RAC Lecture 5 - 1 Lesson 5 Review of fundamental principles...

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Lesson 5 Review of fundamental principles – Thermodynamics : Part II
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.The specific objectives are to : 1. State principles of evaluating thermodynamic properties of pure substances using: a) Equations of State ( Section 5.2 ) b) Thermodynamic charts ( Section 5.2 ) c) Thermodynamic tables ( Section 5.2 ) 2. Derive expressions for heat and work transfer in important thermodynamic processes such as: a) Isochoric process ( Section 5.3 ) b) Isobaric process ( Section 5.3 ) c) Isothermal process ( Section 5.3 ) d) Isentropic process ( Section 5.3 ) e) Isenthalpic process etc. ( Section 5.3 ) At the end of the lesson the student should be able to: 1. Evaluate thermodynamic properties using equations of state, tables and charts 2. Identify various regimes on T-s and P-h charts 3. Estimate heat and work transferred in various thermodynamic processes 5.1. Thermodynamic relations There are some general thermodynamic relations, which are useful for determination of several thermodynamic properties from measured data on a few properties. The following relationships are generally used for the evaluation of entropy change. These are called T ds equations. They are obtained by applying first and second laws of thermodynamics first equation second equation Td s d u pd v s hvd P s =+ =− (5.1) Two more fundamental thermodynamic relations can be obtained by defining two new properties called Gibbs and Helmholtz functions. 5.2. Evaluation of thermodynamic properties In order to perform thermodynamic calculations, one has to know various thermodynamic properties of the system. Properties such as internal energy, enthalpy and entropy cannot be measured directly . Thermodynamics gives mathematical relations using which one can obtain properties, which cannot be measured directly in terms of the measurable properties such as pressure, temperature, volume, specific heat etc.
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1. Thermodynamic equations of state 2. Thermodynamic tables 3. Thermodynamic charts 4. Direct experimental results, and 5. The formulae of statistical thermodynamics An equation of state (EOS) is a fundamental equation, which expresses the relationship between pressure, specific volume and temperature. The simplest equation of state is that for an incompressible substance (e.g. solids and liquids), which states that the specific volume is constant. The next simplest EOS is that for an ideal gas. Ideal (perfect) gas equation is a special equation of state, which is applicable to ideal gases. The molecular forces of attraction between gas molecules are small compared to those in liquids. In the limit when these forces are zero, a gas is called a perfect gas. In addition the volume of the molecules should be negligible compared to total volume for a perfect gas. The perfect or ideal gas equation of state is given by: RT Pv = (5.2) Where P = Absolute pressure v = Specific volume R = Gas constant T = Absolute temperature The gas constant R is given by: M / R R u = (5.3) Where R u = Universal gas constant M = Molecular weight The ideal gas equation is satisfactory for low molecular mass, real gases at relatively high temperatures and low pressures. Ideal gas equation can be used for evaluating properties of
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RAC Lecture 5 - 1 Lesson 5 Review of fundamental principles...

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