Energy, External Work, and Heat Flow

The total energy of all sorts contained within the boundary of the system is called the internal energy of the system, and is denoted by U. From an atomic point of view, the internal energy consists of kinetic and potential energies of all the atoms of the system, or carrying it 

Equation (2.1) is the mathematical statement of the first law of thermodynamics. It is to be noted that both sides of the equation should be expressed in the same units. Thus if internal energy and mechanical work are expressed in ergs, the heat absorbed must be converted to ergs by use of the mechanical equivalent of heat, 

Or if the heat absorbed is to be measured in calories, the work and internal energy should be converted into that unit. 

It is of the utmost importance to realize that the distinction between heat flow n.nd moclinmcal worki which we have made in talking about pnorgry in into n. system, does not apply to the energy once it is in 

The internal energy of a system depends only on the state of the system that is, on pressure, volume, temperature, or whatever variables are used to describe the system uniquely. Thus, the change in internal energy between two states 1 and 2 depends oidy on these states. This change of internal energy is an integral, 

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