Chapter 1: INTRODUCTION AND BASIC CONCEPTS. Thermodynamics = Greek words : therme(heat) + dynamis(force or power)

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1 Chapter 1: INTRODUCTION AND BASIC CONCEPTS 1.1 Basic concepts and definitions Thermodynamics = Greek words : therme(heat) + dynamis(force or power) Note that, force x displacement = work; power = work/time -> Thermodynamics is the science that deals with two energy forms : heat and work. Energy = The ability to cause changes (and has the unit of (mass)*(length) 2 /(time) 2 or equivalent). Different forms of energy : heat, work, thermal, KE, PE, electric, chemical, nuclear,...etc. Macroscopic forms of energy : KE & PE of a system, heat, work, etc. Microscopic forms of energy : energy forms related to molecular and atomic structures, and the degree of the molecular and atomic activities (i.e., KE and PE of individual particles in a system) Internal energy = sum of all forms of microscopic energy. (a) Energy associated with the KE of particles = sensible energy (b) Internal energy associated with the phase of a system = latent energy 1

2 (c) Thermal energy = sensible and latent forms of internal energy. Organized and disorganized forms of energy : Fundamental laws of thermodynamics : (a) 1st law (conservation of energy): Sum of energy of different forms (heat, work, KE, etc.) = constant (if no nuclear reactions) 2

3 e.g., If 1 dollar = 100 yens, then 200 yens = 2 dollars = 1 dollar yens =... etc. (b) 2nd law (increase of entropy): work = high-grade energy; heat = low-grade energy. Thus, 1 J work -> (in a cycle) 1 J heat, but the reversed process is not always possible. 3

4 e.g., 1 dollar -> 100 yens, but 100 yens -> 0.5 dollar + 50 yens, or 100 yens -> 1 dollar is not always possible. -> actual processes occur in the direction of decreasing quality of energy. Important applications of thermodynamics : 2 cyclic devices Q H Q H W W Q L Refrigerator or heat pump Q L Heat engine (a) Heat engines : heat -> work + heat (b) Heat pumps (refrigerators) : work + heat removed from a low-temp. body -> heat received by a high-temp. body. 1.2 Dimensions and units (1) SI units : kg, m, s, N( = 1 kg.m/s2), Pa (= 1N/m2), oc or K, J (any forms of energy) Force = ma (2) English engineering units: lbm, ft, s, lbf (= 1 lbm x ft/s2), psi, of or R, 4

5 Btu (heat), J (electric energy), etc. Force = ma/gc [Examples] Units for # of molecules and atoms : 1 mole = 6.02 x 1023 particles (or molecules); 1 kmol = 6.02 x 1026 particles (or molecules) Weight = mg, where g = local gravitational acceleration [Examples] 1.3 Systems: System : a quantity of matter or a region in space chosen for study. (a) A closed system : a quantity of matter of fixed mass and identity upon which attention is focused for study. (b) An open system (control volume) : a volume in space in which one has interest for a particular study or analysis. Balloon A rigid tank Pipe Water flow Gas flow Gas turbine Closed systems Control volumes(open systems) 5

6 Surroundings = the region outside the system. Boundary= the real or imaginary surface that separates the system from its surroundings. 1.4 States and properties of a thermodynamic system 6

7 Thermodynamic equilibrium = thermal + mechanical + phase + chemical equilibria Property : any characteristic of a system that can be used to identify the state of the system.(must be a point function) Similarities and differences between Dynamics and Thermodynamics: x V y y V z z V x A mechanical system T A thermodynamic system (a) Properties of a mechanical system = velocity components and space coordinates of the system. (b) Properties of a thermodynamic system = p, T,, V, etc. Extensive properties = properties that vary with m(mass), such as V, E, H, U, etc. Intensive properties= properties independent of the mass, such as, p, T,, h(=h/m), u, etc. p v u 7

8 The state of a thermodynamic system = a function of intensive properties. The state postulate : # of independent intensive properties of a thermodynamic system = # of possible work modes + 1 e.g., Ideal gas law for a simple compressible system or substance (work mode = moving-boundary work) : pv = RT 8

9 If p and T are known, then v = RT/p, u = 1.5 RT, h = 2.5 RT (for monatomic gases), etc. 1.5 Directly measurable properties : density, pressure, and temperature (a) Density : = lim V->0, but is still a continuum m/v Specific volume v = 1/ (b) Pressure : p = lim A->0, but is still a continuum normal force/a Abs. pressure = gage pressure + atm. pressure Vacuum pressure = atm. pressure - abs. pressure (c) Temperature : Temperature scales : oc, of, K, and R. Note that, oc and of are based on the boiling and freezing point temps. of pure water at 1 atm. They are not abs. temps. Absolute temperatures(independent of substances) : K = oc R = of

10 Absolute p and T must be used in the equations of state and other thermodynamic relations Other definitions p Proc. A Air Proc. B 2 processes having identical initial and final states. p or V of A = p or V of B but work or heat of A work or heat of B Isothermal (T = const.), isobaric(p = const.), and isometric(v = const.) processes. Cycle : a process in which initial state = final state. Thus, changes of any properties = 0 for a cycle. V 10

11 Quasi-equilibrium process : a process in which departures from the equilibrium state are very small at all times.(e.g., a very slow process, variations at any time << capacities) [Examples] 11

12 Fig Slow and fast processes. 12

13 The zeroth law of thermodynamics : If systems A & C are in thermal equilibrium and systems B &C are in thermal equilibrium -> A & B are in thermal equilibrium. 13