 So in this video we're going to talk about temperature scales. There are four common temperature scales that are used in the United States to do science and engineering. The first of these scales was invented in the early 1700s by somebody by the name of Fahrenheit. And that scale has a particular set of numbers that we normally associate with various temperatures that we're kind of used to. If you hear 70 degrees for the weather outside on a pleasant day in the United States, you typically associate that with very nice weather. And that 70 degrees is in the Fahrenheit scale. So that's the scale represented here on the red. And let me see if I can spell this correctly. Fair and height. Now there's two important numbers that we use to sort of give ourselves a basis in that scale. And the numbers that we're interested in are the freezing point of water and the boiling point of water. And those two numbers in the Fahrenheit scale are typically 32 degrees is when water freezes and 212 degrees is when water boils. So this scale was created before we knew as much detail as we know now about temperatures. And eventually people looked at this scale and said, well, the scale's a little strange. Numbers like 212 and 32, why should we have to remember those scales? Let's create a new scale that uses the same points, the freezing and boiling points of water. But we're going to go ahead and create a scale where 0 degrees is the point at which water freezes. And 100 degrees is the point at which water boils. So we've created a new scale. Sometimes this scale is called a center grade scale, meaning 100 gradations. So 100 units that we break things down into. But this scale is also known as Celsius. So now we have our two common scales. And the nice thing about the numbers in these scales are there are numbers that we feel comfortable with. Numbers between 0 and 100 are what we typically use to measure things. And so we like to basically take our experience and quantify it in numbers that feel comfortable to us. So when we say 70 degrees, you know that's a nice day. Although if you live in Europe, maybe you're talking about a number more like 23 or 24 degrees. Now there's a conversion that happens between these two. So if you want to convert from one scale to the other scale, we need to be able to change the units from degrees Fahrenheit. Let me actually put the little degree circle down here, degrees Fahrenheit and degrees Celsius. And if we want to convert between the two things, there are a couple conversions that are typically used. Here's the first conversion. If I want to get Fahrenheit from Celsius, it looks a little like this. Here's the factor 9 fifths, C plus 32. So the 32 is easy to remember because that's the 32 degrees for freezing that we have to add. And the 9 fifths seems a little strange, but if you look here, you'll notice between 212 and 32. The difference between those is 180 degrees, whereas the difference between centigrade and Celsius is 100 degrees. So that ratio is 180 over 100 or reduces to 9 to 5 ratio. So that ratio you can kind of figure out if you don't happen to memorize the conversion. If we want to go in the other direction and we want to get Celsius from centigrade, we're going to end up flipping over that 9 fifths and there's our ratio 5 out of 9. But now I'm going to put in the Fahrenheit and subtract the 32 degrees, but I do that subtraction before I do the multiplication by 5 over 9. So there are two conversion factors and you can easily convert back and forth between the two things depending on which one you are used to. Again, the imperial system or the system most commonly used in the United States is the Fahrenheit system and the system that's most commonly used in almost every other place in the world is the Celsius system. So now let me ask you a question. Let's say you're outside and the weather is 60 degrees, and I don't care which system, let's say it's 60 degrees, but chances are if you're outside in 60 degrees it's in Fahrenheit because it would be really a lot hotter in the Celsius system. So let's say it's 60 degrees outside and then the temperature reduces by half. The number goes from 60 degrees goes to 30 degrees. Well, what do you expect to happen? Well, on our scale here it would go from 60 degrees which is somewhere around here down to below 30 degrees. Oh, suddenly we're below freezing. It's gotten very cold. So we've gotten colder. Reducing it by half means we've gotten colder. And if we think about temperature as a measure of energy, of the amount of heat that's moving around there, the energy that's in the system, then that makes sense that it would get colder if we were reduced by half. But what if instead we started at negative 60 degrees? So it's negative 60. That's very cold outside. We know it's cold, but if we have that number, negative 60 becomes negative 30, which means when we divide it by two, it actually gets warmer. Well, that's the problem with both of these systems, the Fahrenheit and the Celsius system, is that there are temperature scales where their physical meanings are hard when we start doing division, when we start doing multiplication and division, which we will need to do in some cases when we're trying to relate temperature to other things that we measure. So in order to actually take care of this, we had to stop and think, okay, what does temperature really measure? And if it measures the amount of energy or the amount of heat in something, then maybe there might be a point where if we can completely remove all the energy from the system, make sure that nothing's moving around, nothing's bouncing around, there's no kinetic energy in the system, then we would reach something called absolute zero. Now, this is a theoretical idea that if we keep cooling something down and removing the energy, and removing the energy, and removing the energy, eventually we would get to a point where there would be zero energy, and that there would be no such thing as a negative temperature at that point. Well, these systems are very useful when we're trying to compare, again, temperature to other quantities like volume. So we created two new systems, one that goes along with each of the other systems. Okay? In the case of our Fahrenheit system, we created a system called the Rankine system. And the Rankine system, I'm going to go all the way down here, here is the zero line for the Rankine system, zero degrees Rankine. This is the special value that we call absolute zero. Similarly, the system for Celsius is called the Kelvin system. And if we go all the way down to zero degrees, that's absolute zero in the Kelvin system. The two systems in the middle are what we call relative temperature systems, relative. Those systems have been created to measure temperature relative to something in particular. And that relative thing that we use to make our marking points is the freezing point, or are the freezing point and boiling points of water. The other two systems are what we call absolute temperature systems. In this case, zero has a very special meaning that there is a definitive origin, and it's not created by something relative, but that we've created our origin and that zero value has a physical meaning of zero as well. So now we have four systems, two of them relative, two of them absolute. We also have two systems that are associated with the imperial system, or the system that's using the S, and two systems that are associated with the metric system. Two more conversions that we will need to use. The Rankine system, if we want to convert to Rankine, we take our value in Fahrenheit and add 459.67 degrees. Notice that means if we go in the opposite direction that it's, if we take Fahrenheit at zero and we go all the way down, that zero degrees Rankine is negative 459.67 degrees Fahrenheit. Pretty darn cold. On the other side with Kelvin, it's the same pretty darn cold, but if we compare that to Celsius, our Kelvin conversion looks something like this. We take our Celsius and we add 273.15 degrees. Or to flip that over, absolute zero is at negative 273.15 degrees Celsius. So again, four temperature scales, two of them relative, two of them absolute, two of them imperial system or US customary system, two of them metric. You should be familiar with all four systems if you're going to continue to work with them in science and engineering.