 So we have a collection of objects in front of us. We have some blue squares We have some green circles and we have some yellow circles and let's say I'm interested in these blue squares I want to study them closely. I want to know whether they interact with each other and how they interact with the other shapes So what if I group them by just drawing a boundary around these squares? So by doing this what I've done is I have separated out the portion that I want to study and now I can think of This collection which is within the boundary as a system of objects that I want to study So within this is the system this circle here is the boundary and everything outside this boundary is let's say what we call Surroundings so what I've done is I've made it a bit easier to describe this configuration So this helps set context So let's say you walk up to your friends and you want to discuss some thermodynamics with them as one does of course You want to be able to easily communicate what is it that you're interested in and as soon as you define a boundary You separate the part of your interest which is the system from whatever is not included in the system That is the surroundings and again this choice of boundary is arbitrary So if I say something like my system has yellow circles I can draw a boundary around this and everything within this becomes my system and everything else which is outside This becomes my surroundings So if we go back to our original example, which was the system of blue squares There are some questions that I can ask about this boundary Can any one of the circles enter this system or can one of the squares Exit this system and what if maybe they can't go in or go out physically But there is some exchange of energy at this boundary like if you pick up a glass of hot milk Your hands are touching the glass and not the hot milk directly, but still you feel the heat So if these blue squares are at a higher temperature and all of the circles are at a lower temperature Will there be an energy transfer at this boundary? So to answer these questions we can build on top of this idea And classify systems based on how they interact with their surroundings Let's see how that is done So to study the interactions between systems and surroundings Let's take this bottle with some water in it So in the example we saw before the boundary was imaginary and arbitrary Here let's take the boundary to be the outside surface of this bottle So in this case the boundary is a real physical boundary And what we are interested in is across this boundary Whether matter or energy can be exchanged So in case of this bottle now because it is an open bottle You can add more water into it or you can remove some water from this So we know that in this case matter can be added or removed And so an exchange of matter is possible across the boundary And similarly if we think of the exchange of energy So if this water was at a higher temperature Just by touching the surface outside because of the energy transfer You would realize that the water is hot Which is why in this case an energy exchange is also possible So such a system which allows for the additional removal of matter Or for the exchange of energy with its surroundings Is called an open system But now what if I cap this bottle And now what I have done is I have closed this bottle So after this I cannot add more water into it And I cannot take out water from this So in that case I have prevented the exchange of matter across the boundary But still if the water is hot or cold The outer surface of the bottle will also be hot or cold And so even by capping the bottle Although there is no exchange of matter There is exchange of energy across the boundary And such a system is called a closed system But what if I want to prevent the exchange of matter and energy So let's say what I do is I coat this bottle Or the surface of the bottle with some insulating material And now what I'm doing is because of this insulation I'm making sure that the energy transfer across this boundary is also stopped So what is happening in this case is that there is no transfer of matter Nor there is a transfer of energy And such a system is called an isolated system So you can see that how based on whether matter or energy can be exchanged We can classify the systems as open, closed or isolated But I want to bring out a point here To make the system isolated We assume that we are coating this bottle in some material But usually in real life we would never find examples like this And so the point here is that isolated systems are a hypothetical construct They sometimes help us simplify our calculations Or remove some complicated external effects And we can get good approximations which are useful And there's one more thing here So it's easy to identify whether there has been an exchange of matter Like for example when this bottle was not closed We could easily see how this would be an open system Because we could add water into this bottle or take out water from it But sometimes when we look at the exchange of energy It can get tricky So let's take another example and see this in detail Sometimes when you're trying to identify whether a system is open or closed or isolated Things may get tricky Let's go through one example and try to understand this better Earlier we had seen the system of a bottle with some water in it But now let us look at this system Which is slightly more complicated looking Let me just take you through all the parts of this system So here there is a block And this block is closing off this container Which has some gas inside And you can see that I've drawn these gas molecules And we know that they are randomly moving about within this box But the way I have set up this container Is that I can push on this block to move it inwards And we'll assume that there is no friction along these walls So although the block is still now at this moment We are going to assume that we can push the block from here And it can move downwards And also inside this chamber along all these walls Let's assume that we have coated them with some insulating material And what this does is this insulates all the walls of this chamber So what we're saying is heat cannot be transferred in or transferred out via these boundaries Or another way to say the same thing is that this wall or this boundary is adiabatic So there is no heat coming in or heat going out So this is the setup Now what we want to find out here is what type of system is this Is this an open system or a closed system or an isolated system So to check that we will look at two things Whether the exchange of matter is possible And whether the exchange of energy is possible So first let's look at matter Now because this block is placed here And all of the gas is contained within this chamber Gas cannot escape from this Or more gas cannot be introduced into the chamber And so for this system Matter exchange is definitely not possible So we know that this cannot be an open system Because if matter exchange was allowed It would be an open system So now let's look at energy So we know that this boundary inside the chamber is adiabatic Which means there will be no transfer of heat across this boundary So if we know that there will be no heat transfer across this boundary Can we also say that there will be no energy transfer across this boundary Let's think about this Now we know that this block can be pushed So what if we push this block downwards So when we push this block A part of that mechanical energy went into moving this block downwards And a part of it also got transferred to the gas molecules Because we see that the kinetic energy of the molecules has increased And they start moving faster So that means even when there was no transfer of heat possible across this boundary Energy transfer was still possible And so because we cannot exchange matter but energy can be exchanged We know that this setup is a closed system And the important point I want you to remember from this exercise Is that whenever you're checking for energy transfer You should check for energy transfer across all forms Including heat and mechanical work