Temperature and Celsius

0:00- [Instructor] We use the word temperature a lot in our daily lives like, "What's the temperature outside?" Or when we are sick, we check our temperature. But what exactly is it? Well, we have some idea. If you take a very hot coffee, then we know it's at a higher temperature.

0:14And if you take a cool lemonade right out of the refrigerator, then it's at lower temperature. But, again, what exactly is this temperature, and how do thermometers even work? To answer this question, we need to zoom in. If we zoom in, we can model these things to be made of particles.

0:32So we can say that every object is a system of particles. These particles would be your atoms and molecules that make things up. But in our case, remember, we're dealing with solutions. So these particles represent sugar molecules, water molecules, coffee molecules, and so many other things, all right?

0:46We're just modeling them together as tiny balls. But are these particles at rest? No, they're all moving. They have kinetic energy. Some particles are moving slowly, so represented by smaller arrow marks, so that means a lower kinetic energy. Other particles are moving faster, which has higher kinetic energy.

1:07But what difference do you notice between the kinetic energy of the particles here and particles over here? Well, we notice the kinetic energy of the particles in the hot object is bigger than the kinetic energy of the particles over here, right? Well, we need to be careful.

1:23I mean, there are some particles over here in the cool lemonade, which you can see, that have higher kinetic energy than some particles of this hot object. But what's important is if you were to consider the average kinetic energy of all the particles over here, kind of like calculating your average grade point, then that average kinetic energy over here would be higher than the average kinetic energy over here.

1:52So for a hot coffee, which is at a higher temperature, the average kinetic energy is higher. And for a cool lemonade, which is at a lower temperature, the average kinetic energy is lower. In other words, temperature is a direct indicator of the average kinetic energy of the particles.

2:14I find this mind blowing because what we've been measuring with our big thermometers all this while is actually the average kinetic energy of these tiny microscopic particles, which is pretty awesome if you think about it, right? So that's what temperature is, and that's what we measure with our thermometers.

2:33But now comes the big question. How do these thermometers work? Let's see, thermometers are basically glass tubes with a bulb at the bottom. The bulb acts like, you know, a reservoir where we can store some liquid. Some thermometers use mercury while other thermometers use colored alcohol.

2:49Okay, but how does it work? Well, thermometers work on a fundamental law of nature, which is when you have two systems of particles in contact with each other, energy will flow from a hotter system to the colder system. And this is pretty intuitive, right? This is the reason why if you leave out, if you keep an ice cube on a table, for example, then it'll start melting.

3:12That's because it is colder compared to its surrounding, which is much hotter than it. And, therefore, heat energy starts flowing from there into it. The ice cube gains energy, its temperature starts, you know, rising, it starts becoming hotter, and it starts melting.

3:28On the other hand, if you were to place a hot coffee on a table, then now the coffee is hotter than it's surrounding, right? So relative to the surrounding, the coffee is hotter. And so now energy moves again from hotter to colder region, and now the coffee loses energy, and that's how it starts getting colder.

3:46So the same thing happens when we insert these thermometers over here. Because the coffee is much hotter than the thermometer, energy starts flowing into the thermometer over here, and its temperature starts increasing. But over here, the lemonade is at a much lower temperature compared to the thermometer.

4:03It's just taken out of the refrigerator, it's much colder. And so the energy flows out of the thermometer, making the thermometer colder, reducing its temperature. Okay, so this thermometer is getting hotter, and this thermometer is getting colder. What does that do?

4:17Well, as the thermometer's temperature increases, the average kinetic energy of the particles starts increasing. And so since the particles start moving faster, they end up moving farther away from each other. In other words, the liquid expands. And that's the reason why the thermometer's level starts rising.

4:38The liquid starts rising over here. And you're probably familiar with this idea that as things get hotter, it expands. For example, this is the reason why when, you know, a lid is stuck, a metallic lid is stuck, you can just pass it over, you know, hot running water.

4:52As a result, the lid will expand, and then you can easily open it. This is also why bridges use expansion joints to compensate for the expansion that will happen during the summer. And the same thing's happening over here causing the liquid to expand. And, eventually, when the thermometer has the same temperature as that of the coffee, it'll stop gaining energy and the level will stop rising.

5:15It'll just be stuck over there. And what happens over here? Well, here, because the thermometer is getting colder, its temperature is dropping, the average kinetic energy is getting lower, particles are moving slower, and so the exact opposite, they come closer to each other.

5:31So the whole thing contracts, and therefore the level drops. Again, this happens until it reaches the same temperature as that of the lemonade. This is how thermometers work. But the final question is, how do we put a number to a temperature? Well, for that, we need a couple of reference points.

5:48For example, you can stick a thermometer into freezing water, and we can call that level as zero. And then you can stick a thermometer into boiling water, and you can call that level as 100. And we have now built ourselves a temperature scale. Physicist Anders Celsius recommended to use these references.

6:09And so we call this the Celsius scale. And because the Celsius scale has 100 degrees in between the two reference points, we also call this the centigrade scale. And look, so now we can say that, hey, this coffee is at, I think 90, 80, 70, around 75 degrees Celsius.

6:25And this cool lemonade is at, I don't know, maybe 5 or 4 degrees Celsius. Now, degrees Celsius is part of the metric system, so it's used in science and medicine. But another common scale that we use is the Fahrenheit scale. Now, Fahrenheit, the person who invented the Fahrenheit scale, used very different reference points for his 0 and 100, which are not very reproducible.

6:45So today we define Fahrenheit this way. We assign 0 degrees Celsius as 32 degrees Fahrenheit and 100 degrees Celsius as 212 degrees Fahrenheit. So look, in the Fahrenheit scale, there are 180 divisions between the freezing point and the boiling point. And there you have it.

7:05Temperature is a direct indicator of the average kinetic energy of the particles. Higher temperature means more average kinetic energy. And when you insert a thermometer, depending on whether it gains or loses energy, it'll either expand our contract. And as a result, the level will either rise or fall.

7:22And, finally, we build a temperature scale using reference point. That Celsius scale is built by using 0 degrees for freezing point of water and 100 degrees for boiling point of water. And in the Fahrenheit scale, the freezing point is 32 degrees Fahrenheit and the boiling point is 212 degrees Fahrenheit.