 Hi, I'm Zor. Welcome to Unisor Education. I would like to continue talking about units of measurement. This is a relatively simple topic, and I will spend probably just very little amount of time on each of the units we are talking about. The first lecture of this topic was about basically introduction into the system which we are accepting right now as the standard called C. And at the same time in that lecture I have introduced how we measure time and the unit of measurements is second. Now, the most important and fundamental principle to define systems of measurement was the principle to base all these definitions on some kind of constants which exist objectively in the world. And, for example, for the second we have chosen the resonant oscillation of a particular atom, cesium 133, when it transforms from one particular state to another. Well, this is something which is basically, it exists in nature. It's completely outside of our involvement, and that's why we have chosen this as a base to define our constant for the time. The same thing exactly was with definition of the length unit which is meter. It was based on the speed of light and the second which we have already defined as a time measure. We also defined the kilogram as the unit of mass and that we have connected to Planck constant. Right now we will do the definition for electric current. It's also one of the base units. So, the base units are those which are defined primarily on the level of certain standards, certain objective constants in the world. And then there are derived units which we will talk about later on. So, today we will talk about electric current. The unit of electric current is ampere and we will define it also based on certain physical constant which exists in the world completely outside of our wheel or our machinery. Now, this lecture is part of the course called Physics for Teens presented in Unisor.com. I suggest you to watch lecture from that website because every lecture has textual notes which are basically like a textbook. Also, it's a course which means there is a menu and it's logically connected topics one after another. I base something for every lecture on something which has been already covered before. So, it's very important to basically take the whole course. Also, there is a prerequisite course on the same website called Mass for Teens. You have to know, well, calculus is definitely needed for physics and vector algebra and some other things. What else? Well, the site is totally free, theunisor.com. There are no advertisements, so just pure knowledge for your disposal. Okay, so let's talk about electric current. Now, basically the most important part is to find out some kind of natural constant from which we can derive the value of our unit of measurement of electric current. Well, historically, the ampere, the basically amount of electricity which goes through the wire per unit of time. That's what electric current is. Now, the measurement was based on very interesting phenomenon, if you wish, which I have described in the lecture of electromagnetism chapter of this course. The topic is called magnetism of electric current and particular lecture is called magnetism of two parallel straight current. So, if you have two parallel straight current on certain distance from themselves, between themselves, apparently they have certain force which exists, which either attracting these two wires together, if they are in the same direction, or repelling if they are in the opposite direction. Now, this force can be measured and it depends on the distance between the wires and how strong the electric current is. So, basically the ampere could have been defined using this machinery. Well, it was probably, I don't know why, but it was kind of difficult probably to reproduce it and it's not really based on some natural constant. It was based on certain machinery which we have built to measure the force. Well, instead they have decided to make this kind of a unit of measurement based on some natural constant, which is electric charge of one electron. Now, this is a constant, supposedly, and as a constant it can be used as a base for measurement of amount of electricity. Well, we can always measure amount of electricity in number of electrons, so to speak. But for this, we have to know the charge of one particular electron. Well, and it was really measured. The charge of electron is equal to approximately 1, 6, 0, 2, 1, 7, 6, 6, 3, 4 times 10 to the minus 19 of Coulomb. This is 90. And what is Coulomb? That's amount of electricity. Well, this is one ampere per second. So, having this defined and having established this particular charge of electron, we can now think about how to define the ampere using this as given. So, not very long time ago, actually in 2019, it was decided that, ok, let's just have this by definition exactly equal to this. From which we can derive one Coulomb is electric charge of one particular electron divided by this ugly number times 10 minus 19. Now, instead of divided by 10 to minus 19, we can multiply 10 to 19. So, if we can take the amount of electricity in electron, whatever it is, it's natural constant, multiplied by 10 to 19 degree. And divided by this one point, blah, blah, blah, we can have amount of electricity, which we consider to be one Coulomb. And therefore, if this amount of electricity, e times 10 to 19 divided by 1.602176634, if this amount is per second, so that if this amount of electricity is transformed every second, then this is the current, the current of one ampere. So, we have defined ampere in the way of using the natural constant, amount of electricity in the electron, multiplied and divided by these numbers. If this is per second, then this is exactly one ampere of electric current by definition. So, this is part of the definition of this number. So, as everywhere else, we start from something which is objective, exists in nature without our involvement at all. Using some numbers which we have basically postulated, we have defined as just use these numbers, then that's we will have a unit of electric current. Now, obviously, you can say that, well, whatever amount of electricity is e, well, we don't know, but we can always measure it in any kind of a unit. And that's what basically gives us all the rest of the picture. Okay, so this is a definition, amount of electricity in one electron times this divided by this per second, this is ampere. From this, we define Coulomb as this as a derived unit, but we will talk about this when we will talk about derived unit. Our primary unit, our base unit is ampere and that's what it is, amount of this electricity which is transferred through the current, through the conductor in one second. Now, obviously, as in many other cases, we have fractions of ampere and multiples of ampere. Now, fractions are milliampere, which is 1000 of ampere per second amount of electricity, this amount of electricity per second. So, if it's not 1019, if it's 1016, that would be 1000 times less, that would be milliampere. Microampere is one millionth, nanoampere is one billionth and picampere is one trillionth of ampere. Obviously, the multiples are kiloampere, which is 1000 ampere, megaampere, megaampere. The abbreviations milliampere, microampere, nanoampere and picampere, that's fractions, kiloampere is 1000, megaampere, capital weather ampere and gigaampere, that's millionth of ampere. So, that's what it is, basically. So, we have defined our electric current unit, which is ampere, using the amount of electricity in one electron and some arithmetic. So, this amount of electricity, if you wish, if 10 to 19 divided by 1.62 blah blah blah, if it was an integer number, you could say that this number of electrons per second should go through the wire. Well, this is not exactly this number, so maybe it's something like whatever points something of electron. Electron is not really divided, so it doesn't really make much sense to say that this many electrons per second, because it's actually a fraction of electron. But it doesn't really matter. I mean, our purpose is not this. I mean, we can always multiply it by million and say that this would be million in so many millions per second or not per second. Maybe we will have this, not per second, but another number which would make this integer. And then we can say that so many electrons per the time equal to not like one second, but one point something second or something like this. So whatever it is, this is a good definition based on natural constant. I suggest you to read the notes for this lecture. Basically, it's exactly the same thing. Just to make sure that you have this concept of natural constants as a base for our units of measurements. That's it. Thank you very much. Good luck.