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Current time:0:00Total duration:10:22

Video transcript

so I'm going to ask you what I think is an interesting question have you ever sat in a room at room temperature let's say it's around 70 degrees Fahrenheit and watched a glass of liquid water spontaneously have ice in the middle and I'm guessing that you have never seen that and I'm myself Cove of course you wouldn't see I spontaneously form especially if the room is is at 70 degrees Fahrenheit if it's above the freezing temperature of water but my question to you is why not because that does not seem to defy any of the laws of physics the Newtonian physics or even the first law of thermodynamics and let's just think about how that actually could occur let's imagine a bunch of water molecules in their liquid state so I have a bunch of water molecules in their liquid state I'm going to do a good number of them and they have some temperature remember temperature is just about average kinetic energy but each of these are going to have their own velocities their own Momentum's so they're all going to be bouncing around in different ways and they have their hydrogen bonds between them that's what gives that's why water is its liquid state at room temperature as opposed to a gas so you have some hydrogen bonds between them but I'm not going to get too fixated too fixated on that just yet now you could imagine they're all you know jump bouncing around in random ways but there are some probability that they interact in just the right way that maybe this molecule right over here is able to hit this one in the right way so it transfers most of its momentum to the faster molecule and so this one actually loses some of its momentum and it slows down and just as that's happening in the neighbourhood of its one of one of the other molecules is able to transfer most of its momentum to some other molecule so it - so it - slows down so it - slows down so they all have much smaller momentum and then maybe this one at the exact same time is able to do it so it slows down so it slows down here and then the other ones that are that got they got the momentum transferred to them they're all moving faster now so let's say that one got their momentum transferred to it that one got momentum transferred to it that one got momentum transferred to it that one got momentum transferred to it and this one got momentum transferred to it and now these molecules right over here they their momentum is small enough their velocities are small enough that the hydrogen bonds the hydrogen bonds really take over and they're able to start forming some form of a lattice structure they're getting cold enough you could say to actually freeze so these are turning into ice why can't that happen what I've just described I'm just talking about things colliding and transferring their momentum I'm talking about energy not being created or destroyed so it seems to fit in with the first law of thermodynamics so it seems like theoretically maybe it is possible for for ice to spontaneously form or maybe another way to think about it maybe it is possible to start off with a system that is fairly uniform it has an average temperature here but maybe a cold pocket could form by the rest of it turning hot so maybe initially all of the water is 70 degrees so everything I'm showing you is a neutral 70 degrees Fahrenheit but maybe there's some probability that spontaneously I have no creation or loss of energy but some of the energy from the middle goes gets put into the outside so it warms up so I'm just in a different color so maybe all of this water outside maybe this is a top-down view of the water maybe all of this water heats up maybe all that water heats up and all the water in the middle cools down but they have the same total they have the same total kinetic energy so I haven't created or lost energy it's just happened to be that spontaneously I was able to transfer energy from the middle outwards and even as the middle got a little bit colder I was able to transfer more and more energy from the cold the cold water to the hot water and it gets ordered in this way this is actually it feels possible due to some of the the physics that we already know but some thoughtful folks like these gentlemen here this is Carnot considered the father of thermodynamics Kelvin Rudolf Clausius they repeatedly observed you know this doesn't seem to be happening in nature especially once you get to the characters like Kelvin and classy they're saying hey look it doesn't look like we're we're observing a transfer of heat from cold to hot and since we're not observing it let's just add our own second law of thermodynamics the second law of thermodynamics is really based on empirical observation and the second law of thermodynamics according to Rudolf Clausius and I'm going to paraphrase is is that we don't see spontaneous so let me write this down second second law of thermo thermo dynamics he said we don't see a spontaneous transfer of heat from cold areas to hot areas so second law of thermodynamics so no transfer no spontaneous no spontaneous you can in we can do use work like things like refrigeration equipment to make to make enter to make heat flow from cold to hot and cool something down but no spontaneous transfer transfer of heat heat from from cold to from cold to hot and maybe I'll run underline hot in orange right over here and this was just really based on observation because we don't spontaneously see this happening we don't see the water just or randomly organizing itself into a hot region and a cold region and getting so cold and maybe some of it will spontaneously freeze what we do observe is if I were to put ice water in the middle of a room at room temperature I'm going to see the other way I'm going to see I'm going to see transfer of heat from let me draw the cup here I'm going to see transfer of heat from the warmer regions to the colder regions so this is the if these are these are ice cubes right over here and and this is the water this is the water right over here we're going to see the transfer of heat the other way from the cold regions to the hot regions now this was an empirical observation and it seemed to hold up to experimentation but why do we actually see that and it turns out that there is some super super duper duper small probability this could actually happen remember in real systems that we're talking about in thermodynamics is really the study of systems more than individual molecules that we're talking about any system we're talking about we're talking about way way way way more molecules way way more actors than just three molecules here we could be talking about well if you look at the number of molecules in a glass of water you're looking at things with well you're looking at things with 22 24 25 zeros depending on on the size of your your glass of water so you're looking at a huge huge number of molecules and so statistically and they didn't think about things statistically until Boltzmann comes along but statistically the odds of this happening are so low especially when you're thinking about I'm not talking bout just three molecules I'm talking about I'm talking about way way way more than three molecules that you're just never going to actually see it and you could think about this if we were to allow ourselves to look at the the molecular level of things to not just look at the macro level you could see why this is so if you if you were to have some type of a container let me draw a container here if you were to have a container and you have on the left hand side let's say you start with a bunch of molecules that are hot so they have a high kinetic energy so these are these have a high average kinetic energy here these molecules and on the right side of the container you have maybe some molecules and maybe they're the same type of molecule but they have low kinetic energy so their temperature on average they have a lower kinetic energy they might have a few that have high kinetic energy but on average they have a lower kinetic energy so we see that the we see that the temperature here is lower so let me write this down right now when we're starting off this has a lower lower temperature while the left-hand side has a higher temperature now what's going to happen well these molecules they can interact with each other they're going to bounce into each other the things with high kinetic energy they're going to bump into the things with low kinetic energy and all of these things are also going to get mixed together but if somehow you weren't mixing it these things would be bumping into these and transferring their momentum so as time goes on you're going to have you're going to have a system that looks more like this where all of them are going to have more of a medium or on average a medium kinetic energy they're still going to be differences in their kinetic energies but they're not going to be divided in this way between left and right so you're going to you're going to see you're going to see it all mixed in and you're going to see and you're going to see that neither the left or the right is going to be have a higher temperature and why and so what is the net effect well we had a transfer of energy from the hotter molecules to the colder molecules so that energy that energy that we're talking about that is heat we use Q to denote the heat we have a transfer energy from hot to cold it's statistically unlikely very unlikely bordering on impossible but if there's an infinitely small chance that happens it's just net won't be observed then you could go the other way but that's not what we see when we're talking about many many many not even millions millions of millions of millions of millions of molecules you're going to see the ones with the higher kinetic energy on average mix in and transfer it to the ones with lower kinetic energy and so that's why they were able to say hey we don't see any spontaneous transfer of heat from cold to hot it is always going from it is always going from hot to cold
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