 We will now look at some simple classification of systems. This depends on essentially the type of boundaries that the system has, the behavior of boundaries. In particular what are the interactions which are possible across the boundary and the main interaction which we can look at for classification is the mass flow. For example, let us consider movement of mass across system boundaries. If this is possible then we call the system an open system. However, if the flow of mass or movement of mass is not possible or is prevented then we call the system a closed system. This is the basic classification of thermodynamic system. It depends on the type of boundaries that it has. If a system has a part of the boundary at least across which mass can flow in and flow out, we will call it an open system. If mass flow is not possible or is prevented by some technique then we will call it a closed system. Take for example again this water bottle. Now if it is sealed and the cap of the bottle is closed tight so that it is leak proof then we say that if my system is whatever is contained in the bottle, air and water, boundaries are the inner surfaces of the bottle then it is a closed system. No mass can come in, no mass can go out. However, it is possible that the water is cold, my hands are warm, water is at 27 degrees C, my hands are at 36 degrees C. There is a temperature difference so heat may flow from my hands to the water in the bottle, heating it up. That is an energy transaction. We will study these in detail later. So although it is a closed system, although mass flow is prevented, it is possible that energy transaction does take place. But it is a closed system because mass cannot come in and go out. However, if a part of the boundary is open like this and water can come in or go out, I can drink water out of it. Then during that process and even now the system has defined but now with this top surface as an imaginary surface is an open thermodynamic system. You should understand this basic difference between closed thermodynamic systems and open thermodynamic systems. And you remember that energy transfer in any form is possible in either case, whether the system is closed or the system is open. However, it is possible that the boundaries of a system are such that mass flow is prevented and energy flow is also prevented. In such a case, we call the system an isolated system. No energy transactions, no mass exchange. Closed systems are important because they are simple to study. We do not have to worry about mass coming in and going out. The laws of thermodynamics will be derived and we will study them for closed systems. And then we will transform them so that they are applicable to open systems. So hence forth till about maybe halfway or slightly more than halfway through the course, we would essentially be looking at closed thermodynamic systems. And then we will say that look now we have studied enough of thermodynamics as pertaining to closed systems. Let us now look at open systems and convert these laws into appropriate forms applicable to open thermodynamic systems.