 Welcome back. Now it is time for us to formally begin the study of the first law of thermodynamics. We begin by considering the word adiabatic. This is the first word which is specific to thermodynamics. First thing is adiabatic is an adjective. It applies to a boundary, system boundary or a wall or an interface. It can also apply to a system and can also be applied to process. The word adiabatic is a short form for the phrase one that permits work interaction only, nothing more, nothing less. It can be applied to a boundary or a wall in which case we will say that if that boundary allows only work type of interaction, it is an adiabatic boundary or an adiabatic wall. Using our idea of operational definition, we can at any time determine whether a boundary or the adiabatic or not by doing the following experiment. We create a system where this boundary is part of that boundary. Let the system execute some interaction across this boundary. If we find that the interaction is only of the work type as indicated by the operational definition of work, then it is an adiabatic boundary. If we find that the interaction cannot be fully classified as the work type of interaction, then it is a not adiabatic boundary, not an adiabatic. We will give it a different word later. Just now let us look at adiabatic boundary. Similarly a system which is enclosed completely in adiabatic boundaries will be an adiabatic system. So such a system will be restricted only to do the work type of interaction. Similarly a process in which only work interaction takes place, we can call it an adiabatic process. So this way we will notice that the adjective adiabatic can be applied to a boundary, a wall or an interface, can also be applied to a system, can also be applied to a process. It is a restrictive adjectives which means one that permits work interaction only or one during which like a process only work interaction takes place. Now we are going to look at Carre Theodori's form of the process. This as we will soon see is a statement generalizing the behavior of adiabatic system. Small diversion, why do we consider Carre Theodori's form? We consider Carre Theodori's form because that way we can define energy or energy change, we can define heat transfer without referring to anything which we have not already defined. Because if we use something in our definition which is not already defined, it is possible that we may get into the trap of circular definitions. Thank you.