 Hi, I'm Zor. Welcome to Unisor Indication. We continue talking about units of measurements in physics and today we will measure temperature. This lecture is part of the course called Physics for Teens, presented on Unisor.com. There is a lot of sense, actually, to watch this lecture from the website, even if you found it somewhere on YouTube or wherever. The website is more friendly in a very important aspect. It has a textual component. So there is a video component and there is a textual component right next to each other. So after you've watched the video you can read the text basically. It's like a textbook. Or vice versa, whatever the sequence you prefer. Now there is a prerequisite course, which is called Math for Teens on the same website. Math is definitely needed for studying physics and I do suggest you to either take this course or familiarize yourself with all the necessary mathematical tools for physics. Also there is a very important lecture which was before this one as part of this course. Now there is a chapter called Energy in the Physics for Teens course and you can find it by basically going to Unisor.com opening Physics for Teens menu and you will see Energy as a chapter. In that chapter there is a topic called Molecular Movement and in that topic if you will open that second level menu you will find the lecture Temperature, Pressure and Volume of Ideal Gas. Ideal Gas is a very important substance to study and whatever we will talk about temperature and how to measure it well it's actually measured towards certain constants related to ideal gas you will see. But there is an introduction of certain laws which combine pressure, volume and temperature of ideal gas and these laws are presented in that lecture. So that lecture is actually a prerequisite for the one which we are talking about today. So that's all introduction and now the real talk about temperature and how to measure it. First of all, what is temperature? Temperature is a measure of kinetic energy of molecules if it's water or if it's air or if it's piece of metal whatever it is the way how molecules are moving inside that body maybe not moving maybe oscillating at some frequency if it's a metal they are not really moving very much but they are oscillating. So this is kinetic energy so molecules have kinetic energy and the greater the temperature means the molecules are moving much faster. So if we assign temperature of zero so we will come up with a scale but the temperature equals to zero when there is no movement of molecules at all it's called absolute zero. Well, I mean it's obviously practically impossible to do under our conditions but somewhere up in the space far from all kinds of stars from all kinds of sources of energy body by itself will actually emit all the temperature outside as a radiation and it will remain at absolute zero temperature. The molecules will stop moving. So again we will start with assigning a temperature equals to zero to a state of the object when movement is not really present movement of all the molecules inside that object is not present that's absolute zero. Now then after that no matter what kind of a unit of measurement of the temperature we will choose the temperature will be proportional to average kinetic energy of the molecules. So to have a scale to measure something we need zero and we need a unit. So zero we have basically agreed upon zero is absolute zero when there is no movement of molecules. Now what is a scale I mean what is the unit actually of measurement? Until very recently the unit of measurement in most of the countries was the Celsius system. Now the Celsius system is assigning zero temperature zero degree this little sign is a degree of melting ice and a hundred degrees Celsius is boiling water. Now that's fine. I mean there is another scale which is customary in the United States as a Fahrenheit and there is a connection between Fahrenheit and Celsius which I will talk about a little later. So these both systems are used in the world mostly it's Celsius basically in all countries and in the United States and I don't know where else is Fahrenheit. Fine. So that's how we established a unit of measurement so it's one hundredths of difference between temperature boiling water and melting ice this is my unit this is one degree which we can measure the temperature. So we have an absolute zero as the beginning of the scale and we have a unit one hundredths degree of the difference between these two temperature and that's sufficient to basically assign a temperature to any state of object no matter what this object is and this is so-called Kelvin scale so Kelvin scale is absolute zero Kelvin by the way in Kelvin we don't use this degree sign so this is a temperature of the body with no movement of molecules inside and one degree Kelvin is equal to one degree Celsius that's the same thing, the same unit we're using that was until recently now recently as I was talking about in previous lectures there is a tendency to assign units of measurements in physics related to some kind of world constants, physical constants which we really think are constants and why? Because you see this temperature it's not really very easy to establish standard I mean what kind of a water is this how pure it is or ice it's not really very precise as the precision right now is very important nowadays at the level of our current development of industry and science etc so we need something more precise now so to connect it to some kind of physical constant we really need some kind of a law from which we can actually start and here comes the law which I have presented in the lecture which I was talking about in the very beginning as a prerequisite it's a relationship between pressure of ideal gas volume of ideal gas it's absolute Kelvin temperature in the scale of Kelvin which means absolute zero is where absolute zero is and the degree is the same as Celsius so the law I'm talking about is this one N is number of molecules P is the pressure now how the pressure is measured well pressure is the force per unit of area right so pressure measured in force which is Newton and the measure of area is square meter now Newton in turn is actually kilogram meter to a second square now we have a square meter and meter cancels so pressure is kilogram divided by meter and second square kilogram and second and meter has already been defined before as units based on certain constants that's in the previous lectures before that I put my lectures in logical dependency actually so we define new units of measurement based on some other measurements and some constants okay the volume volume is measured in meters cube right here we have number of molecules which is just a number there is no kind of a units of measurements now what is KB KB is a Boltzmann constant again I did talk about Boltzmann constant in this lecture about temperature pressure volume of ideal gas so this is the law we will base our definition of temperature now using the old Kelvin scale so the old temperature and pressure and volume and this is number of molecules the constant was defined basically experimentally as 1.380 649 10 to the minus 19 of yes so this is the constant now this constant was defined basically was determined experimentally and obviously it's not exactly this because these are kind of old physical units of measurement the temperature especially so what the physicists did to define the temperature now well they just said okay let's postulate we will just assign this is the constant and we will define the temperature degrees temperature unit in such a way that this law actually is true with this exactly this constant so now its temperature is defined based on this so if you want to determine what's the temperature we have to know what's the pressure what's the volume what's the number of molecules and this we already have defined as this exact number and that's how we get the temperature so this is the same reverse logic as we used before from the constant we define the unit of measurements of the temperature okay basically that's it we have defined the unit and now let's go back and define all other units now we have melting ice that has two 73 0.15 units of Kelvin scale boiling water is plus 100 again it's all from the definition and as far as Fahrenheit and how to determine it well that's a different thing if you have a Fahrenheit then Celsius is actually Fahrenheit minus 32 times 5 N and if you want to find out Fahrenheit temperature from Celsius it's correspondingly 9 fifths of the temperature in Celsius plus 32 which is 1.8 degrees of Celsius plus 32 and Celsius in turn is basically Kelvin minus Kelvin is equal Kelvin is equal Celsius minus 273 0.15 so these are conversions basically and Celsius is equal to Kelvin plus 273 15 ok so these are just very simple formulas which are related from the fact that temperature of ice at some point before melting ice was 0 and from this we have determined that absolute zero was minus 273.15 in Celsius so I think I made a mistake it's the other way around Kelvin plus Celsius is Kelvin no that's correct so Kelvin zero plus 273 so melting ice is 273 that's correct so basically these are simple formulas which are converting one into another temperature but most importantly the unit of measurement in the Kelvin scale which is the scale used by scientists it's not really very convenient in real everyday life because you can say basically ok the temperature at 20 degree Celsius it's actually 293.15 Kelvin that's not very convenient I mean we prefer something simpler right but whatever it is if it's not convenient but physicists are using Kelvin degree because it's part of the formula which is basically gives the laws of dependency between pressure and volume and temperature etc so scientifically speaking we are using Kelvin practically we are using Celsius and Fahrenheit in the United States well that's it now that's one of the lectures about units and I will have couple of more and that would be all about so called base units and again I would like you to very to pay a lot of attention the whole philosophy of units of measurements units must depend on certain absolute objective physical constants rather than certain kind of devices which we are using to establish the standards like in case of a kilogram we had a little like cylinder which we have ok this is a kilogram no right now we are basing our units of measurement on some objective physical constants which exist in the world and do not depend on our actions at least as we see them now as not being dependent at least to the degree that we have the knowledge about about all this alright again this is a very short and very simple lecture there is nothing actually to explain or anything it's a definition definitions are not really explained it's a definition right so you can read the textual part of this lecture on theunisor.com and basically that's it thank you very much and good luck