Thermodynamics sign convention

The pump work (negative because of the thermodynamic sign convention) is equal to minus trie friction loss. This is the type 1 problem from the previous list. 

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... 

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 opposite sign convention for work appears if we put a minus sign in Equation (3.3). Such a convention was formerly common in thermodynamics. 

Note that we call this term positive when work is done on the system, just as we make Q positive when heat is removed from the reactor. Thus the signs on Wj and Q are opposite to those of dq and dw, in the thermodynamic equations. We are always interested in shaft work done by a stirrer on the reactor fluid, and we are usually interested in cooling rather than heating. Therefore, we carry these sign conventions so that we can be sure of the signs of these terms. 

It is standard practice in thermodynamics to divide things up into the system and the surroundings (Frame 1). Figure 7.1 shows two examples. In defining the system we define a definitive boundary. Once this designation is made, thermodynamics then considers the transfer of energy (defined as the capacity to do work, w or transfer heat, q) and matter to and from the system. For this we need to adopt a sign convention. 

Accordingly, equations (8.8), (8.9) and (8.10) all represent statements of the First Law of Thermodynamics as it applies to the different scenarios above. Because of the sign convention needed (Frame 7) the equation adopts its varying forms depending on the energy balances which are taking place. 

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.). 

On what law is the first law of thermodynamics based Explain the sign conventions in the equation... 

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

Thus the maximum work obtainable from real (irreversible) processes is always less than the theoretical limit given by the appropriate thermodynamic potential for ideal (reversible) processes. Alternatively, the work or heat input required to make a real process go is always more than the amount given by the thermodynamic potential for the ideal (reversible) process. Don t forget, if the > sign in the work equations looks backwards to you, the sign convention we adopted in 4.6 means that —w is the work obtainable from a system, so that w > Ax is the same as —w < —Ax, and is read —w is less than or equal to the decrease in x . ... 

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 ... 

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