Thermodynamics sign conventions used

The sign convention used here for CFSE follows the thermodynamic convention. 

The simple reaction turbine is analyzed most easily by the angular momentum balance, Eq. 7.65, which, as shown in Sec. 7.7 for a steady-flow turbine, reduces to Euler s turbine equation (Eq. 7.66). To find the power produced per unitjtime, we multiply both sides of Eq. 7.66 by -minus sign is used here tp agree with the thermodynamic sign convention that power... 

Note The sign convention used for EA is in contradiction to that usually found in thermodynamics, an unfortunate historical artifact. 

The sign convention used for electron affinity is the normal thermodynamic one in which a negative sign corresponds to an exothermic process. 

In the case of a flowing fluid the mechanical pressure is not necessarily the same as the thermodynamic pressure as is the case in a static fluid. The pressure in a flowing fluid is defined as the average of the normal stress components. In the case of inelastic fluids, the normal stress components are equal and therefore, with the negative sign convention, equal to the pressure. It is for this reason that the pressure can be used in place of the normal stress when writing force balances for inelastic liquids, as was done in Examples 1.7-1.9. 

Many of the reactions which chemists study are reactions that occur at constant pressure. Because this constant pressure situation is so common in chemistry, scientists use a special thermodynamic term to describe this energy, enthalpy. The enthalpy change, AH, is equal to the heat gained or lost by the system during constant pressure conditions. The following sign conventions apply ... 

The overall reaction of the electrolysis cell (Eq. 7) provides the required stoichiometric coefficients for the products and reactant used in Eq. 8. The sign convention is positive for products and negative for reactants with analogous definitions for AB, AC and AD. Data for the constants A, B, C and D are thermodynamic properties and are reproduced in Table 1 from,19... 

The above sign convention is not universal, and the reader must exercise caution and check what sign convention is being used by a particular author when studying the literature. For example, work is often defined in some texts as positive when the system does work on the surroundings (cf the THEODORE TUTORIAL, Thermodynamics, ETS International, Roanoke, VA, 1995.). 

What is the first law of thermodynamics How can a system change its internal energy, E What are the sign conventions for thermodynamic quantities used in this text ... 

This reaction is always endothermic (IE, > 0). The sign is a convention of thermodynamics some fields use the opposite convention. 

Sign conventions can be troublesome to students, especially when they encounter type (iii) boundary conditions. It is always wise to double-check to ensure that the sign of the left-hand side is the same as that of the right-hand side. Otherwise, negative transport coefficients will be produced, which is thermodynamically impossible. To guard against such inadvertent errors, it is useful to produce a sketch showing the qualitative shape of the expected profiles. 

I define work and heat using the standard sign conventions I define state functions and explain their importance I state the first law of thermodynamics in words and as an equation... 

Heat and work [10 ]. As mentioned at the beginning of the chapter, there is a sign convention for heat and work (6.15) in the energy balance (first law of thermodynamics). Can you use a different convention, for example, by calling heat negative when the temperature of the system is lower than the surroundings ... 

The opposite convention is also used by some authors for the stress, in which case Equation 2.7a would be written cxxx = p + txx- This can be confusing to the student new to the subject, but the convention clearly must be irrelevant to the final physical result. Readers who have studied thermodynamics will recall a similar situation with regard to the sign convention for work work is sometimes defined as positive if done on the system, sometimes as positive if done by the system. 

We shall discuss adoption of a convention for the sign for work of expansion -(Frames 7, 9, 14 and 15) and use it when we discuss in more detail the gas expansion processes (Frame 9). Also (FIRST LAW OF THERMODYNAMICS - see Frames 2, 8) the internal energy change, A U for the overall process in Figure 1.1 (i.e. gas at Vf and 7j -> gas at Vf and Tf) (being a state function) is identical for both paths between the SAME initial and final states (and so is route independent). 

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