 This is a quick introduction to temperature scales and conversions as used in physics. We've got three basic temperature scales we're going to be using. The Fahrenheit scale, the Celsius scale, and the Kelvin scale. In general, the symbol for temperature is a capital T. If we're having problems where we're going between different temperature scales, we'll often give them the subscript of either F, C, or K in order to show which one we're using. Although if you see just a plain capital T, that's often the Kelvin temperature because it's the absolute temperature scale as used in physics. If we look here at a couple of examples of thermometers, we're going to notice a few important points here. Over here we've got centigrade, also known as Celsius, and over here we've got Fahrenheit. The first thing I want you to notice is that the size of the degrees over here in the Celsius thermometer are much larger than the size of the degrees over here on the Fahrenheit scale. The numbers are closer together here. And this is a scale difference between the two different scales. As a matter of fact, for every five degrees we have over here on the Celsius scale, you can fit in nine degrees over here on the Fahrenheit scale. And we'll look at a few different examples of that. The other thing I want you to notice is that they don't have the same zero point. The zero point on the Celsius scale is the freezing point of water. The zero point on the Fahrenheit scale is much colder than that. As a matter of fact, the zero in Celsius is equal to 32 degrees in Fahrenheit. Now that's a convenient number because most of us here in America remember that, and so we can remember this 32 degrees Fahrenheit. And that's going to come into play in some of our calculations later. Now because of both the offset here on the zero point and the fact that we've got different sizes of our things, by the time we get up to the boiling point of water, our two temperatures are very much different from each other. The boiling point of water in the Celsius scale is all the way up here at 100. And the boiling point on the Fahrenheit scale is at 212 degrees Fahrenheit. Now if we were to look at the difference between that in Celsius it goes from zero to 100, so there's 100 degrees in that whole range. If I were to look over here on the Fahrenheit scale going from 32 up to 212, that's a difference of 180. Now if you divided 180 by 100 you get 1.8, which is the same factor as that 9 Fahrenheit to 5 Celsius we talked about earlier. So having seen those differences let's see how that comes into play in our equations. If we're converting between Celsius and Fahrenheit, there's two equations that you'll often see. One equation if you've got a Celsius temperature and you want to get a Fahrenheit temperature. And then another equation if you've got a Fahrenheit temperature but you want a Celsius. Now the first thing I want you to notice in here is some of our familiar numbers have shown back up. So our 32 degrees that's our offset here. And the 9 fifths or 5 ninths that's our scaling factor. So for every 5 degrees Celsius I've got 9 degrees Fahrenheit. So when I put in a temperature in Celsius the Celsius degrees are going to cancel each other out and leave us with something in Fahrenheit and then we correct for our offset of our 32 degrees Fahrenheit. If I've got the degrees in Fahrenheit I want to take care of that correction first while there's still both Fahrenheit degrees. And then my 9 degrees Fahrenheit here on the bottom is going to cancel with my Fahrenheit on top and my 5 degrees Celsius on top is going to mean I end up with a temperature of 5 in Celsius degrees. Now I've got one more scale though. Here's my Fahrenheit, here's my Celsius, here's the new one which is Kelvin. Now these thermometers again show some of our points like the zero degrees Celsius is 32 degrees Fahrenheit, 100 degrees Celsius is 212 degrees Fahrenheit. But we've also got our new numbers down here. And this Kelvin scale came about when they were doing some other experiments with temperature and pressure. They found that everything converged that if I got down to this temperature of minus 273.15 Celsius the pressure went to zero. And we came to understand that in terms of the jiggling of the atoms that that's the coldest we can get. So that point gets labeled as absolute zero and that's our zero point on the Kelvin scale. Now they kept the size of the Celsius degrees the same when they made the Kelvin scale. So if we were to go between zero and 100 well that's 273 to 373 it's the same 100 degree offset between those two points. So if I were to look at the size of a Celsius degree it's the same as the size of a Kelvin degree. Fahrenheit's are still smaller degrees where we get nine of them to fit into every five we'd have on either one of those two scales. So if we take this 273 or 273.15 as our important number we can then come up with our equations to convert between Celsius and Kelvin. Because they're the same size degrees there's really sort of a factor out front here that you can think of it as one degree Kelvin and one degree Celsius are the same size but one divided by one isn't going to change your number any. So to go from Celsius to Kelvin we add our offset of 273 and to go from Kelvin to Celsius we subtract our offset minus 273. Now you could come up with a similar set of equations to go from Kelvin to Fahrenheit or Fahrenheit to Kelvin but most of the time we just use Celsius as an intermediary so we take it from Fahrenheit to Celsius and then Celsius to Kelvin or take it from Kelvin to Celsius and then Celsius to Fahrenheit.