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

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

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

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

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

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.

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

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!

High temperature Reservoir High temperature Reservoir QC QH W QC Low temperature Reservoir Low temperature Reservoir

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

a-b : Isothermal Expansion at TH. |QH| = proportional to TH d c 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. p V

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 :

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?