SPH 302 THERMODYNAMICS

Transcription

1 THERMODYNAMICS Nyongesa F. W., PhD. 1 Course Outline Lecture 1: Thermodynamic concepts & Zeroth Law Lecture 2: 1 ST Law Lecture 3: 2 ND Law Lecture 4: Entropy and 2 ND Law Lecture 5: Thermodynamic potentials Lecture 6: Phase Changes & Equlibria Nyongesa F. W., Ph.D 2 1

2 Prerequisites SPH 203 Calculus ODE Nyongesa F. W., Ph.D 3 Introduction & Overview Before industrial revolution, machinery were powered by animals 2

3 James watt Steam Engine 1 ST Engine (invented1790) to convert steam to mechanical work - lead to industrial revolution 3

4 Steam Engines of Industrial Revolution Steam powered cars 4

5 Steam powered Industries 5

6 By 19 th cent, a new science was born THERMODYNAMICS GOAL: Asses efficiency of devices that convert heat work e.g., heat engines Steam (Heat) Work output Nyongesa F. W., Ph.D 11 Nyongesa F. W., Ph.D 12 6

7 1 ST Diesel Engines -1930s Hybrids & gas turbines 7

8 15 What is Thermodynamics? Thermo Energy transfer in form of heat Dynamics Energy transfer in form of mechanical work Thermodynamics = science that govern energy conversions processes (heat work) Nyongesa F. W., Ph.D 16 8

9 Why Thermodynamics? It gives Laws that govern energy conversion processes Solar Chemical ENERGY Mechanical Thermal Energy 1 (heat) Relationship? Laws of Thermodynamics Energy 2 (work) Nyongesa F. W., Ph.D 17 Food (Energy) input Work out Waste Nyongesa F. W., Ph.D 18 9

10 waste W Nyongesa F. W., Ph.D 19 Heat pumps Work in Heat out Nyongesa F. W., Ph.D 20 10

11 Laws of Thermodynamics Energy conversions processes are governed by 4 Laws Zeroth Law: Gives condition for thermal equilibrium between 2 bodies Nyongesa F. W., Ph.D 21 First Law : Defines relationship between heat input (dq), work (dw) and internal energy (du) in a system dq = du + dw dw dq du Nyongesa F. W., Ph.D 22 11

12 No animal works continously without eating or wearing out 23 Second Law: (i) Defines efficiency of processes that convert heat work. i.e., No heat engine with 100% efficiency. If you eat, you must shit. No exhaust Engine Fuel Tank 24 12

13 Food input) Work out Impossible 25 (ii) Gives direction of natural occurring processes - Arrow of time Naturally processes proceed in a direction such as to increase Total Entropy (disorder) of system Disorder 26 13

14 Disorder 27 Third Law: Explains behaviour of systems as T absolute zero As T 0, Entropy change (ds) 0 system becomes orderly E.g., water ice Nyongesa F. W., Ph.D 28 14

15 Lecture 1 Thermodynamic Concepts & Zeroth Law Thermodynamic concepts Ideal Gas Laws Zeroth Law Nyongesa F. W., Ph.D 29 Objectives Define thermodynamics concepts Explain ideal gas Laws Explain Zeroth Law and its significance Nyongesa F. W., Ph.D 30 15

16 Thermodynamic concepts Energy conversion processes are studied using concept of thermodynamic system System Part of material universe that can be isolated completely from the rest for investigation SYSTEM + SURROUNDING = UNIVERSE sorrounding system boundary Nyongesa F. W., Ph.D 31 Types of systems 3 main types (a) Open systems:- Allows exchange of both heat and matter through the boundary. E.g. human digestive system. Nyongesa F. W., Ph.D 32 16

17 (b) Closed systems:- Allows only exchange of heat with surrounding E.g. refrigerator or Engine Nyongesa F. W., Ph.D 33 (c) Isolated System:- No heat or matter exchange occurs with surrounding. The walls are Adiabatic (Adiathermal) E.g. Vacuum flask. Nyongesa F. W., Ph.D 34 17

18 Thermodynamic Variables:- parameters that describe behavior or state of system i.e., P, T, V & composition ( ) Extensive variable:- Dependent on mass/size of the substance present in the system e.g., U, S, V etc. Intensive variable:- opposite of Extensive Nyongesa F. W., Ph.D 35 Heat:- Transfer of thermal energy between systems as result of temp difference Work:-Transfer of mechanical energy Both Heat & Work = ways of transferring energy Nyongesa F. W., Ph.D 36 18

19 Thermodynamic equilibrium:- When system experiences thermal, mechanical and chemical equilibrium S.T. state variables (P, V, T & ) are const. Nyongesa F. W., Ph.D 37 Working substance:- Fluid enclosed in the system that either receives or transfers energy to the surrounding in the form of heat or work. In Thermodynamics, we use ideal gas as the working substance. WHY? Nyongesa F. W., Ph.D 38 19

20 Activity Name the working substance in these systems Nyongesa F. W., Ph.D 39 Lecture Evaluation Explain the difference between Open & Closed system Adiabatic & Diathermal Wall Extensive & Intensive variables State function and state variable Heat and work Nyongesa F. W., Ph.D 40 20

21 1.2 Behavior of Ideal gases An ideal gas is an abstraction whose properties represent the limiting behavior of real gases at very low densities/high P. Assumptions in ideal gas (i) no intermolecular attractions S.T internal energy is entirely K.E Temp (ii) Molecules occupy negligible volume Nyongesa F. W., Ph.D 42 21

22 Behavior of ideal gas depends on T, P and V and obey gas laws: (a) Boyle s law: Compressing a gas increases P 1 P V PV C P PV = c V Nyongesa F. W., Ph.D 43 (b) Pressure law: Heating a gas increases P P T P T C (c) Charles Law: Heating a gas increases V -273 K P V T V T V T C -273 K Nyongesa F. W., Ph.D 44 T 22

23 (d) V No. of moles (n) of the gas i.e. V n Combining (a) to (d) gives PV = nrt Equation of state..(1.1) State of a system is given by Eqn of State Nyongesa F. W., Ph.D 45 P-V Diagram From PV = nrt Behaviour/processes of ideal gas is represented on PV diagrams P PV = c V Nyongesa F. W., Ph.D 46 23

24 Non Ideal (Real) Gas Characterized by Gas cannot be compressed to zero volume since gas molecules occupy finite volume and liquefy at low P Intermolecular attractions Nyongesa F. W., Ph.D 47 Eqn of state of real gas is given by P v a 2 V nb nrt Where a & b molecular attractions & v = molecular volume Nyongesa F. W., Ph.D 48 24

25 Activity State difference between Ideal & Nonideal gas Nyongesa F. W., Ph.D 49 Zeroth law of Thermodynamics A hot object looses heat to attain thermal equilibrium with surrounding Nyongesa F. W., Ph.D 50 25

26 Statement of Zeroth Law "If objects A and B are separately in thermal equilibrium with third object C, then A and B are in thermal equilibrium with each other. A C B 51 Significance of Zeroth Law Gives condition for thermal equilibrium between 2 bodies Introduces concept of temperature and how it is determined. Temperature = measure of degree of hotness Nyongesa F. W., Ph.D 52 26

27 Lecture -Evaluation 1. Explain an ideal gas and ideal gas laws 2. State Equation of State 3. State Zeroth Law and its significance Nyongesa F. W., Ph.D 53 27