 Let us now look at thermodynamic interactions. The scheme of thermodynamics is to consider the behavior of two systems. Let us call it system A. Let us call this system system B. You may call this system A as system, system B as surroundings but let us continue with this nomenclature system A and system B. What these two systems do is they transfer energy. They have a transaction of energy transfer between them and because of this energy transfer, system A undergoes a change of state for a process from an initial state say A1 to a final state say A2 and system B also undergoes a process and its state changes from an initial state B1 to its final state B2. These energy transfers require that two systems are involved and second one, the energy we have not exactly defined what is meant by energy but let us use our feeling from our study of physics is transported from one system to the other across the boundary that separates them. So we can consider a boundary separating the two and what transaction takes place must be across this boundary. These energy transfers are known as interactions and the whole scheme of thermodynamics is to study the interactions, the type of interactions, the quantification of interactions and relate them to change in the state of system A and the change in the state of system B. In this course we are going to look at two types of interactions and thermodynamics allows only two types of thermodynamics interactions. The first one is the work type of interaction and this interaction we will define but this interaction is a primitive in the sense that it is defined in other branches of physics. Right from our high school days we know what is work and how work is defined and how work is major but thermodynamics defines another type of interaction which is the heat type of interaction and this is defined by thermodynamics. Our next major topic would be a study of the work interaction. Thank you.