7/22/2013 Thermodynamics • Deals with very practical phenomenon, Chapters 12-15 e.g., engines, power generation systems • The study of heat and work and the Thermodynamics transformation of one into the other • Based on observations of energy exchanges between macroscopic systems Introduction • Macroscopic – systems with very large numbers of particles in 1 m3 there are over 1025 air molecules 1 4 Equilibrium of mechanical systems: Macroscopic Parameters the concept of temperature • The macroscopic parameters are those that can be measured directly – • Three parameters were needed to temperature, pressure, volume describe the motion and equilibrium states • The macroscopic parameters reflect the of bodies length, mass, and time average behavior of the microscopic • To describe thermal or heat effects in constituents of the system, for example, systems we need another fundamental the velocities of the molecules which parameter temperature: T cannot be directly measured • Newton’s laws cannot be used to solve problems involving 1023 particles 2 5 Example: thermal equilibrium Pressure of a gas • Block of ice and the balloon are each in mechanical equilibrium: F = 0 • The pressure on the walls • Now put them in contact of a container is due to the • Both systems now undergo changes impact of molecules on the • The volume of ice decreases (it melts) walls. and the pressure and volume of the • The impulse on the wall balloon decrease due to one molecule is small, but the average ICE • The come to a final state of thermal effect of all the molecules equilibrium which must be character- in the box can be large ized by a new physical property called temperature T • In thermal equilibrium: Tice = Tballoon 3 6 1 7/22/2013 Heat Engines Temperature • Thermodynamics deals with devices which use heat to do work --- e.g., a steam engine. • Temperature is a macroscopic parameter that is a measure of the average KE of the molecules in a system. ENGINE HEAT WORK • Temperature is the macroscopic measure of the internal energy of a system work out Efficiency = heat in The laws of thermodynamics place limits of the efficiency of engines 7 10 Laws of Thermodynamics Heat • First Law • Heat is the energy that flows from one The transformation of heat into work obeys the system to another system because one is Principle of conservation of energy hotter than the other • Second Law • Heat flows from the hot system (lowering – The heat going into an engine cannot all be its internal energy and temperature) to a converted to work, some heat is wasted cold system (raising its internal energy and – Heat flows spontaneously from hot to cold, temperature). (ice always melts when placed in water); and • Heat stops flowing when the two systems work must be done to make heat flow from reach a common temperature. cold to hot (refrigerators) 8 11 Internal Energy Temperature Scales • Temperatures are reported • The molecules in a system, like a gas, are in degrees Celsius (C) or degrees Fahrenheit (F) in constant motion. Each molecule has a • These scales are based on kinetic energy ½ mv2. The KE of an the freezing and boiling individual molecule cannot be measured points of water • 100 C = 180 F, so that • The internal energy of a system is the 1 C = (9/5) F sum of all the kinetic energy of all the • The C and F scales are also molecules in the system offset by 32 9 12 2 7/22/2013 Temperature Conversion Kelvin scale Formulas 5 • One degree C = one degree K TTCF32 9 • Scales offset by 273.15 9 TTFC32 5 • T(K) = T(C) + 273.15 • Note that there is no particular significance to 0 on either the F or C scales • Is there a temperature scale where 0 is the minimum temperature, i.e., there are no negative temperatures? • YES--- the Kelvin Scale 13 16 12.2 The Kelvin Temperature Scale Example: Liquid N2 A constant-volume gas •TF = 321 F thermometer. •TC = (5/9)(TF 32) = (5/9)(321 32) = (5/9) ( 353) = 196 C Gas filled container •TK = TC + 273 = 77 K 14 17 12.2 The Kelvin Temperature Scale 1x104 Thermometers Gas A 8x103 • Certain materials have properties (thermometric properties) that change with 3 6x10 temperature --- these can be used to make Gas B 4x103 thermometers – Length of a column of liquid 2x103 Gas C – Thermocouple – electrical ---digital 0 thermometer – Infrared radiation detectors – thermometers -300 -200 -100 0 100 200 300 that are placed into your ear 273.15 T (C) absolute zero point = -273.15 oC 15 18 3 7/22/2013 Thermocouple Metal B 97.3 F Metal A Two different metals bonded together behave like a battery and produce a current. The current depends on the temperature, and thus it can be calibrated to form a thermometer. Thermocouples can operate over a large range of temperatures and respond quickly to temperature changes. All digital thermometers use thermocouples. 19 4.