 Hi, I'm Zor. Welcome to Unisor Education. Today I would like to start a new chapter in this course. The course is Physics for Teens, presented on Unisor.com, and the new chapter is about alternating current. I do recommend you to watch this lecture from the website from Unisor.com because every lecture has a textual part, notes, and this is basically like a textbook. So you have the video presentation and the textbook, which basically accompanies this. So you can always, you know, read it again and check whatever you have missed, maybe. The site is free, by the way, and also there is a prerequisite course called Math for Teens, which is absolutely necessary for learning physics. I mean, not necessarily that particular course, but math in general is necessary for understanding what's going on with physics. Okay, Alternate Current. Now, alternating current is what we are using at home, usually, and what's used in plants, industrial facilities, etc. Now, first of all, let me just tell you right up front that alternating current is not the same as direct current. Direct current is when you have a constant flow of electrons in one direction with the same intensity, so to speak. So the current itself, which usually is denoted with a letter I, is constant. Alternating current is, well, it's alternating. It's alternating in the magnitude and direction. And it seems to me that this actually is more, it's simpler, right? So why do we have to bother with alternating current? Well, let me start with mentioning, it was a movie. I think it's called ACDC or something like this. It's basically two different technologies based on direct current and based on alternating current with two very important scientists and inventors, Edison on the direct current side and Tesla and Westinghouse on the alternating current side. They were actually competing in the United States for putting the electricity into the homes. And so considering such an important figure as Edison was proponent of the direct current, it's not, you know, so simple to say, okay, this is definitely better than that. No, it's that simple. However, alternating current is still more convenient. And that's what I'm going to talk about right now. Okay, first of all, why is it more convenient from a theoretical standpoint? Now, you know that direct current produces magnetic field around it, right? So if you have some kind of a conductor, a wire, and there is a direct current in it, then there is a magnetic field around it. Or if you will put it in a loop, for instance, this is the battery. So you have a direct current, constant flow of electrons, then you have magnetic field around it. So it actually acts as a permanent magnet with a north and south pole, which are perpendicular to the area of the loop. So we know that. So we know how to convert electricity into magnetic field. What alternating current does, it produces the variable magnetic field, right? Since we are changing the current, direction, magnitude, whatever, there is a change. There is a change in flux, magnetic flux, and change in magnetic flux can generate electricity somewhere else. That's called induction, you remember? Faraday's law, etc. So what's important about alternating current is not only it can produce a magnetic field, but it can produce magnetic field, which can produce electricity again using the induction. So that's very important, because by changing from electricity to magnetism, from magnetism back to electricity, from electricity back to magnetism, etc., we can actually change certain parameters and I will talk about which ones. And at any point, we can connect some kind of a consumer, which is the most convenient for that particular voltage on the emperature, whatever else. So let me start from the beginning. How do we generate electricity? Well, we have to generate it, right? So we have power plants. Power plants usually, we're not talking about solar power plants, which are important and they do produce electricity, but it's a direct electricity and it's relatively low in power. The most powerful power plants are based on transforming mechanical movement. It might be the falling water and hydroelectric station, or it can be steam in nuclear station produced by the heat. So we are transforming some other energy sources, like heat or mechanical movement, into electricity. And basically, the mechanical movement at the very end is the most important part. How did we do it? Well, again, this is based on magnetic induction. What we usually do is, well, the simplest movement which we can really use in our practical life is rotation, right? So if you have a rotating frame and it's rotating around this axis and you have some kind of a magnetic field here, uniform magnetic field. So what happens with this wire frame? Well, as we know, we have the magnetic flux which is equal to intensity of the magnetic field B times the area through which this magnetic field is flowing. But since my area is changing, in this particular case, area is zero, right? Because magnetic field lines are going, they're not really crossing the area. But if it turns, then magnetic field will actually go through the frame. And if it's a perpendicular in this position, the whole area of the wire frame will be crossed. So this is variable. Now, how is this variable? Well, if S is the real frame, then you have to multiply it by, let's say, cosine of W omega t, where omega is angular speed, right? It's very easy to understand. At t is equal to zero, well, it's probably better to have sine here. It all depends on the angle. So my position is, if my position is this initial position at t is equal to zero, then we have to put sine here because sine of zero is zero. So that's why you have zero. But if omega times t, if angle is 90 degrees, then the sine would be equal to one and you have the full area to go through the magnetic field. So this is magnetic flux. And we know that the generated EMF, electromotive force, well, actually the voltage in this case would be also depending on time minus derivative of the flux, which is equal to B S cosine and omega omega cosine omega t. So this is variable. So as we are turning, rotating our frame, and that's exactly how electricity is produced by the power plant, they use whatever the mechanics are to rotate certain frame in the magnetic field, or, which is more practical actually, they rotate magnetic field around the frame or inside the frame. It doesn't really matter. I mean, how do we arrange it? The most important is that the magnetic flux is changing. So something is supposed to be rotating relative to something else. Field source of the magnetic field relative to the permanently positioned frame or frame in the magnetic field doesn't matter. The result will be the same. It will be variable flux, variable flux generates my EMF. And the EMF can be used as a voltage if you connect consumers to this. So this looks like it's easier to, from purely practical purposes, from engineering standpoint, it's easier to produce alternating current. Now alternating, you see, it's a cosine. So you have a sinusoidal graph of voltage produced. It has certain maximum, which is equal to B times S times omega. Minimum is minus BS omega. And it's pulsating in between these from U max to minus U max in a sinusoidal way. Now, obviously, since we have the Ohm's law, the form, the shape of electric current will be exactly the same. Variable sinusoidal. Now, again, the fact that this is a cosine or sine, it doesn't really matter. It all depends on how do we start the initial moment, T, at the moment when the frame is this way or the frame is this way. So the most important is the character. The character of a sine and the character of a cosine are exactly the same. Just shift in time by half a period. Okay. So we have concluded that generation of electricity is easier when we're generating the alternating current. So again, this is sinusoidally changing electric current is called alternating. So if you will measure the electric current in, let's say, outlet at your home, if you will be able to connect, let's say, a scope or something, some device, which will show you exactly the flow of electrons, if you wish, then you will see exactly the graph like this. The sinusoidal. So to generate is easier, it's simpler. Can we generate direct current? Well, we can, but it's much more difficult, mechanically speaking. From the position, let's say, of battery, for instance, the chemical energy converted into electricity. Actually, it's a direct current in this particular case. But mechanically speaking, when you have really a rotation as the source of mechanical energy, you cannot avoid this. I mean, engineering wise, this is the easiest solution. And that's exactly why AC is dominating the power. I mean, what do you have for direct current? Very few things. Well, obviously, remote control and TV, you have whatever the battery operated. Well, you have cars, there is a direct current in the car because there is a battery. And the battery produces, let's say, 12 volts constant electricity. But in real industrial capacity, we are using alternating current. And again, my number one reason is it's easier to generate using the mechanical source, not chemical, mechanical source of energy to produce electricity using induction. So that's the main method. Another advantage goes step further. Now, what do we do with electricity? Well, you have a power plant, but you don't have it in every apartment or every building or whatever else. We have some centralized power plants, and we have to transport electricity from the power plant to consumers. So now let's think about it. How do you do it? Well, you just, this is the power plant and this is the consumer. Well, you have wires. Sometimes wires are miles and miles and kilometers and kilometers long, and they have some resistance, electrical resistance. How do we reduce loss of electricity due to resistance? I mean, you know, we have a drop of the voltage, right? So if you have some voltage here, and then you have a very, very long wire, which has certain resistance, the voltage here will be less. There is a voltage drop. Now, there is a voltage drop, and there is a basically loss in the power. So if you remember, there is a amount of heat produced by electricity, which depends on the resistance. So basically, the power consumed by resistor R is this. I is the current, and the R is the resistance. So we know that from the previous part, where we were talking about electricity. So we are losing every second. This is the power consumption. Obviously, the energy will be my times T, where the time, T is the time, which we are losing. But per second, we are losing this. So how can we reduce these? I mean, this is just complete waste. I mean, why do we have to heat up our wires? There is no need for this, right? Well, we can reduce R, or we can reduce I, the current. Well, reduce R is kind of questionable. I mean, how can you reduce R? You can increase the diameter of the wire. Well, it will reduce resistance, but it will tremendously increase the price and weight and stuff like this. So not an option. So you have to somehow reduce I. So if you have developed certain power, electric power here, what's the power which you are actually producing? Well, that's this, voltage times current. And this variable, because this is variable. Now, now you have to transfer it over there, without losing power, basically, or with as small loss of power as possible. Well, the power is produced, but maybe we can change the same amount of power by increasing voltage and decreasing current. The result of multiplication will be the same. Let's say we double the voltage and have the current. It will be the same thing, but we need this device, which will do this type of thing. Because if we will do this, my voltage will be high, my current will be less, and my loss of power will be less. So we need the device which would, without loss of power, increase voltage and decrease the current. So it will be high voltage, lower current, it will be here. Now, this is the consumer. You don't want 6,000 volts coming into the apartment. So now you have to really do inverse operation. You have to reduce the voltage and increase the current to feed all the devices which we have, right? So we need another device which again does this power transformation, increasing the current and decreasing the voltage. But these devices do exist. These are transformers and they're working also only on the alternating current. Now we did talk about solenoids and we did talk about two loops. If one has a variable current in it, like alternating, well any variable, it produces certain variable magnetic field. So the B which goes from this would be variable. Now if you have this loop nearby, since B is variable, this is just permanently positioned. My flux would be different and since my flux is different, that would be induced EMF, induced electromotive force. Since the flux is different, the derivative would be greater than 0 or less, not equal to 0, right? And it will produce electric current here. So if this is variable, let's say it goes this way and then this way, this way and then this way, my flux would be this or this, this or this, changing sinusoidally again. And it will produce sinusoidal electric current in this, alternating current. So alternating current can be transformed from here to here. Now if you have not just one loop, but a solenoid here and here, a lot of different loops on the same wire. So every loop will produce whatever the magnetic field actually is and every loop here would consume. And it all depends on the number of times this loop actually goes around. So if you have a certain number and one of loops here and two here, so this will be proportional to N1, this will be proportional, the result will be proportional to N2 because every loop generates the same thing, right? Every loop here generates the variable magnetic field. Variable magnetic field generates in every loop of these corresponding variable electric alternating current. So it's proportional here, it's proportional here. So by changing N1 and N2, we can actually vary the current which is generated here relative to here and voltage here versus voltage here. Now we will go through these calculations in another lecture. It's an important topic how transformers are actually arranged. But this is actually how transformers are made. Transformers which can increase the voltage, decreasing the current on this end or vice versa, decreasing the voltage and increasing the current on the consumer end. So it's all done using transformers and transformers are working only with variable electric current. So alternating current not only easier to produce but also easier to transform in the most convenient fashion with less power loss. That's why Edison has lost his bore with direct current against Westinghouse and Tesla which were proponents of the alternating current. Okay, what else? Okay, now we have basically concluded this, that we have sinusoidal voltage and sinusoidal alternating electric current and I will just mention some terminology. So you have U as a function of T, it's equal to it's some kind of a maximum times sine of omega t or cosine of omega t. Sometimes people using this or that and again it doesn't really matter because it all depends on where do we start the time. Since it goes all the time infinitely left and right you can start. Now this is the sine and this is cosine. So one is just shifted relative to another. So it's the same behavior all depends on when do we start our time. So we will use most of the sine, doesn't really matter. And the U maximum is, well basically it all depends on geometry of this wire loop and the magnetic intensity etc. etc. Doesn't really matter. It's all in the power plant, somehow it's done. There is certain magnets which produce certain magnetic field intensity, B. There is a loop which has certain area S and there is a mega which is angular speed. So this is something which you probably just you know think about everything. All these details are hidden here but the behavior is here. Now obviously the current using the Ohm's law would be similar. What is I max? Well this is U max divided by R where R is the resistance of the circuit where the whole thing actually is happening. So whenever you have a regular let's say device something like a lamp which you connect to electricity at home. So you have alternating current going through this lamp. Well if the current goes one way then the current goes another way. Again this way and that another way in the sinusoidal basically character. Now the U max is called amplitude of the alternating current. What else do we have? Well we have frequency. Now what is frequency? Frequency is number of, well this is the period, number of periods per second. Well one period is 2 pi angle in radians. Omega is number of radians per second. So if we want number of rotations per second it's basically omega divided by 2 pi. This is number of radians per second. This is one full circle. So if you want to know how many circles per second you have to divide. Now the period is equal to t. It's equal to well that's basically 2 pi over omega. Right? So if you have so many periods per second then one period is one second divided by the frequency which is I think it's f. Period is t. So this is just terminology. So you have period, you have frequency. Frequency is more often used in regular you know conversation about electricity. In the United States the frequency is 60 periods per second or hertz. That's unit. That's number of periods per second. One period per second is one hertz. So the frequency in the United States is 60 hertz. In Europe it's 50 and I don't know about other countries quite frankly. But probably it's either 50 or 60 depending on which one they, which technology they have adopted. As far as the voltage is, the voltage generated at the power station, well I point here because I remember I was actually drawing the power station here and consumer there. So on the power station whatever the voltage is generated immediately from from the power plant before the first transformation is irrelevant. But after the first transformation the voltage which is actually transmitted through the wires across the continents even, well that's hundreds of thousands of volts. Now compare it with the consumer voltage which is in the United States it's usually 110, 220 in Europe, it's 220 usually. Well hundreds of thousands versus like 200, 100, 200 whatever it's a big difference. So first we are increasing the power, increasing the voltage sorry power doesn't change obviously we have the energy conservation. So transformer increase the voltage and decrease the current to reduce the loss. So the high voltage goes with a very low current and that's why very low losses in between and that high voltage goes to consumers where it actually is reduced to whatever we are having here. And by the way the whole thing of increasing the voltage and reduction of the voltage is not like instantaneous it goes in steps like from hundreds of thousands we might have some local transformation center whatever where it's from hundreds of thousands goes down to thousands and then from thousands goes let's say near every building in the city you have a small transformer which will transfer from thousands let's say two hundreds of volts and that's what actually goes to apartments. Well basically that's it it was an introductory lecture very very small amount of mass here about alternating current and its properties. Now as far as the properties again you have the maximum you have which is amplitude you have the frequency which are important. Now the question is when we are saying there is a 220 volts what does it mean actually in the outlet well we know that it's changing right so it's not like permanent is it like amplitude is it the maximum no it's not the voltage which is usually the one which is mentioned when we are talking about alternating current is in some way a mean voltage. I will talk about this in the next lecture it's actually the voltage which allows the corresponding current to produce as much heat let's say as much power if you wish as if corresponding direct voltage and direct current would be supplied so if you have an incandescent lamp now if you connect it to alternating current it will increase the temperature the heat will be increasing and it will start basically emitting light right so the electricity the direct current which will produce him about the same amount of light or amount the same amount of heat would be actually the number which is associated with alternating current so you have alternating current and you have a direct current so alternating produce some heat and direct produce some kind of heat so the amount of direct current the value of direct current which is producing exactly the same amount of heat as alternating is actually quoted as the voltage or or amperage of a particular device whatever it's consumed when we are talking about alternating current so we are not talking about absolute values but we are talking about effect which alternating current produces with let's say lamp or or motor or whatever else well that's it i do suggest you to read the notes for this lecture you have to go to unison.com choose physics for team course that's electromagnetism and that's where you will find properties of alternating current thank you very much and good luck