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### Thermal interactions between macroscopic systems

To begin with we first define the macroscopic state or macrostate of a system. Macrostates of a system can be defined by specifying the external parameters of the system (for example volume of any system) and any other conditions or constraints imposed on the system for example consider an isolated system , macrostates of this system can be specified by specifying the parameters of the system like volume of the system and total energy of the system which is constant.
For discussing thermal interactions between two systems let us consider two macroscopic systems S and S' which are allowed to interact with each other in a way that they both can only exchange energy but we must keep in mind that total energy of combined system S+S' remains constant. For pure thermal interactions to take place between two systems external parameters of the system remains constant so that energy levels of the system does not change. Because of thermal interactions between two systems energy transfer takes place between two systems and the mean energy transfer takes place between two systems i.e., from one system to another. This mean energy transfer between two systems as a result of pure thermal interactions between these two systems is known as HEAT.
Consider change in mean energy of system S is known as heat absorbed by the system. We all know that heat transfer between two systems can be negative as well as positive. The quantity we say -Q is called heat given off by the system. Since combined energy of both the systems is constant from this statement we can conclude that
Q+Q'=0 or, Q=-Q'
From above expression we can draw a conclusion that heat absorbed by one system is equal to heat given off by another system. Thus due to thermal interactions between two systems S and S' transfer of heat takes place from one system to another and remember that in case of pure thermal interactions external parameters of the system always remains constant.