 Hi, I'm Zor. Welcome to Unisor Education. We continue talking about different units of measurements in physics related to the standard system, which is called C. Today we will talk about the electricity and basic concepts of electricity and how they are measured. Now, this lecture is part of the course called Physics 14 presented on Unisor.com. I suggest you to watch this lecture from this website because the lectures are hierarchically arranged, well, logically, sequentially, so to speak. And in this particular part of the course, when I'm talking about units of measurements, I'm introducing new units always based on whatever has been introduced before. So it's a hierarchical kind of a building, like we built the building, first fundamental kind of basement, whatever, and then the first floor, the second floor, etc. So everything is based on whatever has been done already. And in this case, I will use the concept which have already been introduced before as the base to introduce new units of measurements for new concepts which we are talking about, in this case, electricity. Okay. Now, the first concept we will talk about today is electric charge. So, electric charge, well, electric charge is basically related to electrons as we know. If number of electrons exceeds the number of protons in the atom, we are talking about negative electric charge. And if, on the other hand, there is a deficiency of electrons, this is called the positive electric charge. Now, question is how to measure these charges. Well, some time ago, people wanted to measure them just by the number of electrons. So a certain number of electrons in excess of number of protons in that particular object would be a unit of measurements of electric charge. Later on decided to do it differently. We have already introduced a concept of electric current and the unit ampere, one ampere. So, using this particular unit of measurement, the amount of electric charge would be measured by the electric current and the time during which it actually exists. So, if you have a conductor and there is a current which is one ampere, amount of electric charge going from one place to another in one second is one cologne. So, one cologne is one ampere times second. Now, to tell you the truth, personally I consider a certain number of electrons to be a unit, like one cologne, would be more natural for me. But in any case, physicists decided to do it from the other way. And then you can define ampere as cologne per second. But they have decided to do it the other way around for different purposes. Primarily, because I think there are very nice and precise tools to measure the current and how can you count the electrons? That's not very easy, right? So, probably because of this, I'm not really sure. They have decided first we will define as a primary, as a base unit, an ampere, and then times a second that would be the amount of electric charge. Now, if electrons are going away, so this is an object, and electrons are going away with certain current, that means that this object is being charged positively. So, if this flow of electrons is one ampere, then during one second this thing will have plus one cologne. If the other way around, if electrons are going into the object, making axis of electrons, which means negative, so if the flow of electrons is one ampere, then in one second it will be minus one cologne. So, this will be plus one cologne, this will be minus one cologne. So, this is all about electric charge. So, basically the unit is cologne, and cologne means certain ampere and times time, amperage and current, electric current times number of seconds basically. So, if five ampere is acting during 10 seconds, it will be 50 colognes. Okay, so that's all about colognes, electric charge. Next is voltage. So, how can we define a volt? Well, let's think about this way. Let's consider you have electrostatic field. For example, you have some negatively charged point object, so around it we have electrostatic field. Now, if you have certain test object and it has certain charge, now if it's negative, now my source of this electrostatic field is, I was saying, it's negative, right? And if my test object is also negative, there is a repulsion, certain force, right? Now, if my test object is positive, there is an attraction, also the force. So, there is a force acting on this object. So, if it moves, it does certain amount of work. Now, this work can be performed by the field itself, or it can be performed by outside force against the field. It doesn't really matter. Somebody or something does the work. If the charged object moves in the electrostatic field, either the field or some outside force forces it to move, something does the job, something performs the work, which can be measured. And that's very, very kind of familiar now, familiar grounds. So, we have joules as amount of work performed. And we have coulombs as measure of how much electric charge this test object has. Well, great. So, one volt, one volt is a difference in potential, it's a potential energy, right? Because at every point, object has a potential energy because, again, if we release it, it will move either towards the source of the field or outside. So, there is a potential energy. And so, what we are saying is that one volt is a difference in potential energy if one particular joule of energy is needed to move from A to B, one coulomb of electrically charged object. So, if you have a test object of one coulomb, and it takes amount of work, one joule to move it from point A to point B within electrostatic field, now we are saying that there is a voltage, there is a difference in potential between these points A and B. Voltage is just a different world, it's different of potential energy, same thing. So, there is a voltage which can be measured as it's equal to one volt. If one joule is required to move from A to B, one coulomb of electricity, then we are saying that the difference in potential is equal to one volt. So, this is basically the definition of the unit of measurement of the difference of electric potential or voltage between two different points. Okay, so that's done. So, we always think about voltage as certain amount of work needed to move from A to B certain charge. Okay, now the last thing is resistance. Now, imagine you have a conductor and for whatever way we want, we maintain certain difference of potential between two ends. So, you have A and B. Now, this is conductor and there is certain voltage here, constant. How we do it doesn't really matter. Now, why do I need conductors? Well, because electrons are moving. So, our object, so to speak, would be electrons. Now, imagine what happens with electrons as they are moving. Well, they actually meet certain resistance from whatever is in the conductor. It's not easy to move in between the atoms when atoms actually trying to grab you, etc. Just imagine the small electrons and nucleus all around. So, what we are saying is the following. If the current observed between A and B is equal to one ampere and the voltage is equal to one volt, well, pay attention, we have already defined the volt before. So, that's always like this, we define something and then define something else based on whatever we have already defined. So, volt was defined, ampere was defined, that's the base unit, a few lectures ago. So, if the difference of potential of one volt produces one ampere of current within this conductor, we are saying that it has one ohm, OHM, and this is the Greek letter omega, capital. We are saying that resistance of this particular conductor is equal to one ohm. So, basically we are talking about the Ohm's law, which means R is equal to U over I, remember? The difference of potential of voltage, this is the current and this is resistance. We are talking about this one, we are talking about electricity properties of the currents, etc., etc. So, again, I would like to basically tell you where the units are coming from. The unit of Ohm comes from unit of current and unit of voltage. So, again, it's all hierarchically built, one unit based on whatever has been already defined before. That's how the whole mathematics is built, axioms, then the first kind of level of theorems, then the theorems based on the previous theorems, etc., etc. Same thing in physics, new concepts are introduced based on whatever has been already introduced and the new units of measurement are strictly arranged in the same logical sequence. That's why it's important to watch these lectures about units in the sequence presented on Unisor.com. And by the way, Unisor.com is totally free website. You can take exams by the way there if you want to. You don't have to sign in if you don't want to and there are no advertisements. So, I do suggest you to watch all these lectures from Unisor.com. That's it for today. Thank you very much and good luck.