 Hi, I'm Zor. Welcome to Unisor Education. Today we will talk about electric current, which is basically a movement of electrons. Now, this lecture is part of the course, Physics 14, presented on Unisor.com. I suggest you to watch the lecture from the website, because the lecture has notes, very detailed notes, like a textbook, basically. And all the lectures are actually arranged in some logical order. There are exams, if you want to take them. The site is totally free, no strings attached, no advertisement, etc., so enjoy it. Now, speaking about electric current. Well, electric current is related with movements of electrons. Now, before, when we were talking about electric field, we didn't really consider any kind of a dynamics. Basically, it's like electrostatics part. Now, we are talking about certain dynamics. Okay, so movement of electrons. Well, let's just think about this. If electric current is a flow of electrons, then we have to explain how they move and why they move. Because electrons are associated with nucleus of atoms, so they are actually comprising the atom. Certain number of protons and neutrons in the nucleus, plus certain number of electrons, which is equal to the number of protons, make an atom. So, why would electrons which are attracted to nucleus and they are circulating in certain orbits, why would they fly away and start moving into whatever the direction is? Well, the answer is we need certain external force to force them to move. By themselves, they don't really move. If you take, let's say, a copper wire, just hold it in the thin air. Well, electrons would not move from their orbit. They will still be circulating around each nucleus, and there is no current, there is no ordered movement of all or part of the electrons from one end to another. So, we need certain external force. This external force, electric field. So, if one end of the copper wire is attached to, let's say, a negative charge, and let's say another with a positive charge, what happens with electrons? Well, if the charge is really strong, then the force of repelling electrons to electrons, the charge with negative electricity has extra electrons. So, these extra electrons would repel the electrons inside the copper wire. On the other hand, the other end, if it's connected, let's say, to a positive charge, that means absence of electrons. It attracts electrons inside the wire. So, this is repelling and this is attracting. So, electrons will move from repelling to the attractive side, obviously. So, we need these external charges to force the electrons to move from one place to another. Now, we can consider two diametrically opposite kind of materials. There are certain materials, like metals, for instance, which do conduct electricity quite well. Now, what it means is that their electrons, which are orbiting the nucleus, they are easier pushed away from the nucleus and obey the external forces. On the other hand, there are certain other materials, which are arranged differently and their intra-atom relationship between electrons and nucleus are stronger, in which case it's much more difficult to force them to move. So, we need maybe extra force on the external field or they might actually be completely dielectric. Now, dielectric or insulators actually means the same thing. These are substances which are very reluctant to move their electrons from one place to another. Well, the perfect insulator is actually vacuum because there is no electrons at all, obviously. And here is a very important thing. There is something which is called electric field and it propagates through vacuum, through anything. And there is something which we call electric current, which is flow of electrons. These are two different things. The propagation of electric field and movement of electrons, which we are calling electric current, are two distinct things, so we should not really mix them together. Okay, so let's just consider a particular conductor of electricity, like copper, for instance, we were just mentioning copper wire. Well, it has 29 electrons orbiting each nucleus. In the nucleus we have correspondingly 29 protons and certain number of neutrons, like 34, 35, 36, depending on isotope. Now, the electrons are obviously attracted to the nucleus because protons are positively charged and neutrons and electrons are negatively charged, so they attract each other. Now, if there is an external force, the outermost orbits, because electrons are actually orbiting in different orbits, different distance from the atom, from the nucleus, so the outer electrons on further most, farthest most orbit, they are easier to be pushed out from the attracting forces of the nucleus. So if there is certain electric charge from outside, then electrons can actually be relatively easily moved from this outermost orbit of each atom. And the electrons are moving from negative to positive charge. Now, here is an important thing. What is the speed they are moving? Now, the electric field propagates almost instantaneously, actually with the speed close to the speed of light. Electrons do not move inside the copper wire with the speed, not at all. The analogous situation is, if you have a water pipe, let's say, it's filled with water and you have some kind of a pump which pushes the water in. Now, as soon as the pump starts working, it doesn't mean that the water from here goes immediately to that. However, we do have immediate flow from the other side, right? Because the water which this pump pushes inside the pipe, this water pushes the next, these molecules, push the next molecules, then they move the next, etc., etc. So the push is propagated, but not exactly the molecules. Now, the push is propagated, well, basically with the speed close to the speed of sound in the water, for instance, in this particular case. So this movement is related to certain properties of water. We know that water really is almost impossible to squeeze, right? So whenever we are pushing on one side, water immediately goes out into another side. But the molecules which we are pushing here, the new molecules which pumps in, they are not exactly moving with the same speed, they are much slower. Same thing with electrons. As soon as you turn on, let's say, you have this copper wire, and you have this end with a negative and this end with a positive, as soon as you turn on the electric field, the electrons from negatively charged end will start pushing electrons which are immediately next to it. They push others, etc., etc., so it propagates until the whole flow is established. But the first electrons which are leaving the wire will be those guys which are old electrons which are here. It's because the push goes really fast. The push goes with the speed of light. This is the propagation of electric field. But the electrons themselves are much slower, and obviously it depends on certain other conditions. Okay. Now, about insulators or dialectrics, well, obviously, as I was saying, vacuum is the best one, but there are some other relatively good insulators, like for instance glass or porcelain. Now, it's still a big question which I didn't really touch right now and I'm not going to, why certain things are more conductive than others. It is related to certain internal structure of the electrons, how they are arranged in molecules. They are sharing electrons between two different atoms within the molecules. For instance, glass is mostly silica O2. It's a molecule and the way how molecules are kept together by certain links between the atoms, this actually dictates how easily it is to move the electrons out from the corresponding orbits. So we're not talking about this internal structure of these molecules. Let's just take it as given that there are certain materials which are easier to use to push the electrons through, like copper for instance, or aluminum or silver is one of the best actually. And there are those materials which are much more difficult, like glass for instance. Okay, what's next? Okay, next we will talk about current. Now, as we were saying before, the current can exist only if there are external forces, in the electric field which actually pushes the electrons from one end to another. So that's fine. Now, we have to somehow measure this current. And this is obviously something which people were thinking about for a long time. Now, think about analogy with the water pipe. How do we measure the strength of the current in the water pipe? Well, very simply by amount of water which goes out of the pump or at the other end of the pipe per unit of time, right? So we have certain quantity per certain unit of time. With electricity is exactly the same. We know how to measure electricity, how to measure electric charge. Well, electric charge is basically electrons, right? So number of electrons defines per second, let's say, defines the speed of the current, right? Well, obviously we don't measure it in number of electrons. We are measuring in number of coulombs of electricity. And as you remember, coulomb is just a charge which is equal to certain number of electrons. That's basically it. So we will have a bigger unit. We don't want to measure in single charge of one single electron. So we charge in a bunch of electrons. This is our unit. The same thing as the length we don't measure in molecules, right? We measure in meters, for instance, or in feet. So we have the quantity of electricity. Let's say one coulomb per second. This is a unit of measurement of the strength of the current. And it's called ampere. Now, as usually the unit is called in honor of physicists by the name ampere, obviously. So as I was saying before, all physicists, all famous physicists, immortalize themselves or somebody immortalize them to name units of measurements correspondingly to their names. So the person whose name is ampere was definitely one of those researchers of the electricity. And in his honor, if we are transferring one coulomb of electricity per second, that means that the current is one ampere. And sometimes we are talking about strength of the current or amperage, which basically is exactly the same thing. So amperage means the speed in amperes, so how many coulombs per second are going through the wire. So this is the unit. Now I would like to connect this unit with other interesting thing, other unit, actually. Now we have introduced, when we are talking about the electric field, something which is called a volt. Now what is volt? Volt is a difference in potential between two points, such that to move one coulomb electricity requires one joule of work. So one volt is a joule per coulomb. It signifies basically the difference in potential between two points of electric field. There is one point and another point. Now to move certain amount of electricity from one to another, well, to move one coulomb electricity from one point to another is actually a difference in potential. And again, if one coulomb requires one joule, that means that the difference in potential is one volt. Is the transferring of one coulomb electricity to another point requires 25 joules, it means that the difference is 25 volts. Okay, that's fine. Now, speaking about joule, remember what is joule? Joule is work and work has certain definition. For instance, work is force times length, distance. Let's put distance. Now, what is power? We are talking about mechanics. Power is work per unit of time, right? Or force times distance divided by time. Now, what is distance divided by time? That's a speed, right? Now, we are obviously talking about infinitesimal pieces of time and distance, etc., or average if you wish, if you have it longer. This is obviously not a pure theoretical physics. This is conceptual view of what is power, which can be actually expressed as average force times average speed, something like this, okay? Now, let's talk about this one. Now, joule again is work. Now, coulomb can be expressed as amperage times time, right? So, I can say that this is equal to 1 joule divided by 1 amper times 1 second. Now, what is work per time? Well, work per time is power. So, it's measured in watts, by the way. Power measures in watts, right? So, this is 1 watt divided by 1 ampere. So, what do we see here? Voltage times amperage, 1 volt, times 1 ampere equals to 1 watt. So, voltage times amperage. Voltage is analogous to the force and amperage is analogous to the speed. Well, amperage is analogous to the speed because it's amount of electricity per unit of time. Now, speed in mechanical thing is basically distance covered per unit of time. So, again, it's some quantitative characteristics per unit of time. So, I just wanted to make certain analogy that the voltage between two points is basically similar to the force which electric field applies against the probe of electric charge. And when we are talking about the flow of electrons, the amperage is basically analogous to speed because amperage is amount of electricity per unit of time. Okay, so these are just analogies, but this is a very important thing. Amperage times voltage is wattage power. So, that's a very interesting relationship between electric units and mechanical units. You see, electrical units were chosen in such a way that if you're using the same unit of the same system of units, like C, for instance, in this system, this is something which was defined mechanically, but it's the same, the units are chosen in such a way that we still have one unit by one unit is equal to still one unit. One amperage times one voltage is equal to one wattage. Same is one unit of force, like one newton times one unit of speed, which is one meter per second, gives you also one watt, right? So, that's very convenient kind of thing. Okay, now we were talking many times about this analogy of the pipe and with the water coming, with the pump, let's say, and coming out from another side. Now, the electricity in the copper wire, when one end is positive, another is negative, is basically analogies to one pumping water and another sucking on another side. So, that's kind of important thing. Now, can we connect this wire to two negative charges and will that be any electricity flow? Well, it depends. If these charges are exactly equal, then the forces will be the same and they will neutralize each other. If both negative charges are not equal to each other, then one which is stronger will push the electrons out to the other end of the copper wire, stronger than that one pushes it back. So, the flow will still be. So, the most important thing is the difference between them. If there is a difference zero, then there is no movement. If one is negative and another is negative, then obviously they neutralize each other. But if one is negative and another is positive, they add together. And let me just give you one example. In the apartment building, usually there are, well, more than two, but at least two different kinds of things. There is something which is called zero and something which is called phase. And usually there are two phases. Number one and number two. So, one is positive and another is negative. Now, if you connect this and this, it will be certain, let's say this is 110 volts and this is zero volts. Then the difference will be 110. This is the voltage. And there will be certain flow of electricity which is defined by this voltage. But you connect this to this. One is positive and another is negative. Then there will be 100 and plus and one end and minus 110 volts and another. It will be 220. Right? So, the difference in potential will be twice. And that's why most of the, like, lamps or something like this are attached to the wire which is between 110, between the phase and zero and certain, like, devices which require a lot of current, something like electric stove. They usually are defined, they are usually connected using the wire between these two, which gives you 220 difference between two different ends, which means it will be twice as much electricity coming per unit of time. So, what else is important? Let me check. Now, what is direct current? So, we have something which is called direct current and alternating. Alternating will be later. Direct is right now. Direct current is when you are maintaining certain polarity, like, always positive on one end and always negative on another end. So, the current goes always in the same direction. Now, how can you maintain this? Well, let me just, again, go to certain examples. For instance, you can consider with water. It's a very convenient example. Consider the water slide. So, you have certain slide and the water actually goes down. Now, what's here is some kind of a pool. And the water pump sits back to some kind of reservoir on the top. So, this pump maintains constant difference in potential between the water which is on the top and the water which is in the pool. So, this pump is very important. This is the source, basically, of the movement of the water and the source of maintaining the difference of potential between this one which is higher and this one which is lower. Same thing in electricity. So, if you would like to have certain current here, you obviously need certain device, generator or whatever else, which basically separates, that's the normal way of doing things in practice. It separates electrons from the protons, well, from the rest of the atom, thus generating negative on one hand and positive on another. So, absence of electrons here and axis of electrons here. So, the electrically neutral something is somehow processed in a way using certain energy, obviously, like hydroelectric station, for instance. Using some energy, it separates certain electrons and pushes them to this end. So, whatever left will have absence of electrons will be on the right. And that's how the difference in potential is retained and electrons within this wire are moving from here to here. And again, back here, here we are neutralizing basically everything. Now we are again processing somehow this electrically neutral thing separate again and the whole thing is repeated and it's a constant movement of electrons from here to here to here to here, et cetera, et cetera. Now, let's just think about this. As electron leaves the orbit, well, what's remained is actually a hole in this particular play. Well, we call it a hole sometimes, right? So, electrons are going this way. Now, what happens if you have certain conductor and let's say you have all neutral thing. Now, we have an electron from here to here. What happens? Well, there is a hole remains here, right? So, then if the same electron moves here, then the hole actually becomes here and here. So, the more and more holes are moving one direction and electrons are moving to another direction. So, absence of electrons is a positive and axis of electrons on a negative side. What's interesting is that electrons were discovered later than all these things about electricity were researched, investigated, uncovered, et cetera. So, when people were actually talking about this electric current, they didn't know what caused the flow of electricity. They did not know what electrons actually are about. So, by definition, they called something which is, which they called positive and there is something which they call negative. Let's say if you are, let's say, rubbing a silk against amber. So, something which is on the amber was always, I don't remember quite frankly, one is positive and other is negative. So, the similar things everywhere were positive or negative. So, they decided that the flow of electricity when it really flows from positive to negative. Now, physically we know that this is actually electrons which are moving into opposite direction from negative to positive. But the history is a history. So, by definition, when we are talking about current, let's say we have just put this arrow. What does it mean? Well, it doesn't mean this is a movement of electrons. This is a movement of basically holes what remains the absence of electrons. Electrons are moving to a different direction, the opposite direction. So, this is always direction from positive to negative by definition. It's not because something behind it. It's just a definition. People stick to this and that's what it is. That's basically it. Right. So, direct current exists in the conductor whenever we maintain certain constant difference in potential between two ends of this conductor. Let's say one is positive and other is negative. Usually of the same magnitude, but obviously one is positive and other is negative and they are different. Well, sometimes we have one is negative, let's say, and another is zero. One is positive and another is zero. Zero is usually associated with ground. We have a ground wire in like every receptacle which we have. It's related to the earth. Earth is so big that no matter how many electrons we push in, it will still be neutral. Generally speaking, neutral. So, that's between negative and zero. Or between negative and similar positive. Like if you have a battery, for instance, you have, let's say, 3.5 volts, the voltage between two poles of the battery. Well, that's where it is. That's the voltage. It defines the strength of the battery. Okay, so that's basically it. The whole lecture is more conceptual. And the only little kind of mathematics here was that one volt times one ampere is equal to one watt of the power. Okay, so that's it. Thank you very much and good luck.