Chapter 15: Thermodynamics

1. Chapter 15: Thermodynamics • Thermodynamics: how heat is converted to and from other forms of energy, especially mechanical energy. • Heat engine: a process or system which converts heat into mechanical energy. High temperature Reservoir 1. Heat (QH) is absorbed from a source at high temperature. 2. Mechanical work (W) is done (by converting some of the absorbed heat to mechanical work). 3. Heat (QC) is given off at a lower temperature QH W QC Low temperature Reservoir

2. The first law of thermodynamics: • Energy is Conserved! • Net heat input = change in internal energy + net work output • Q = U + W • Cyclic Processes: • repeating process in which the system or heat engine returns to the starting point (same thermodynamic state) each cycle. • A Cyclic Process is necessary for most practical heat engines. • Over each complete cycle • U = 0 • net heat input = net work output

3. Refrigeration: getting heat to flow from cold to hot requires work! High temperature Reservoir QH 1. Heat (QC) is absorbed from a source at low temperature. 2. Mechanical work (W) is done on the system (work is input). 3. Heat (QH) is given off to the higher temperature reservoir. W QC Low temperature Reservoir

4. Work done during volume changes • Expanding gas in a piston • Force and pressure • p = F/A => F = pA • Work = force x distance • W = Fs = pA s • but A s is just the extra volume of gas, so • W = pV

5. Isobaric process: process at constant pressure • W = p(V2V1) • Other processes: • W = area under the curve on a pressure-volume (p-V) diagram

6. Example 15.1: The heat of vaporization of water at atmospheric pressure is Lv = 2260 kJ/kg. How much of this heat represents work done to expand the water into steam against the pressure of the atmosphere? At T = 100 ºC an p = 1 atm, the density of water is 1.00x103 kg/m3 and the density of steam is 0.600 kg/m3.

7. p p V V p p V Indicator Diagrams: p-V diagrams used to analyze cyclic processes which use a gas in a heat engine. Work done by system Net work done by system equals enclose area Work done on system V

8. The Second Law of Thermodynamics • The Natural tendency of all physical systems is towards “disorder” (increasing entropy) • The entropy of a closed system can never decrease! • The natural direction of heat flow is from a reservoir of internal energy at a high temperature to a reservoir of energy at a low temperature. • Heat flow from Hot to Cold! • Major Consequence: • It is impossible to construct a heat engine which operates in a cycle that does nothing other than take in heat from a source and perform an equivalent amount of work! • => no 100% efficient heat engines!

9. High temperature Reservoir High temperature Reservoir QC QH W QC Low temperature Reservoir Low temperature Reservoir

10. The Carnot Engine Cycle • some types of processes • Isobaric process: occurs at constant pressure • Isochoric or Isovolumetric process: occurs at constant volume • Isothermal process: occurs at constant temperature • Adiabatic process: occurs with no heat transfer • Carnot cycle is made with only reversible processes => “most efficient heat engine possible” p V

11. a p b d c V • The most efficient engine cycle operating between two specified temperatures: Carnot Cycle • a-b : Isothermal Expansion at TH. • |QH| = proportional to TH • (absolute temperature!) • b-c : Adiabatic Expansion to TC. • c-d : Isothermal Compression at TC. • |QC| proportional to TC • d-a : Adiabatic Compression to TH.

12. Engine Efficiency • net mechanical work comes from net transfer of heat • W = QHQC • Efficiency is the effectiveness with which supplied heat QH is converted to work :

13. For the Carnot Engine only: Q is proportional to T for both isothermal processes, so Example: Steam enters a steam turbine at 570 ºC and emerges into partial vacuum at 95 ºC . What is the upper limit to the efficiency of this engine?