Chapter 15 The Laws of Thermodynamics

Thermodynamics n The study of the processes in which energy is transferred as heat and as work heat--transfer of energy due to a temperature difference work--transfer of energy not due to a temperature difference

Thermodynamics System--any object or set of objects that we wish to consider closed system--a system for which no mass enters or leaves (but energy can) open system--mass as well as energy can be exchanged with environment isolated closed system--neither mass or energy passes across system boundary

15.1 The First Law of Thermodynamics n based on the law of conservation of energy n the change in the internal energy of a closed system, (  U), will be equal to heat added to system minus work done by the system n  U = Q - W

15.1 The First Law of Thermodynamics n  U = Q - W where: n  U, Internal energy (the total of all energy of all the molecules of a system) n  U = positive when internal energy of system increases n  U = negative when internal energy of system decreases

15.1 The First Law of Thermodynamics n  U = Q - W where: n Q = heat n heat added to the system is positive n heat lost from the system is negative

15.1 The First Law of Thermodynamics n  U = Q - W where: n W = work n work done by the system is positive n work done to the system is negative

15.1 The First Law of Thermodynamics n For isolated systems: n  U = 0 so n 0 = Q - W and n Q = W = 0 n so no work is done by an isolated system

15.1 The First Law of Thermodynamics n The amount of internal energy is a property of a system; heat and work is not n a system doesn’t have a given amount of heat or work; rather they are added or removed form the system by the change of state of the system n so heat and work are part of a thermodynamic process that can change a system from one state to another not a characteristic of the system such as P, V, T, n, or U.

15-2 The 1st Law of Thermodynamics Applied to Some Simple Systems Simples Processes: n Isothermic Process n Adiabatic Process n Isobaric Process n Isochoric (iosvolumetric) Process

Isothermic Process n Idealized process carried out at constant temperature n for ideal gases; PV=nRT so PV= constant

Isothermic Process n Example: ideal gas in a cylinder fitted with a movable piston n gas is in contact with a heat reservoir (body of mass so large that there is no significant  T to it when heat is exchange with it) n assume that compression or expansion is done slowly enough that all gas stays at equilibrium at same constant T

Isothermic Process n If heat (Q = +) is added to the system & T=constant then the gas must expand and do work on the surroundings (W = +) Heat added to system Work done by system

Isothermic Process n Since T is constant there is no  U (  U=3/2nR  T--Ch-14) so n  U = Q - W or Q = W n so in an isothermic process work done by the system is equal to the heat added to the system Heat added to system Work done by system

Adiabatic Process n Process in which no heat is allowed to flow into or out of the system n Q = 0 n can occur if process happens so quickly that heat has no time to flow into or out of system

Adiabatic Process n Since Q = 0 then n  U = - W n U decreases (  U = -) if gas expands and does work by the system (W = +) n temperature also decreases as well (  U=3/2nR  T) decrease in internal energy Work by the system Temperature decreases

Adiabatic Process n If gas contracts work is done on the system so U and T increases n diesel engines compress fuel mixture by a factor of 15+ so it ignites without spark plugs increase in internal energy Work on the system Temperature increases

Adiabatic Process n Example: n hold a rubber band loosely with hands n gauge temperature by holding to lips n stretch band suddenly n quickly touch to lips n temperature increases, WHY?

Isobaric Process n Pressure is kept constant n since P = constant, P = F/A so F = PA n W = F x d so W = PAd n since V = Ad n when a piston expands: W = P  V

Isobaric Process n  V = V f - V i n so expansion of gas causes work done by system (+W) n compression of gas caused by work done on system (-W) n and this work is W = P  V Gas expands Work by the system

Isochoric Process n Isovolumetric n one in which volume doesn’t change n since W = P  V and there is no  V process does no work on surrounding n all heat added changes internal energy n  U = Q Heat added No movement of piston Internal energy increases

PV Diagram n Graph of the relationship between pressure and volume at various temperatures isotherms--lines (curves) with same temperature n area under the curve (P  V) gives you work done as conditions change for the various processes

PV Diagram pressure volume isothermic adiabatic isobaric isochoric

15-3 Human Metabolism and the First Law Metabolism--the many energy transforming processes that occur within an organism n  U = Q - W n our bodies does work on environment and loses heat so to maintain our internal energy we must increase  U by eating metabolic rate--rate at which U is transformed inside our body (kcal/h or watts)

15-4 The Second Law of Thermodynamics n The Second Law of Thermodynamics n it as many equivalent statements n Clausius statement--heat flows naturally from a hot object to a cold object, but not spontaneously (by itself without input of some kind of work) from cold to hot n the study of heat engines (any device that changes thermal energy into mechanical energy) will help develop a more general form

15-5 Heat Engines n Any device that changes thermal energy into mechanical work n the basic idea behind a heat engine is that mechanical energy can be obtained from thermal energy only when heat is allowed to flow from a high temperature to a cold temperature

15-5 Heat Engines n Heat input (Q H ) at a high temperature (T H ) is partly transformed into work (W) and partly exhausted as heat (Q L ) at a lower temperature (T L ) n T H and T L are called the operating temperatures of the engine High Temp T H Low Temp T L engine QHQH QLQL W

15-5 Heat Engines n We will examine heat engines that run in a repeating cycle; continuously n ****Note*** New Sign Convention: Q H, Q L, and W are always positive now Two types of practical engines: n Steam engine n Internal combustion engine

15-5 Heat Engines Steam engine n Steps (OH): n high pressure steam is produced by combustion or nuclear fission n steam turns turbine or expands piston (work) n low pressure steam is condensed and re-vaporized

15-5 Heat Engines Internal combustion engine steps (four-stroke engine)(OH): n intake stroke--fuel mixture enters cylinder n compression stroke--mixture is compressed by piston n power stroke--fuel mixture is ignited and expanding gases force piston down (work) n exhaust stroke-exhaust gas pushed from cylinder

15-5 Heat Engines n Why is  T needed to drive a heat engine? n Efficiency of heat engine e = W/Q H n Q H = W + Q L (energy conserved) n so W = Q H - Q L n e = W/Q H = (Q H - Q L )/Q H = 1- Q L /Q H

15-5 Heat Engines Carnot Engine n ideal engine n each process of heat addition and exhaust, expansion or compression would be reversible (each step done slow enough to achieve equilibrium before next step occurs) real engines--process acts quickly, irreversible (due to friction and turbulence)

15-5 Heat Engines Carnot cycle four step cycle (OH) n gas is expanded isothermally with addition of heat (T constant) n gas expands adiabatically (no Q added but T  ) n gas compressed isothermally n gas expands adiabatically until its at original state

15-5 Heat Engines Carnot efficiency (ideal) n e ideal = (T H - T L )/T H = 1- T L /T H n theoretical limit to efficiency n 60-80% efficiency is excellent

15-5 Heat Engines Second Law of Thermodynamics Kelvin-Planck statement n no device is possible whose sole effect is to transform a given amount of heat completely into work n only possible if T L = 0K Third Law of Thermodynamics n absolute zero is unattainable

15-6 Refrigerators, Air Conditioners, and Heat Pumps n Operation of these devices is the reverse of a heat engine n by doing work heat is taken from a low temperature region to a high temperature region High Temp T H Low Temp T L engine QHQH QLQL W

15-6 Refrigerators, Air Conditioners, and Heat Pumps n Refrigerators and Air Conditioners process n inside (low T) expanding gas absorbs heat; liquid  gas n condenser/pump; pumps warm gas outside and compresses it so that it loses heat; gas  liquid

15-6 Refrigerators, Air Conditioners, and Heat Pumps n Refrigerators and Air Conditioners coefficient of performance (CP) n CP = Q L /W = Q L /(Q H - Q L ) n CP ideal = T L /(T H - T L )

15-6 Refrigerators, Air Conditioners, and Heat Pumps n Heat Pump n device that heats a house by taking heat from Q L (low T) outside house and delivering heat Q H (high T) to warm inside of house n works like refrigerator and A/C, can be reversed to cool in summer CP = Q H /W

15-7 Entropy and the Second Law of Thermodynamics Entropy (S) n is a state function of a system n we deal with a change in entropy (  S) during processes n according to Clausius, the change in entropy of a system when an amount of heat (Q) is added to it by a reversible process at a constant temperature (K) is:  S =Q/T

15-7 Entropy and the Second Law of Thermodynamics Entropy (S) n if the temperature change is not too great an average value for temperature can be used, if not that’s what they made calculus for Second Law of Thermodynamics (again) n the entropy of an isolated system never decreases. It can only stay the same or increase

15-7 Entropy and the Second Law of Thermodynamics Entropy (S) Second Law of Thermodynamics (and again) n the entropy of an isolated system never decreases. It can only stay the same or increase n for real processes;  S>0 n if system is not isolated:  S =  S s +  S env  0

15-7 Entropy and the Second Law of Thermodynamics Entropy (S) Second Law of Thermodynamics (and again) n the total entropy of any system plus that of its environment increases as a result of any natural process n only in ideal processes is  S = 0 n entropy is not conserved it is always increasing

15-8 Order to Disorder Entropy--a measure of the disorder of a system Second Law of Thermodynamics (once again) n Natural processes tend to move toward a state of greater disorder n Examples: salt/pepper, broken cup, mixing hot/cold

15-8 Order to Disorder n An increase in entropy corresponds to an increase in disorder (randomness) n Information theory n built on the idea that the more orderly an arrangement the more information is needed to specific it or classify it n hot/cold liquids  cool liquid

15-9 Unavailability of Energy; Heat Death n In the process of heat conduction from a hot body to a cold one entropy increases and order goes to disorder n the separation of hot/cold objects can serve as high/low temperature regions for a heat engine and be used to obtain useful work n when the two objects reach the same temperature no work can be obtained

15-9 Unavailability of Energy; Heat Death Second Law of Thermodynamics n in any natural process, some energy becomes unavailable to do useful work n in any process energy is never lost but it becomes less useful to do work n energy is degraded as it goes from more ordered forms (mechanical) to less ordered forms (internal to thermal)

15-9 Unavailability of Energy; Heat Death n A natural outcome is that as time goes on, the universe will approach a state of maximum disorder n matter will become a uniform mixture at one temperature, no work can then be done n all energy would be degraded to thermal energy and all change would cease (Heat Death) n this is based on the assumption that the universe is finite

15-10 Evolution and Growth; “Time’s Arrow” n Evolution of organisms and entropy n Why has entropy called time’s arrow?

15.12 Energy Resources: Thermal Pollution Know this: n greenhouse effect n thermal pollution Processes for electricity generation n Fossil-fuel steam plants n Nuclear energy n Geothermal energy n Hydroelectric power plants n Tidal energy n Wind power n Solar energy (solar cell/photovoltaic cell)