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Current time:0:00Total duration:10:56

Video transcript

let's talk about something called as thermal expansion thermal expansion in this video we'll first understand what this is then we'll look at a couple of examples and then finally we will try to dig a little bit deeper and understand the reason behind this phenomenon okay so let's begin what comes to your mind when you hear the word thermal for me thermal is something like heat and expansion means something is getting bigger right expanding bigger so we would say getting bigger so the idea is if you heat something or you increase its temperature then it'll expand it will get bigger in size and similarly if you cool it down if you were to decrease its temperature then it will shrink in size it will contract and most things really do that I say most because there are some exceptions to it but most things around us really experience this phenomenon so let's first take a couple of examples to convince ourselves this is really happening around us I think the most a most common example can be seen in a thermometer it consists of a glass tube which is filled with some kind of a colored liquid so we can see that usually it's mercury sometimes they use other stuff as well and so if you take this thermometer and you hang it in your room let's say and if the room gets hotter then the mercury the liquid inside also gets hotter and as a result this mercury will expand now as it expands you will see the level of the mercury rising like this and similarly if the room were to get cooler or maybe you were to put this thermometer in some ice-cold water or something then the mercury would get cooler and it would contract it would shrink in size and therefore the the level now will start dropping and again you can see this and therefore the clever application is just by looking at the level of the mercury you can sort of understand the temperature of the temperature of the surrounding another example can be seen in this picture take a very good look at this picture can you see something weird over here the railway tracks have buckled side words what do you think has happened over here well my guess is that this may have been an extremely hard day due to which the tracks expanded a lot but there is no room to expand along the length over here and it's for that reason the tracks ended up expanding towards the side and this is extremely bad this is very dangerous for the trains and so engineers have to take care of this so what do you think they do well if you've ever taken a walk along a railway track you may have noticed that there are some gaps left in between these gaps are left just to make sure this doesn't happen so that just in case if the temperature goes very high and if the tracks expand a lot then they can just expand into this gap and not side words and this buckling can be avoided so again another example that we can relate to in our daily life and there are plenty of other very interesting examples and I really encourage you to explore them by going over the Internet or discussing with your friends and teachers but now for the rest of the video let's try and dig a little bit deeper and find out what's the cause for this thermal expansion why do things expand on heating and a contract on cooling alright let's figure this out using an example imagine we have a a metallic bar with us let's say we have gold yeah we'll assume that someone gifted us this big gold bar and we are going to use this to understand the secret of thermal expansion now to find out the secret to figure out the secret we have to look into the microscopically microscopic picture of matter so imagine we zoom in zoom in this is a magnifying glass so if you zoom in a lot what would we see well recall that all matter is made up of is made up of atoms so we would probably see gold atoms so maybe something like this and our gold bar is sitting on our table right now at room temperature so the temperature is say 27 degrees Celsius and so what are these atoms doing at room temperature are they addressed well it turns out that they're not addressed they're sort of like vibrating they're sort of jiggling around they're pretty much localized but they do jiggle a bit okay all atoms are doing that and it's for this reason we will see that if you pick any two atoms so let's say you pick these two neighboring atoms the distance between them keeps changing sometimes they come close to each other sometimes they go farther away and we can simplify this by assuming that the atoms are fixed at some average distance between them so even if the distance between them is changing and the atoms are vibrating we'll assume that they are there at rest they're fixed at some average distance apart and so now the big question is what happens when we increase this temperature what will happen if we increase this temperature well if we do that then the atoms jiggle even more so the atoms will imagine this is the vibration at 27 degree Celsius as we heat it up as we heat up our gold bar we will see that the atoms of the gold bar end up vibrating more rigorously all right all atoms do that and as a result what we will find is that all the atoms end up going farther away from each other the average distance between them starts increasing let me write that down all right so when temperature increases when temperature increases we will see that the atoms vibrate more atoms vibrate more they get more kinetic energy and as a result the average distance between them the average distance between them also increases and it's for that reason if if we were to heat this up then we would see probably something like this the same atoms would now be much farther apart so what would be here like this there'd be all much farther up and you may be wondering well why is that if the atoms vibrate more that the average distance must increase and that would be a good question but turns out to be a little bit more complicated really interesting we thought that we live it more complicated and so that deserves a separate video all by itself so maybe we'll look at that in another video but over here which is gonna accept the fact that if the atoms vibrate more than their average distance increases and from this we can pretty much conclude what's going to happen if all the atoms go farther away from each other and that's the key to understanding this if all the atoms go farther away then they end up occupying more space and the whole gold bar the material as a whole will end up expanding and that's really what's going on over here so let me just keep that somewhere over here all right so that's the true reason for thermal expansion because the atoms vibrate more and they start going farther away from each other and we can also see now what happens when you cool this gold bar if you were to cool this thing down then the atoms get less kinetic energy they vibrate less and as a result they come closer to each other occupying less space and so the whole material ends up contracting and so this is the secret behind thermal expansion all right so let's now look at a classic a classic question that's usually asked when it comes to thermal expansion I'll go something like this imagine we have some kind of a material so let's say we have we have some kind of a metallic sheet over here so this is some metallic sheet at some temperature and let's imagine we punch out a hole from it so imagine we take a nice circular hole and we punch it out and suppose we heat this thing up heat it up in it the question now is what's going to happen to this hole as the material expands due to due to increasing temperature do you think the hole gets bigger in size or reading the hole gets in size I want you to really pause this video for a while and think about this come up with an explanation in your head as to what's going to happen alright okay let me tell you the most common misconception over here we may think that as the material expands in all directions as the material expands this way in all directions the whole thing could also expand into the whole and therefore the whole size would get smaller something like this but guess what that's wrong that cannot happen and they're very very easy way to think about this is we go back and we remember that as temperature increases remember over here as temperature increases the average distance between all the atoms in the end up increasing and so if we pick if we pick a couple of atoms over here let's say one over here and one over here the average distance between them should increase when the material expands but if the material expands into the hole or if the whole hole size shrinks then we would see that the atoms have come closer to each other and that's not possible we just discussed that and as a result of this we could now conclude that if all items go farther away then the only way that can happen is that the hole size should increase and this is very important all right so the hole Oh gets bigger just like that Oh gets bigger and let me tell you the best way to think about this the best way I like to think about thermal expansion in most cases is just imagine that we take a photo and we just stretch it like this you just stretch it and when you stretch a photo you can clearly see that everything inside the photo becomes bigger including including that hole all right so don't fall into that trap of thinking that the hole gets smaller in size