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7/22/2013

Thermodynamics

• Deals with very practical phenomenon, Chapters 12-15 e.g., , generation systems • The study of heat and and the transformation of one into the other • Based on observations of exchanges between macroscopic systems Introduction • Macroscopic – systems with very large numbers of particles  in 1 m3 there are over 1025 air molecules

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Equilibrium of mechanical systems: Macroscopic Parameters the concept of • The macroscopic parameters are those that can be measured directly – • Three parameters were needed to temperature, , 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 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

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Heat Engines Temperature • Thermodynamics deals with devices which use heat to do work --- e.g., a steam . • 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 of a system work out Efficiency = heat in The 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 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)

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Internal Energy Temperature Scales • 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 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

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Temperature Conversion Kelvin scale Formulas

5 • One degree C = one degree K TTCF32 9 • Scales offset by 273.15 9 TTFC32 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 .

•TC = (5/9)(TF  32) = (5/9)(321  32) = (5/9) ( 353) =  196 C Gas filled container •TK = TC + 273 = 77 K

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12.2 The Kelvin Temperature Scale

1x104 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 – – electrical ---digital

0 thermometer – detectors – thermometers -300 -200 -100 0 100 200 300 that are placed into your ear 273.15 T (C)

point = -273.15 oC 15 18

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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. can operate over a large range of temperatures and respond quickly to temperature changes. All digital thermometers use thermocouples. 19

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