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Current time:0:00Total duration:8:32

Second Law of Thermodynamics and entropy

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Video transcript

the second law of thermodynamics one statement of it is that the entropy of the universe only increases and I put an exclamation mark here because it seems like a very profound statement and on a lot of levels it is and just to get us in the right frame of mind I have this image here from the Hubble telescope of of the night sky and each of these dots these are not stars these are galaxies that's a galaxy that's a galaxy there that's a galaxy and so hopefully this gets you in a little bit more of a of a cosmological scale but let's think about what this is actually telling us the entropy of the universe only increases so entropy we can we can define that as the disorder of a system and we're really talking about the number of states that a system could take on and then we're saying the universe but we could also say the entropy of a closed system only increases a system that is fully contained it's not interacting with its surroundings because the universe is the ultimate closed system there's nothing for it to outside of it to interact with thermodynamically and I'll do a quick review of open closed systems just so we make sure we understand that so if I had a campfire so I've some logs and I add my the flame going right over here so that's the campfire if I were to just look at the logs in the fire that's going to be an open system because it's clearly interacting thermodynamically with its surroundings it's releasing Heat it's warming up it's warming up the air molecules around it it's releasing light out into the universe there could be interactions from the from the rest of the universe into the system so it isn't isolated from the rest of everything else but a closed system it is isolated and there are it's very hard to create a true closed system in our everyday life but we can approximate it and the one that you've probably experienced not to not in the not-too-distant past is a nice cooler a nice cooler we're at least attempting to thermal thermodynamically thermo thermodynamically isolate isolate the inside of the cooler from the outside from the rest of the universe so this is and the way we do it is we have some type of an insulating material maybe some styrofoam and we could put you know we'd use it to maybe store ice and but it's not a perfect closed system because eventually the from the rest of the universe will warm up the walls of the cooler and eventually that heat will warm up will be transferred to the ice and it will warm it up and it will and it will melt it so it's not a perfect closed system but it's a good approximation because we're at least attempting to isolate isolate it thermodynamically from the rest of the universe and I can even make a little cover of this just so that if we really wanted to isolate it and in research labs you'll see things that are much better approximations of closed systems but even those at some level are they're going to interact with the rest of the universe the ultimate closed system so this is a closed system is really is really the universe nothing to interact with outside of it thermodynamic thermodynamically so let's let's think a little bit about this definition the entropy of the universe only increases why does this make intuitive sense well the best example I can think of is just straight up diffusion so if I were to have let's say have a container so I have a container and I'll make it a I'll make it a closed container I will say this is some type of theoretical ideal closed system here now let's say I had some ideal gas so I had some ideal gas molecules right over here they have some average temperature but that means they all each have their own individual their own individual kinetic energy they're all down cing around in different ways what's going to happen over time well over time the ones on the left here they're going to bounce off this wall and then their eventual going to go in this direction and so over time you're going to have a situation where the system is going to look something more like this so the system is going to look more like this we're instance see this is six particles these six particles are going to diffuse throughout the container so they're going to diffuse throughout the container they're going to take up they're going to take up more of the space of the container now what just happened in that process well when you knew that the particles were confined to this little section of the container there were fewer possible States you had lower entropy then when you are here when you know that it's filled up the container there's more possible locations more possible orientations for it and so you're going to have more states you have higher entropy higher higher higher entropy and in general these processes where you have the entropy increasing we call these irreversible processes irreversible irreversible processes and why is it reversible well there's some probability that these molecules might just gather back into this corner of it but it's very very low probability and this is when we're dealing with 6 molecules but in real systems we'd be dealing with a much larger than six molecules we'll be dealing with millions of millions of millions of millions of of molecules so things with between 20 and 30 zeroes of molecules and there it's very unlikely that they just all bump together in the right way to start taking a smaller volume when they could actually fill the container and so that's why you don't see that's why you don't see what you know smoke just naturally turn into some type of shaped particle or or take up less space as opposed to filling its container so this is irreversible because you went from you went from a fewer number of potential States in the smaller volume to a higher number number of potential States and the universe is constantly doing this that's why the entropy of the universe is only increasing now there's some processes that it feels like the entropy isn't increasing that much so if you were to take one billiard ball right over here and you were to roll it you were to roll it into another billiard ball right over here and transfer the momentum to that one it feels like that could go the other way around like that that other billiard ball could hit this one and go backwards and at a macro level it feels like this is a reversible process and people will tend to call this reversible but if you really were to go on a microscopic level and and it looks like the entropy isn't increasing that much but if you were look at it on a microscopic level and just to be clear the entropy you know when when this ball is moving and this is stationary going to a state where this is moving in this stationary it doesn't look like the entropy is increasing that much and so that's why they tend to call this reversible because you tend to observe things where maybe this one it could go backwards this could hit this one and then this one could go you can kind of run the film in rewind but even there if you were to look at a microscopic level you would see that some heat is being generated and then some molecules in the ball are getting are getting excited as they collide and as they have friction with the air and as they as they as they roll on the ground over here and you're never going to get those molecules to go back into the state that they were before that you actually do have the entropy increasing in the system so even when in our everyday lives people tend thermodynamics see people talk about reversible processes they're only approximately reversible and that the entropy is only increasing a little bit it's not like there's zero increase in entropy irreversible reactions these are the ones diffusion is a very obvious one where it's very clear that you have an increase in entropy and it feels like it's a very very low probability or almost zero probability of this thing ever going back to where it was and you won't observe it because we're talking about that many molecules something with 20 or 30 zeroes of molecules the odds of all of them just doing the right thing you could wait around a very long time and never actually observe that happening and so hopefully this makes sense that the disorder in this way the number of states only increases as you have more and more interactions and a lot of that is coming from heat everything you're doing right now when I'm making this video my body is generating heat that heat is dissipating into the universe that is adding to the number of states that the universe can actually take on as I move my hands that my my little digital pencil that I'm using it's causing friction that's releasing heat into the universe my computer is running releasing heat in the universe you watching this releasing heat in the universe the electrons traveling on the wire to your computer releasing heat in the universe and all of that is increasing the number of states so if you're thinking on the molecular level of of everything
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