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General Form of the First Law

Let us consider the general case in which a change of state is brought about both by work and by transfer of heat. It is convenient to think of the system as being in thermal contact with a body that acts as a heat bath (one that transfers heat but does no work) as well as in non-thermal contact with another portion of the environment, so that work can be done. [Pg.38]

The value of A17 in the change of state can be determined from the initial and final states of the system as well as from a comparison with previous experiments that used only adiabatic work. The work W can be calculated from the changes in the environment (for example, from the change in position of a weight). The value of Q is determined from the change of state of the heat bath, which was also previously calibrated by experiments with adiabatic work. [Pg.38]

Although the change in state of the heat bath, hence the value of Q, usually is determined by measuring a change in temperature, this is a matter of convenience and custom. For a pure substance the state of a system is determined by specifying the values of two intensive variables. For a heat bath whose volume (and density) is hxed, the temperature is a convenient second variable. A measurement of the pressure, viscosity, or surface tension would determine the state of the system equally as well. This point is important to the logic of our development because a later dehnition of a temperature scale is based on heat measurements. To avoid circularity, the measurement of heat must be independent of the measurement of temperature. [Pg.39]

Although the proportions of the energy change contributed by heat and by work can vary from one extreme to the other, the sum of the heat effect and the work effect is always equal to At/ for a given change of state, as indicated in Equation (3.13). [Pg.39]

That is, Q and W can vary for a given change of state, depending on how the change is carried out, but the quantity (Q + W) is always equal to At/, which depends only on the initial and final states. [Pg.39]


Note that the total differential d Ne) on the Ihs differs from the terms dN and dNp on the rhs that is, (2.4.10) allows for the possibility that the amount of material in the system can change during a process. This most general form of the first law is always true. However in many situations, this general form simplifies because some contributions are zero or are negligible compared to other contributions. For easy reference, we collect many of its useful forms here. [Pg.58]

In this chapter, we briefly describe fundamental concepts of heat transfer. We begin in Section 20.1 with a description of heat conduction. We base this description on three key points Fourier s law for conduction, energy transport through a thin film, and energy transport in a semi-infinite slab. In Section 20.2, we discuss energy conservation equations that are general forms of the first law of thermodynamics. In Section 20.3, we analyze interfacial heat transfer in terms of heat transfer coefficients, and in Section 20.4, we discuss numerical values of thermal conductivities, thermal diffusivities, and heat transfer coefficients. [Pg.568]


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