 Hi, I'm Zor. Welcome to Unisor Education. I would like to complete this chapter of the course, which is related to units of measurements, with the last lecture about units which are used in research of atoms. Now, this lecture is part of the course called Physics for Teens, presented on Unisor.com. I suggest you to watch this lecture from the website, because this lecture is part of the course, and the course has menu on this website, sub-menu, etc. Everything is logically kind of related to each other, plus every lecture has textual part, so at the same time you can watch the lecture and follow along the notes. It's very complete notes, like text book basically. Plus the course has other kind of functionalities. For instance, you can take exam in certain cases. The study can be supervised in which case you will have to sign in, but in any case everything is free. There are no advertisements, so you can just study yourself or under supervision, whatever. Speaking about units used in Physics of Atom. Well, I will talk only about two units which are, well, generally speaking, are used. Everything else probably is more or less the same as all other parts of the Physics. So the first unit I'm talking about is called Unified Atomic Mass Unit. Unified Atomic Mass Unit. Sometimes, well, in C, usually it's just lowercase U. In older scientific articles it's usually AMU, Atomic Mass Unit. And sometimes it's DA, stands for Dalton. That's the name of yet another physicist who was very much contributing and basically suggested to use this unit. So what is Atomic Mass Unit or rather unified Atomic Mass Unit? Well, as far as the exact definition, it's one-twelfth of the atom of isotope of carbon, carbon-12. Now this is a precise definition. Why one-twelfth? And why carbon? And why carbon-12? Okay, this is basically, again, first the physicists have decided that that would be something very convenient but then they wanted to make it like a real science and they have a definition. So what's the convenient thing? Convenient thing is basically to count how many protons and neutrons are inside the atom's nucleus. Obviously it's a very important characteristic. So you remember that number of protons and number of electrons is supposed to be the same and that characterizes the main properties of the element. Number of neutrons usually exceeds the number of protons or equal to number of protons. And they needed to basically keep the nucleus together, these neutrons. So basically the number of protons and neutrons it's like a weight, so to speak. More precisely atomic mass. However, how can we deal with this? Because different elements have actually not only different number of protons and neutrons but number by itself is not really precise. Because first of all protons and neutrons they are close in mass but not exactly the same. So we needed some precision. Okay, so how can we get to precision? Well, first of all they have chosen to use the carbon as extremely widely populated in our environment element. Carbon 12 is something which is the most popular if you wish. Most frequently occurring isotope of carbon is like 98% of all the carbon on Earth is carbon 12. And carbon 12 has 6 protons, 6 neutrons and 6 electrons. Now electrons are very small so they are not really participating in the total weight but number of protons and neutrons is exactly 12. So if we will divide by 12 we will have the average. Basically it's an average mass of proton plus neutron over 2, right? That's the average weight, average mass of nucleon. Nucleon being either proton or neutron. So that's why it's about one nucleon in mass. Unified can be used for everything else. And that means that C12 has exactly 12 unified atomic mass units of measurement. Well, because one unit is 112 so the whole thing is 12 units. Alright, now how can we measure it in kilograms? Because the kilogram is actually the C unit. Now this is also kind of a mass but it's just a different unit of mass. Much smaller obviously, right? So how can we figure out exactly? Now the help is from Avogadro number. Avogadro number. Now you remember that Avogadro number is defined. It's just a definition, exact definition. As 6.02214076 times 10 to the 23rd degree. This is number of atoms in a mole of the element. Now the mole is, by definition, is number of grams which corresponds to its atomic mass, right? So one mole which means 12 grams, exactly 12 grams. Because this is exactly 12 units of carbon 12 is one mole. And this is 6.02214076 times 23 atoms. Okay, so one atom weights how much? 12 divided by this number, 6. blah blah blah, 10 to the 23rd. This is one atom. And one atom is 12 units. Now this is gram, by the way, this is gram, 12 gram. Okay, so one unit is what? 9 by 12. And this is grams, so we need kilograms, so it would be 1,000 factor in the denominator. So that's basically what one unit is. Now if you will divide it approximately, and this is approximately because this is a division, division is not exact. When I put this number, this is exact number, this is a definition basically of a mole, right? Now when I'm really dividing it and put it in decimal, that would be a different number. It would be 1.66053921 kilogram. So this is one unified atomic mass in kilograms. So it's a straight direction, just a number, just a factor. Sorry, 10 to minus 27. Because this is 10 to 26 in the denominator, so that's why it goes 10 to the minus something, and 1 over 6 would be... I multiply by 10, so that's why it's minus 27. So it's 10, and that's 27, and that would be 1.6 blah blah. Okay, so basically that's it about atomic mass unit, or Dalton, or unified atomic mass unit. That's how it is in kilograms. Okay, now what's next? It is used in atoms, which you might actually be interested in. Okay, next is unit of energy called one electron volt. So if you read about different researches, energy which atoms are exchanging between themselves, you might actually find that this energy is not measured in joules, as C actually prescribes. It's measured in electron volts. So what is electron volt? Now imagine you have two points, A and B, and the difference in potential is one volt. Now we know what the volt is. That was part of the previous units we were discussing. Everything has a definition, and the sequence of lectures, which I'm actually doing, is exactly in the way so I can use something which has been already defined before to define something new. So we have the difference in potentials between two points in space. Let's say it's an electrostatic field, for example. It has obviously potential at some point, and the difference in potential would be one volt. Now what if you have an electron here at rest? Now let's assume that the difference is in such a way that this electron will go this direction. So let's say this is plus, this is minus, now electron is minus, so it will go this direction. It will, in vacuum let's say, it will gain certain speed, right? It will gain certain energy. So this energy which it gains by moving from a point A to point B, if the difference in potential is one volt, and this is, we are talking about one electron. So one electron will gain kinetic energy equals to one electron volt. So one electron volt gains its energy gained by electron moving from a lower potential to a higher potential, and the difference in potential is one volt. Now if an electron moves by itself, if this is plus and this is minus, this is one volt which field actually is, well, helping electron to move. If it's the other way around, if this is plus and this is minus, and by itself electron from here will not go there because plus is attractive and minus is, which means if we want to move it, then we have to spend that energy. So the energy B spent to move it from point to point should be equal to one electron volt. In any case, we have the conservation of energy law, and this conservation of energy tells that somebody has to spend an energy. If you have to move from A to B, either the field spends or we spend against the field. All right, so we have defined what electron volt is. Now how can we explain what's the relationship with Joules, with the unit of measurement of the energy, so this is energy. So how can we relate it to energy? Energy or work, which is basically the same unit. All right, so let's just think about it. Now what is one volt? One volt was actually defined as the difference in potential such that if we are moving one cologne of electricity and we spend one joule of energy, then this is one volt of difference in potential. Okay, so if we move one cologne across one volt difference, we spend one joule of energy. Now if we move one electron across the same distance, we will have something like a fraction of joule. Now what is the fraction? Well, the fraction is how many electrons are in one cologne, right? We are reducing the electric charge by certain factor, which means we have to reduce the work by the same factor. So what is this factor? How many electrons are in one cologne? Okay, now for this, we have a definition of cologne. Now from definition of cologne, if you will go to the lecture which is devoted to the units of measurements of electricity, you will see that one cologne is such a measurement that one electron charge is 1.602166342176 7634 times 10 to the minus 19 cologne. That's basically direct consequence from the definition of cologne. Cologne is actually an ampere second. An ampere times second. That's what one cologne is. And one ampere was defined using the same constant. So basically that's what immediate consequence, which I put in that lecture, is that one electron is exactly this number of cologne. Okay, basically that's it. So this is the number. This is basically the ratio between electron charge and one cologne. That means that we have to divide our one cologne. Remember, one cologne, one volt is one cologne, right? So basically since we reduce this to this, we have to reduce by the same number. So one electron volt is equal to this number, 6021763410-19 joules. So that's the connection between electron volt and joules. And again, physicists like to measure inner reactions between atoms in electron volts basically. So that's basically the relationship to joules. This is measure of energy. For example, let's say if you break a nucleus of uranium, then parts are scattered around. Now they have certain energy. So the measure of energy is in electron volts if you're talking about one particular atom. Okay, now that's it. That's all I wanted to talk about, units of measurements. Basically I think it's the last, I believe it's the last lecture of this course because if I will think about something else, I'll add obviously, mostly I'll probably add to exams maybe some problems. But generally speaking I have covered, I think everything which would be probably interesting for the person who is studying physics, not on a professional level, but on a level of the person who really is interested in what exactly our universe is made. So then I will start some other probably courses. I'm just thinking about maybe relativity or something like this, but that would be in the future. So thank you very much and good luck.