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Unformatted text preview: M. S. Shell 2009 1/14 last modified 10/7/2009 The first law and reversibility ChE210A So far, we have discussed the properties of equilibrium states and the relationship between the thermodynamic parameters that describe them. Here, we will discuss what happens when we change from one thermodynamic state to another. That is, we will discusses processes. The first law for processes in closed systems An important concept in the discussion of processes is that of a path . A process can be defined by the path of changes that take a system from one state to another. State variables are independent of such paths. That is, they only depend on the initial and final states in the process, and not the intervening steps. State variables include g,G,, ,,, . Now we will introduce the very important first law of thermodynamics , in differential form: G You will often see this equation using , rather than G , notation for the internal energy. There is no difference in the significance of the first law equation in this case; rather, it is simply a matter of notational convention. The first law is essentially a statement of the conservation of energy. Recall, from Newtons equations of motions for the time evolution of classical molecular systems, that the total energy is conserved. Therefore, if the energy G of such a system changes, then it must be accounted by a mechanism of energy transfer between the system and the surroundings. The first law defines two mechanisms by which this can occur: measures energy transfer due to work . Work represents some kind of ener- gy transfer that we can measure (and potentially use) at the macroscopic level. This could be the movement of a piston to perform mechanical work, but it could also be a number of other measureable changes, like the production of an elec- tric current (electrochemical work). measures energy transfer due to heat . Heat is essentially the part of the change in energy that cannot be accounted for at the macroscopic level in terms of work. It is energy that is distributed to molecules in a random fashion that produces no net macroscopic work. In this sense, represents the microscopic component of energy transfer. M. S. Shell 2009 2/14 last modified 10/7/2009 The first law simply states that changes in the energy of a system must exactly balance with any transfers of heat and work. Before we continue, we will comment briefly about sign conventions. Sometimes the first law is written with gG . This is simply a difference of convention that changes the significance of positive and negative values of work. You can rationalize signs on the basis of physical intuition about energy transfers. In our positive convention, The system does work on its surroundings simply means that some energy left the system and went to the surroundings in the form of work. Thus, gG 0 , since the energy of the system decreased....
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- Fall '08