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Transcript
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 151c15:1 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 151c15:2 Refrigeration: getting heat to flow from cold to hot requires work! 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. High temperature Reservoir QH W QC Low temperature Reservoir 151c15:3 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 = pV 151c15:4 Isobaric process: process at constant pressure W = p(V2 V1) Other processes: W = area under the curve on a pressure-volume (p-V) diagram 151c15:5 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. 151c15:6 Indicator Diagrams: p-V diagrams used to analyze cyclic processes which use a gas in a heat engine. p p Work done by system V V p p Net work done by system equals enclose area Work done on system V V 151c15:7 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! 151c15:8 High temperature Reservoir QH High temperature Reservoir QC W QC Low temperature Reservoir Low temperature Reservoir 151c15:9 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 p V Carnot cycle is made with only reversible processes => “most efficient heat engine possible” 151c15:10 The most efficient engine cycle operating between two specified temperatures: Carnot Cycle p 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. a b d c V 151c15:11 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 : QC W QH QC Eff 1 QH QH QH 151c15:12 For the Carnot Engine only: Q is proportional to T for both isothermal processes, so TC QC TH QH so Eff carnot TC 1 TH 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? 151c15:13