. That was the idea. They didn't know about electrons and protons at the time. Now we do. We know that the thing that was called positive charge is actually a proton. There are other things that are positive charge, but almost everything in our experience that has a positive charge is a proton. The negative charge is an electron. And there are some things that have no charge. What one of them is the neutron. Inside the nucleus, there are these neutrons. They exert essentially no force on an electron or on a proton. Well, if they get really close, they will exert a force on the proton from the nuclear force. This is a different kind of force. The neutron does experience the nuclear force. It has gravity, it just doesn't have an electric force. So what is an electric charge? Deep down what is it? Well, that's a question that has no answer. Try to imagine what kind of answer you would like. Some people want to explain this in terms of little mechanical things, because they feel mechanical things are things I understand. And so if they could explain this in terms of mechanical things, but it turns out to be the other way around. Mechanical things can mostly be understood in terms of electricity. Electricity is one of those things that's so fundamental that almost everything including the atoms are explained in terms of electricity. So if you think, I don't understand anything less than mechanical. You're going to have a hard time with electricity. Is it some kind of mechanical things that can be explained in terms of electricity? And here are the fundamental properties, positive things, attractive things, but like charges, repell whether or not two electrons or two protons. And the force can be measured between these things. You take two protons and you hold them next to each other and there will be this big force of repulsion, big compared to an electron. And you can measure that. And if you double the distance, the force is one quarter. It's just like gravity. You know, you get two things that are attracting each other from gravity and you go ten times as far away. And the force is one hundred. It's ten squared, smaller. Same thing of electricity. Whether it's repulsive or attractive, the force is what we call an inverse square. You go three times further away. The force is nine times weaker. So these are the basic properties of the electric force. I can design something that will continuously rub. Still disappointed this hasn't worked better. Sometimes it's just a very damp day and the electricity tends to be lost. And the electrons fly off into the air. When that happens, you know, electrons left. You do, well, there's an ice repulsion. Stop it. What's supposed, what I did is I had something that rub continuously. Had a little machine to rub continuously. And this thing would pick up some charge. And this thing would pick up some charge. They'd be opposite. What's happening is we're rubbing electrons off. These atoms with a mechanical, some of the electron stick. Let's say they stick to this. This thing comes up. Now, if it comes up another property of electrons, defines what we mean by a metal. Metals can almost be defined in terms of their electrical properties. It's one of the definitions you can come up with. A metal is something that acts like an electron pipe. When an electron gets through a piece of metal, it can move freely within the metal. Move freely inside of this thing. That's amazing by itself. But the combination of electrons and metals means wires are possible, which is what enables us to send electricity so conveniently. The way it works in a metal, you have all these atoms and they have electrons in the last electron on each atom. And move easily from one atom to the next. So if a one end you take an electron off, and the other end you put an electron there, they'll all jump over one atom. These electrons can move around. They're actually jumping from atoms to atoms, but for all practical purposes, it says if they're moving freely within the metal. You put too many of them in the metal and they start repelling each other. Normally a metal has a nucleus and the electrons, and so there's no net charge. If you have a positive proton and an electron moving around it, and you come over here with another charge, it'll be repelled from this and attracted to this and the other effect will be zero. So normally you will see an effect not much of an effect. But if the electrons can move around freely, they can move from one to the other in a metal. So let me get back to where I was. Here I have this experiment where what I'm trying to do is I have something that rubs against this. It gets some charge from the rubbing. I bring it up here to all touches a piece of metal. The electrons can jump to the metal and I'll put this on top of the big metal sphere. Then this will come down and rub again. Come up to the metal, the opposite electrons. If I get this thing going, I can have something that's continuously rubbing, not just once, and putting all of its electrons on the metal. So I have to take a belt that rubs against something like this and then comes in contact with the metal. Oh, look at that. Here we have one. Inside of this, there is a belt. It rubs again something down there, brings the electrons off onto the metal and then they stay up here. The belt comes down and gets another one goes on up. So let's see if this works. Put some electrons on it. Now the belt's running. Opposite from the direction I thought it was. And so if that's happening, then there should be, ooh. Sparks. You might think I'm undergoing excruciating pain with these sparks. Actually, I'm not. I've done this demo now for five years and so each spark jumps to a permanently scarred and insensitive part of my hand. And so the pain is actually quite favorable. I wonder if we could turn the lights down just to see these sparks better. You're not a quite big. I'll get bigger ones if I take another piece of metal. Thanks a lot for the chart. Oops. Thanks a lot for the chart job. You get that spark. Those of you over there can see this. Some of you heard thunder this morning. It's amazing that people didn't realize that the thunder is lightning is basically a big spark. That was the hypothesis of Benjamin Franklin. He convinced everybody in the world that that's what was going on. A big spark. How does that happen? Well, the mechanism is actually very similar. You have water droplets rubbing up against the air and then moving to a higher part. It looks like I'm going to hear that thing. I get a little spark. It's actually quite that sparks are this kind of electricity is often described in terms of the voltage. And one of the things I want you to understand is what does it mean to be volts? So these sparks are probably about 100,200,000 volts. 100,000 volts. What does a volt mean? A volt is the energy of the electron. That's what it means. So these electrons when they come holling off to hit my hand have an energy. The term is usually called an electron volt. Let me talk about an electron volt. One electron volt is an energy off and here's a number you don't need to know. 1.6 times 10 to the minus 19 joules. What I would you need to know is it's pretty tiny. That's pretty good. It sparks. And here we have something which is now basically 100,000 volts. Why do I say volts? Volts instead of electron volts. Typically if you have a lot of electrons and they all have the same energy you say this is the bolts, the bolt of the collection of electrons. But each electron has an energy of this. The total energy that you have it depends on how many electrons you have. So in fact the energy is equal to the number of electrons times the volts. Now give it an electron volts. You have to multiply by this if you need joules. But you don't need to know that number. You always look it up. Physics students get to know that number because they use it in homework exercises. But I don't see any reason why you have to know that. What you need to know is it's tiny. Now how do I get a tiny number into a large number of joules? The answer is you have a lot of electrons. Now Avogado's number which I wrote down here wrong one. Six times 10 to the 23rd. That's the number of atoms. In one what's called a mole for hydrogen it's one gram. For carbon it's 12 grams. You look up the atomic weight. It's a few grams for most things. And that's how many atoms you have. You take that many atoms and you multiply by this many joules. And you get a number that's a few joules. See this is a small number but you have a lot of them. So if you want a lot of energy you want joules like of energy and you need a lot of electrons. And that's what we use in these lines. A lot of electrons. Typically Avogado's number worth of electrons. So electricity is moving electrons. Let me take this thing. We used to do this with a student you find a student who likes light hair. When I was a kid that usually used to be a woman. Long hair and you put that. Now what happens is some of the electrons will jump on this thing. So this thing gets the electrons and since it's touching you get electrons too. And They repell each other. Imagine how much more interesting this was when you had someone with long hair holding your hand on this. And what happens if I put my hand in here to touch these things? It's grabbing me. Why is that? My hand isn't charged. So why is it going to my hand? And in douce charge. Let me show you how this works. My hand is actually a fairly good conducted. Electricity flows through my blood fairly easily. Suppose I have this big thing here. That's full of electrons. I put my hand here which I'll think of as a piece of metal. So joke about physicists always assuming everything is a sphere because that's the only thing they know how to calculate. I bring my hand up here to this. Now what happens? There are electrons in here in their positive charges. The positive charges are in the atoms they can't move but the electrons can move around. The electrons are repelled by these charges. So the electrons move all electrons move over to one side. They're positive and negative charges everywhere. The electrons all move over here. We're large number that move over there leaving behind positive charges. Now because they're further away suppose I have something on here that also has a negative charge on. It's attracted to the positive charges and it repelled by the a negative negative charges. But the negative charges are further away. As a result, you're pulled to my hand. Let me demonstrate that again. Here we have the hair. Really standing up nicely. I have my metal sphere. The metal sphere has electrons in it but the electrons are being pushed away by those electrons. So you have more electrons over here leaving behind the positive charges. The positive charges then attract the negative-le charged hair. They're delicious spark. Not much of a spark. This in charge is up quickly today. Hence if you do very well on a dry day. When the electrons leave this thing, these things are no longer repelled by the sphere. They come down and lose their charge too. So, isn't that a music? What are you going to do with this thing? Any practical use? Some day you'll text it. This one is cute. The bill who sets up this demo for me doesn't like me to do this one. That's because he sweeps up afterwards to clean it up. Little bits of puffed rice. By the minute you've got a grocery store, they're getting charged and they fly away. My hair, I can feel my hair being pulled. Okay, electricity. What good is it? It's the basis of all high tech. When electrons travel through the metal, they don't go in straight lines. They bang into the atoms, but the electric force pulls them across. So if I have an electric charge down here, that's attracting the electron, the electron will come. But it doesn't go in a straight line. It bounces from one atom to another. That's called resistance. And the result is you'll notice it's not going any faster than the bottom than it is at the top. It picks up energy from falling, from being pulled by the electric field, by the electric charge down here. But as it's falling, it bangs into another atom, gives up that energy. Then it starts falling again. I call it falling. What I mean is it's being attracted by the opposite charge down to the other end in this wire. So here it is coming down, but it's losing its energy. It picks up energy from the attraction, and then loses it on this. That's what makes this thing heat up. So this gets hot. That's what makes the electric light bulb work. We just draw a little more explicit diagram for the electric light bulb. Electric light bulb basically works like this. You have a piece of metal at the bottom, which you screw into. And in the middle here, you have another conductor. The rest is not conducting. This is the conductor. The rest is just plastic or glass. So this is all connected together, but it's plastic. So wire coming from this becomes up here. And the wire connects down to this. Now when you plug this in, it makes contact with the plug. You screw this in, and here's one electrical contact, and here's the other electrical contact. And the electricity is made to flow from one to the other. Now electricity has to go in a loop. The reason is, if the electrons don't have any place to go, they'll just start piling up, and pretty soon they'll repel other electrons from coming. So unless the electron is going in a loop, the electron will stop itself. It's like water. And the analogy between this and water is a very good one. If you have a water pipe, you can start filling up your bulb, but pretty soon you won't be able to put anymore in it. Yet it's the flow of electricity that you need to heat up the material that's going through. So electricity has to flow in a circle. You'll sometimes see the so-called high tension lines that are supported on pieces of glass. They go over you up in the air. Electricity flowing through them. There has to be a way for that electricity to come back. Down here at the end, you may hook up through the wires of your house and electric light bulb, but then the wire has to come back. And how does it come back? It could be a separate wire, or sometimes the back path is actually just through the ground. And if electricity flowing through the ground works both ways, you can have two wires, so you can have it come back through the ground. But it has to come in a loop. If the wire comes back this way instead of going through the ground, we have a separate wire, come back like this. Electricity is going in a circle. The electrons are losing energy as they go through here, but you don't want them to lose too much. You have to have a metal that doesn't heat up very much. And that's an important issue in designing electrical power lines, which we'll get to. The trick turns out to be the used very high voltage. I'll explain why a little bit later. To the electricity travels this way, and then comes back this way. Why doesn't it jump across? Why does it go all the way to the end and to the light bulb? Why not just come straight across here with air there? And air does not conduct electricity very well. So it goes along the wire instead instead of coming back the other way. Again, imagine it like water pipes. These water pipes going here. In the end the water pipe goes through something. It maybe turns a windmill or windmill. It wouldn't turn a windmill, but turns a mill of some sort. That's some work and then comes back. What if the water had a way of breaking across, it wouldn't go through this resistance, it would come right back. Now, if a bird is sitting on this thing, it's a bird. Sitting on the wire, what's going to happen? Some electrons will flow into that bird. And the bird will come charged up. Just like some electrons flow on here, just a little bit. Pretty soon there are some electrons in that bird. When that happens, they repel additional electrons. So no more electrons go, and they electricity continues to flow this way. They can't go up into the bird because the bird doesn't complete the circuit. There's no way to lose those electrons. The bird just picks up the electrons. Likewise, if you're, you could stand on the wire. And you'd be there and a little bit of electrons, but it just takes a few. To come in and pretty soon they repel the other electrons in your state. However, if the bird happens to go from here to here, but one foot on each wire, if they live in ostrich. Once for any wire, then the electrons come here. And they don't have to stay there. And repel the other electrons. They come right across and come back this way. So now there's nothing to impede it. You could get a lot of electrons flowing. That electrons will start heating up the bird. And we could do this at San Quentin, even. You should be done that way. It's a story behind that that's in the text that I don't know if we'll really touch on. About the debate that took place between Nicola Tesla and Edison early in the century. They both believed that electricity was the high coming high tech. I mean, there's so many high techs that's beginning of the century. I think that time was perhaps more disruptive than the high tech right now. With the advent of electricity and airplanes and automobiles and gasoline and radio, this is all happening around the early 1900s. Very, very disruptive time and technologies were growing very quickly, very rapidly spreading into many new areas. Right, electricity has to flow like that in order to keep on flowing. Otherwise, it just stops. You charge this thing up and it'll charge up to a certain amount and then it'll stop charging. Because electrons here will repel. Electrons up here will repel the other electrons and keep them from jumping off. This rubber belt. Electrons have to flow in circles. Let me show what resistance looks like. When you have a lot of electrons. A feature of electricity, which is, turning that to be very important, is this resistance is very useful in a light bulb because you heat up the wire. But it's very bad in these transmission lines. Because you don't want to heat up the transmission line, you just want that electricity to flow. So you try to make transmission lines to have as little resistance as possible. There is a development that could truly transform our technology. So far, it has had just a little bit of an impact. That's called a superconductor. In a superconductor, this would be similar, except it wouldn't lose energy as it's moving. It can keep up its velocity and not change at all. That's what a superconductor is. Something where electrons flow, but give no energy at all to this. Now, superconductors, there are two ways to get good conductors. First, let me mention, if I want to slow this down, there's a way to do it. Shake it. If I shake it, as this electron is coming down, it's more likely to hit something if these things are shaking. So imagine the following, imagine you are an electron. And as an electron there, on the other side of the dance hall, you find a positive charge, something that truly attracts you. And so you are drawn to it and you're a pole to that, but there's a dance floor here. Now, I'd like you to imagine whether you will get their quicker if it's a fast dance or a slow dance. Imagine it's swing or something where people are spinning all over the place and dancing. You try to work your way there. And you'll take you a long time, if it's a slow dance. And you just sort of get there right away. The fact is resistance is higher when the molecules are moving a lot. When these molecules are bouncing around, it's harder for the electron to get through. That means that if you cool this thing down, so the molecules are not vibrating as much and the resistance goes down. It's something we'll find that backwards. I think if you heat things up, the electrons move faster. Actually, most of the electron motion is just bouncing. A little bit of drift in the forward direction. So if you cool something down, the resistance goes down, but a remarkable thing happens when you get to very low temperatures. And that is for some metals, the resistance disappears all together. That's called superconductivity. I cannot explain superconductivity to you based on the physics of electricity. It requires a concept of quantum mechanics. Let me just state what that is. It'll be exposed to it. And we'll come back to this when we explicitly discuss quantum mechanics later in the semester. For some materials, when you pull them down, they're develops what's called an energy gap. An electron can have a very low amount of energy, or it can have an energy which is higher than that, by what's called the gap energy, but nothing in between. That's a quantum mechanical effect. We'll be talking a lot about that when we come to quantum mechanics. One of the key elements, the key elements to quantum mechanics. One is that electrons are behave like waves. And the other is the development of these energy gaps. Now the fact that the energy gap develops means the electron, when it bounces into an atom, it can't change its energy. Unless the energy is equal to the gap energy. As a result, it doesn't lose energy. It just keeps on going. Superconductors right now, it's looking up as we have all required very low temperatures. At least as low as liquid nitrogen, that liquid air that I pour around here. If that's a lot colder than any of your refrigerators, we have no superconductors that work at room temperature, no superconductors that work even at ordinary refrigerators, they have to get down very, very cold to work. We understand that for the simple superconductors. We don't truly understand all superconductors. There's something called high temperature, superconductors. Once you've been know about these, I temperature. High temperature means basically liquid air temperature. It's not really high compared to anything you experience in your life, a lot colder than your refrigerator, but it's not as close to absolute zero. Typically, a hundred degrees above absolute zero instead of 0.73 above absolute zero, which is the freezing point of water. These so-called high temperatures, superconductors are not really understood. We don't understand in detail why they have such a, why they develop this energy gap at such relatively high temperatures. If someday we do understand these, maybe then, we'll be able to make superconductors that will actually operate at room temperature. That is a revolution. If we start having electricity, superconductors that will operate at ordinary, living temperatures, you will see a transformation that is enormous in the way we conduct our lives. Everything from light bulbs to electric engines to electric transmission. We'll talk more about that as the class goes on. I want you to be aware of that. Magnetism was always thought to be something different. Magnetism is also mysterious. It's first discovered in natural rocks. It's what's called a loadstone. It's a piece of rock that is magnetized. Nobody knew what that meant to be magnetized, but here's a needle. That's also magnetized. And if I put a magnet in here, try, it's magnet. It repels, the tracks. This is a big effect compared to electrical. But people are very excited about magnetic forces. You haven't played with magnets. You should go to a toy store and buy some magnets. That was demonstrating this one for class. This is a magnet going around here the table. Well, my magnet just stuck to something under the table. Now we have these magnets that stick to metal. This is such a strong magnet here that it's actually well, these are pretty good magnets too. Look at this. Of course. That magnet is going crazy. Well, sounds were discovered. They're receiving like this before. Where would you think of it? Right. They're only magnets. To me, magnets are one of the most mysterious things in physics. It's force. At a distance, you think we'd have some practical use for that? Look at these. They kind of float. Magnetic levitation. Of course, magnetism was a great military secret for a while. The fact that if you took a magnetic load stone and hung it like this, it would point in the same direction. Eventually, you spin it and wind up pointing in the same direction. Once it settled down, I became the magnetic compass. People used to know what it pointed at. People said maybe it pointed at the North Star. Well, it doesn't point at the North Star. It turns out that the Earth is a magnet. Magnets are lack of electricity except it seems to be more powerful. It's a bigger effect. Real things. I mean, big forces. Magnets are also mysterious. We now know that magnetism is actually an aspect of electricity. It comes about from moving electrons. This is strange, but this is what you need to understand. You have two electrons here and they will repel each other. But if they're moving, there's a slight attraction. You notice, the repulsion will be less. As they're moving, you're saying force that depends on their motion. This is weird. You don't expect the force to depend on the motion. But a moving electron will develop this additional electron force that we call the magnetic force. If they're moving fast enough, the faster they go, the more the force is. As they get up to near the speed of light, the force of attraction from the emotion is as strong as the force of repulsion. But if they're moving much less in the speed of light, the effect is less. I here have two wires. I'm going to have electrons move in those two wires. So let me have the electrons move in the same direction in the two wires. Actually, I have quite a few electrons going in here. We have this hooked up to a big battery. And they repel each other. You see that? When I have electrons going the same, they repel each other. It's just a motion of the electrons. The force that comes about from moving electrons. That's only there when they're moving is actually magnetism. That's what magnetism is. It's a different aspect of the electric force. Permanent magnets like this. Whoa, or an electron's moving in this. It's just sort of sitting there. Why should this have? Why are there forces with these things? So the electrons aren't moving. Turns out electrons are moving in this thing. Well, of course, they're moving in the atom. But the part of the electron motion that gives rise to what we call permanent magnetism is from the discovery that every electron is spinning. Every electron moves in it. Well, it stays in the same place, but it spins. And so the electric charge is moving in a circle. And that makes for this magnetic force. So typically in most materials, the electrons are all spinning. But they're spinning in random directions. Up here, the electron might be going this way, this one may be going this way, this one may be going this way, and so on. And it all cancels out. But when you have a permanent magnet, what you do is you take all those electrons, and you all line them up, so they're all spinning in the same way, so the magnetism adds up. And that's what a permanent magnet is. From electron spin. Spin is another one of these truly mysterious things. We know the electron spinning from the old angle of momentum. Remember, a little momentum. We didn't discuss much about it. But we had that seed here when you're spinning like this, and you watch the Olympics. And you're going like this, and then you bring your hands in. You go faster. That's angular momentum. And when you pull it in, you have to go faster, have the same angular momentum. And the skinny ice skaters, it go really fast. It's after pull all their mass in real close to get going so fast. It was sort of fun to watch that. If you're a hunkier ice skater, you can't keep going so fast, because you can't pull all your mass into the same line. But the electron is also spinning. We know this because it has this angular momentum, it can change its angular momentum. The electron is spinning, and that's what the electron is trying to do. But we can also have electrons moving in wires. And that gives rise to magnetism. Here repelling each other. If I have them going in opposite directions, they attract each other. I could have electrons moving in a circle like this. If they move in a circle, it's almost like an electron spinning, except here it's not the spinning. It's moving in a circle. And that creates magnetism inside of this thing. And I could try that here. So here I have another little magnet. And I'll stick it inside. This thing's called a solenoid. Whenever you wind wires in a circle like that, and look at that. When I have electricity flowing in it, it suddenly makes the magnet going one direction. Suppose I have the electricity going in the opposite. See, the little red tip is always going onto your left. Suppose I make the electricity going in the opposite direction. I think that goes the opposite way. So here are the key things that I want you to know. Magnetism comes about by the motion of electric charge. Static electricity is just when the charge is sitting there. Like charges will repell, opposite charges will attract. The charges are moving. There's another force. That force will cause the electrons to attract or repell, depending on whether they're moving in the same direction or the opposite direction. That's a magnetic force. The magnetic force for permanent magnets comes about from electron spin. But a magnetic force can also be made by winding a coil like this. This is a very practical device. I'll show you something next time, where you take a coil like this, and you use it to open and close your car doors. Seriously, that's how they work. They have a coil of wire. Here's the lock. You can open the door or you can lock the door. I'm getting mechanical motion. I'm getting mechanical motion by changing electricity. So by using a switch, I can make this thing a one way or the other. That's the way car doors work. You push a button and you click as your door is locked. Most cars these days have these magnetic switches. You can imagine making a motor out of this. You have some electricity. And if you do this, just right. And switch it just right. You can make something turn. Make a motor. It's actually how motor is work. Most electric motors work on magnetism. Here's an example. We have these magnets here. The electric motors in you just drive. So you're all all iPods. The ones that had disk drives in them. They would have magnets permanent magnets and electricity flowing. Right now there's no electricity flowing. So let me make electricity low. You probably have to turn on some power here. Let's see. Do you have that on do it? That'll be awesome power. I do this just right. Right on. Oops. I have to get it just right. Get it working. Yeah. Get just working right. I might put it to sing to make a motor. Well, after my timing, just right. So that what I want is the magnetism that's created here to be repelled by this. So it goes away. And it's attracted by this one. But when it comes over here, I want to switch the switch. So now it's repelled. And it's attracted to this one. And then switch the electricity. So it's attracted. And then repelled. If I do that just right, I have what's called an electric motor. Now, there is a way to make the switching work just right. And that is if I automate it. If I design this thing, so the switch happens automatically, every time it goes halfway around. And it'll get going faster and faster and faster. So maybe I'll set up one of those for next time, where I can get that happening automatically. That's one of the electric motor does. And electric motor uses electricity to create a magnetism. And then uses that magnetism. These days, often, with permanent magnets. But sometimes, instead of having permanent magnets here, we'll have as another coil. Electric motors are using electricity to make something move, typically, in a circle. That's the idea of an electric motor. We're turning electricity into mechanical motion. This thing is now pointing back in the same direction as it was. It's pointing back in that direction because the earth is a giant magnet. When we talk about the earth's magnetism, so here's the earth. If you have a little magnet like this, the end at point north is sometimes colored blue, as it is here. Sometimes it has a little end on it. The other end has a little south end thing on it. It's a magnet. Like this. The plot, plot that points to the north is called the north point again, or often called the north pole of the magnet. Now magnets unlike electricity have a peculiar feature. Remember I pointed out that when I put my hand near that electric source, this thing has a lot of electrons on it. That some of the electrons move over to this side leaving positive charges behind. There's no net charge on this thing, but the charges of separate. Magnets are all the same way. If I break this magnet in half and take away, just part, I still have a north end, and now this thing becomes the south end of the magnet. Magnets are all dual-ended like this. This is this little charge, and one will be a north side, the other will be a south side. Here is the rule for magnets. If I take two magnets, I can get rid of them, and I take two north poles, and they bring them close to each other, they will repel. They will repel. If I take two south poles, this is a try to over here, two south poles, they likewise will repel. But if I take a north pole in the south pole, they attract, or a south pole in the north pole, they attract, very much like charges. A magnet that's had the same property that north poles like poles, they would call them poles, because they pointed the north pole. Here's the pole. This is the north pole. The earth is a big magnet. Here's a little bit of irony, and you can have fun with your friends, or parents, or anybody else you want to tease. The north pole is attracted to this part of the earth. So what kind of a magnet is that part of the earth? Well, if the north pole of your magnet is attracted to it, it must be a south magnetic pole. Down here, there must be a north magnetic pole, and here's your big magnet for the earth. So the fact that some people, this terminology can get very confusing. You call this the north pointing pole, but the common terminology, it's the north pole of the magnet. Because it points to the north. If it has a little bit too much friction here, but this one here will point to the north pole of the earth. This is the north pointing magnet. It's pointed to the north pole, because the north pole of the earth has magnetism that attracts it. Our definition, the magnetism at the north pole, is a south magnetic pole. And the magnetism at the south pole is a north magnetic pole. So it attracts the south end of the magnet. So no matter where in the earth you have these things, they tend to point in this direction, unless there's a lot of metal nearby. You take a magnet like this, and it'll point to the north pole as long as the earth's magnetism was what dominates. If I happen to have a piece of metal nearby, it's a single point in some anomalous direction. And the magnetistic compass will not work. We'll not give me the direction that I want that I need to know. Why is it when you break a magnet? You can't break off the north pole and just have north poles. The answer is because a magnet is actually a permanent magnet that comes from the spin of the electrons. So let me draw that over here. Here I have a piece of metal, typically iron. In iron you can get the electrons at least one electron to reach out on all spinning in the same direction. So they're moving in circles. They're spinning like little tops. Let me draw little tops here. You're all lined up like this. Because you're spinning, it's hard to believe that this thing has all this internal spin in it. But when you have these magnets and you have the electrons all spinning in the same direction, deep inside of it, this thing is spinning. The outermost electrons are spinning. Because you're spinning, it's like electricity moving in a circle. And so here we have electricity moving in a circle. That makes a magnetism on the inside. If I break this in half, it's still a magnet on the inside with the electrons causing a magnetic magnetism to go in this direction. If you break this in half, and you still have that property that now you still have the electron spinning like this. This will become the north pole. This will become the south pole. That basically means that if you put a magnet, it'll tend to be, suppose I take two of these magnets and put them together like this. These two north poles will repel each other. But this north pole will be attracted to this south pole. That'll tend to pull them together. This north pole will be attracted to this south pole. That'll tend to put them together. So the duty of these magnets attract a repel. The answer is they will repel because the north poles are closer. And because this thing is further away, therefore the force is weaker. So these behavior of magnets, like here, if I put the north, and the south pole against each other, they attract. At the same time, this is repelting this. Watch, see? It repels it. But it's so far away now that the attraction is more important. So the magnetic recording is based on the fact that you can make a piece of metal magnetic by putting another magnet next to it. In your hard disk drive, in your computers, it records signals using magnetism. Let me explain a little bit how that works. Let me use this one. There's a lot of research that's done into finding the best possible magnetic materials. And there's so much money in this business that better magnetic materials are being found all the time. Let's just take a very simple magnetic material such as iron. And we'll make a disk. Out of a thin piece of iron. In this thing, in this thin piece of iron, let's look at this thing up close. Here it is. The iron will have the electron spinning in many different ways. If you want to record a signal on this, these are recorded on these things in terms of signal as either one or zero. And you put a lot of ones or zeroes on here using magnetism. And the entire digital signals made of ones and zeros. You can encode letters of the alphabet that way or brightness is with one and zeroes. The way this works is they take a coil of wire. Send electricity through it, turning this into a magnet. So you're making a magnet by electricity flowing in a wire. That magnet will cause these magnets to be, if this is the south pole, they may be repelled. There's an earth pole, it might be attracted. It'll cause these magnets to flip over. So when you send electricity to this, this will turn into the magnet's being aligned. So you put the magnet down here, you turn it on. It puts a force on these things, causes them to flip over. So in fact, turning on this magnet, making this rotate. The magnets will rotate to the little point in the same direction. Then you remove this magnet. Actually, you don't remove it. What happens to the disk is moving. And as it passes over this magnet, you turn the magnet on. These individual electrons line up. Then they move away from the magnet and they're left lined up and they stay that way. If the north pole is down here and the south pole is up here, it'll be one signal. Maybe that'll be a one. If the south pole is down here, the north pole is up here. If you're spinning in the opposite direction, you can call this a zero. So this is what a magnetic recording does. Is this the best way to record signals? If you would ask me 20 years ago, I probably would have said no. It's hard to predict the technology that will turn out to be the most practical for recording information. What they've been able to do is to make these regions extremely tiny. And by making them extremely tiny, they can make them so you can store a lot of signals on the disk. And as the disk is spinning, what you can do now is put a wire up here. And you can sense the magnetism of this little region. I didn't talk about that yet, but we'll come to that on Thursday. So as this thing spits around, you can record on it by having a magnet. And then you can read it off with another magnet. As this thing is spinning, every time the disk comes to where your sensor is, you detect whether it's a north or a south pole and then you know what was stored there. Then you go reset it. You can change this and store a new information on it. This is what's happening on a hard disk drive. You have these thin plates of magnetic material. You have wires that create magnetism. And the magnetism is a way of storing information. You're only storing an upward down. You're only storing a north or a south pole. That's it. But the amazing thing is, if you store ones and zeros, you could let, for example, every three letters. One one zero versus one one one. This could be a code. This could indicate the letter A, this could indicate the letter B. And the pattern that you leave there could lead to that information. Or you could have these could be numbers that represent the strength of a field, the strength of the sound, for example, the loudness of the sound, and then you're recording music. This is what happens with magnetic recording. What do I want to cover in the last five minutes? Let me talk about transformers. I said that this thing becomes a magnet when we have electricity flowing through it. Let me use a little bit of relativity theory. Relativity is something we're going to get to. More and more is this semester goes on. And one question is where does the term come from relativity? You'll see that in a moment. Suppose I have electrons flowing through a wire. That creates magnetism around that wire. That magnetism can exert a force on another magnet. And a electron, see, I don't want to say this. Let me choose the concept of a field. It comes across as this fourth gravity. It makes more sense or becomes more important to think about for electricity. That when you have electrons here, how do they exert a force on another electron? There's no mechanical linkage or is there. Or is that a distance? Where does that really come from? It's a very abstract motion and it's hard to really know. But the following approach turns out, makes some predictions that are verified. And so this is now the standard way of thinking about it. And electron, let's just sitting in one place. Creates around it. It changes the space around it. It fills that space around it with something that we call an electric field. Electric field gets weaker as you get further away from it. Gravity doesn't the same thing. You say that here's the earth. And the gravity has a gravitational field. The way you could tell that there's a field here is if you put an electron right here, another electron that the field exerts a force on the electron. An old way of looking at it is you say an electron exerts a force on an electron at a distance. But we've been driven to believe that that's not the way it works. So if the electron creates a field, a field is just a name for an alteration of space. So this space over here has a property that if you put an electron that'll be pushed in that way. I tend to think of a field as filling up of space with some kind of an energy that can exert forces on other electric charges. The same field on a proton, we pull the proton this way. On a neutron, the neutron doesn't sense the field at all. Some people think of a field as it looks as similar to a stretched piece of rubber. This electron is distorting the space out here in some way. The reason that would force to come up with this notion is because if you put the electron here and it's repelled and you have luck thrown here and it's sitting here creating a field and this electron is being repelled, suppose you suddenly take this electron and remove it. This is the force here go away instantly. The answer is that to be no. The reason is if you remove this instantly, the field doesn't change right away. The field is still there. It's a property of the space and once this electron is gone, the field will begin to disappear, but the disappearance of the field only goes with a certain speed. The speed for the collapse of that field, think of it as if you have a big stretched rubber membrane. And you're holding down something here and it makes the whole rubber membrane tilt. So something here will start rolling towards it. If you remove your fist suddenly, as the rubber membrane spring right back right away, it takes a little while. You put an electron in space and suddenly remove it. The electric field doesn't disappear. First disappears over here and then it spreads. The disappearance spreads with time. It moves with a certain velocity. That velocity turns out to be the speed of light. The field does exist even when the electron is not there. In fact, if you take an electron and shake it, you can shake that field, hold the electric field. That shaking electric field becomes what we call an electric wave. An electric wave, the microwave, it's radar, it is visible light.