cmt458 lect3

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Terms & definitionsTerms & definitions•System – Part of the universe that is under investigation. A system can absorb /lose heat, can do work or can have work done on it.

•Surroundings – region ouside the boundary of the system

e.g system – ball, air + earth = surrounding

Analyse: how air & earth affects motion of ball

e.g. Gas in piston-cylinder arrangement

Analyse how pressure affects volume of gas

•Universe = system + surrounding


Terms & definitionsTerms & definitionsWith a closed system, no transfer of mass is possible: internal energy may only change due to heat and work.

With an isolated system, no change in the internal energy is possible: heat, work and mass transfer are all impossible.

With an open system, the internal energy may change due to transfer of heat, mass and work between system and surroundings.


Terms & definitionsTerms & definitionsHomogeneous system,

a single-phase system where the property of system is uniform over the system (same value regardless of where it is measured)

Heterogeneous system

a multiple-phase system

the measured property varies with location where it is evaluated


Terms & DefinitionTerms & DefinitionIntensive Property – A property that is independent of amount of matter. It is non-additivee.g density, temperature, pressure, specific heat capacityExtensive Property - A property that depends on amount of matter. It is an additive property.e.g mass, volume, heat capacity, enthalpy

Extensive/extensive = intensive property

Note: specific quantity = property/masse.g specific volume = volume/mass specific internal energy = internal energy/mass specific heat capacity = heat capacity/mass


Relations among temp scalesRelations among temp scalesCelsius Kelvin Fahrenheit Rankine

Absolute zero

Ice point

Steam point

-273.15 C -459.670 K 0 R

0 C

100 C

273.15 K

373.15 K

32 F

212 F

491.67 R

671.67 R


Temperature ScalesTemperature Scales Temperature- a measure of kinetic energy

- degree of hotness of a subs Temperature Scales:

Kelvin, Rankine, Fahrenheit, CelciusE.g

T(oF) = 1.8T(oC) + 32 0 oC = 32oF T(K) = T(oC) + 273.15 0 oC = 273.15 KT(oR) = T(oF) + 459.67 212 oF = 671.67 oRT(OR)= 1.8T(K) 0 K = 0 oR Note:


StateState Some terms associated with ‘state’Some terms associated with ‘state’

StateState Change of stateChange of state Equation of stateEquation of state States of matterStates of matter State/Path functionsState/Path functions


StateState State – A system is in a certain state when State – A system is in a certain state when

all the properties of a system are fixed ie all the properties of a system are fixed ie the values of V, T, P etc are fixed. the values of V, T, P etc are fixed.

Change of state – when a system goes Change of state – when a system goes from some initial state to some final state.from some initial state to some final state.E.g PE.g P11VV11TT11 to P to P22VV22TT22


Equation of state (EOS)Equation of state (EOS) An equation that describes the PVT behaviour An equation that describes the PVT behaviour

of a gasof a gas The simplest equation is the ideal gas equationThe simplest equation is the ideal gas equation

PV = nRTPV = nRT P = pressure, V= volume, n = moleP = pressure, V= volume, n = mole T = temperature, R = ideal gas constantT = temperature, R = ideal gas constant Will discuss other examples of EOS in future Will discuss other examples of EOS in future

lectureslectures


States of MatterStates of Matter Solids – has definite volume &shapeSolids – has definite volume &shape Liquids – has volume no definite shapeLiquids – has volume no definite shape

They flow and can be pouredThey flow and can be poured Gas – no definite volume and no def. Gas – no definite volume and no def.

shape –takes the volume and shape of shape –takes the volume and shape of containercontainer

Plasma? – No def. volume or shapePlasma? – No def. volume or shape Composed of electrically charged particlesComposed of electrically charged particles


State/Path functionsState/Path functions State functionsState functions

Differential Differential changechange in property = infinitesimal change in in property = infinitesimal change in the propertythe property

Identified as points on graphIdentified as points on graph Represents a property of a system and always have a valueRepresents a property of a system and always have a value The cyclic integral of a state function is zeroThe cyclic integral of a state function is zero

Path functionsPath functions Infinitesimal Infinitesimal quantitiesquantities of heat and work of heat and work Represented by areas on a graphRepresented by areas on a graph Work and heat appear only when changes are caused in a Work and heat appear only when changes are caused in a

system system


Thermodynamic function that is Thermodynamic function that is independent of path. They do not depend independent of path. They do not depend

on past history on past history


State functionState function


ProcessProcessIsothermal (T=constant)Isothermal (T=constant)Isothermal systems have walls that conduct heat and their surroundings Isothermal systems have walls that conduct heat and their surroundings have to be at a constant temperature. have to be at a constant temperature. T=TT=T22-T-T1 1 = 0 (finite change)= 0 (finite change)

dT=TdT=T22-T-T1 1 = 0 (infinitesimal change)= 0 (infinitesimal change) Boyle’s law Boyle’s law PP11VV11 = P = P22VV22

Isobaric (P=constant)Isobaric (P=constant) P=PP=P22-P-P1 1 = 0 (finite change)= 0 (finite change) dP=PdP=P22-P-P1 1 = 0 (infinitesimal change)= 0 (infinitesimal change) VV22/T/T22 = V = V11/T/T11


ProcessProcessIsobaric (P=constant)Isobaric (P=constant)

Constant pressure processesConstant pressure processes take place in systems take place in systems having flexible walls (think balloon) whose having flexible walls (think balloon) whose surroundings are at a constant pressure. A typical surroundings are at a constant pressure. A typical example is the path taken by a process that goes on in a example is the path taken by a process that goes on in a flexibly-walled system surrounded by the atmosphereflexibly-walled system surrounded by the atmosphere P=PP=P22-P-P1 1 = 0 (finite change)= 0 (finite change)

dP=PdP=P22-P-P1 1 = 0 (infinitesimal change)= 0 (infinitesimal change)

VV22/T/T22 = V = V11/T/T11


ProcessProcess Isovolumetric, isometric, isochoricIsovolumetric, isometric, isochoric Constant volume ProcessesConstant volume Processes are obtained by having rigid are obtained by having rigid

walls around the system. The walls may or may not walls around the system. The walls may or may not conduct heat. conduct heat.

V=constantV=constantV=VV=V22-V-V1 1 = 0 (finite change)= 0 (finite change)

dV=VdV=V22-V-V1 1 = 0 (infinitesimal change)= 0 (infinitesimal change)

PP22/T/T22 = P = P11/T/T11

IsentropicIsentropicEntropy is constantEntropy is constant

S= 0 (finite change)S= 0 (finite change) dS= 0 (infinitesimal change)dS= 0 (infinitesimal change)


ProcessProcess Isenthalpic (Constant enthalpy)Isenthalpic (Constant enthalpy) Cyclic – a process is a cyclic process if it returns Cyclic – a process is a cyclic process if it returns

to the starting initial stateto the starting initial state AdiabaticAdiabatic

An An adiabatic processadiabatic process takes place in a system whose takes place in a system whose walls are impermeable to heat. No heat passes into or walls are impermeable to heat. No heat passes into or out of the system. Typically an insulated bottle or out of the system. Typically an insulated bottle or vacuum bottle is used to carry out an adiabatic process. vacuum bottle is used to carry out an adiabatic process.

Diathermic boundary – heat can flow through that Diathermic boundary – heat can flow through that boundaryboundary


Polytropic processes (PVPolytropic processes (PVnn =const) =const)

ProcessProcess nn

Isothermal (T = const)Isothermal (T = const) 11

Isobaric ( P =const)Isobaric ( P =const) 00

Isochoric ( V = const)Isochoric ( V = const)

Adiabatic (no heat transfer)Adiabatic (no heat transfer) = Ratio of heat capacities= Ratio of heat capacities


EquilibriumEquilibrium• The central concept of thermodynamics is

equilibrium

• Thermodynamic state quantities are defined (and measurable) only in equilibrium.

• Equilibrium state is static on the macroscopic scale but dynamic on the microscopic scale.

• The state that is automatically attained by a system after a sufficient period of time.

• At equilibrium, there is no net driving force for change. i.e all opposing forces are counterbalanced.


EquilibriumEquilibrium• Thermal equilibrium

• A system is in thermal equilibrium when its temp is uniform throughout and equal to the temp of its surroundings.

Zeroth Law of Thermodynamics: All systems which are in thermal equilibrium with a given system are also in thermal equilibrium with each other. If A and B are in thermal equilibrium, and B and C are also

in thermal equil., then A and C are in thermal equil. Consequence of the oth law:

B acts as a thermometer; A, B and C are all at the ‘same temperature’ If there is a temp. gradient, heat flows until temp

difference disappears


EquilibriumEquilibrium• Mechanical equilibrium

• A system is in mechanical equilibrium when it has no unbalanced force acting on its surfaces.

• Chemical equilibrium • A system is in chemical equilibrium when its chemical

composition remains unchanged with time. • Every system that has not reached equilibrium is

changing continuously toward such a state with greater or less speed.

• Systems that are already at equil: • Disturb slightly – return to same state of rest• Disturb large – new condition of equil.


Reversible ProcessReversible Process In thermodynamics, many situations are assumed to be

ideal situations in order to simplify problems e.g reversibility

A process is reversible when its direction can be reversed at any point by an infinitesimal change in external conditions.

A reversible process never moves more than differentially away from equilibrium.

A process is reversible if the work and heat effects from the process are sufficient to restore the system to its original state.


Conditions of ReversibilityConditions of Reversibility Absence of dissipative processes such as friction The existence of the system in equilibrium state s at all

times. The maintainence of only infinitesimal differences in

thermodynamic potential between the systems and its surroundings

A reversible process produces the maximum or requires the minimum amount of work

For a reversible expansion/compression of a gas, the external pressure is approximately the same as the pressure of the gas i.e Pext =Pgas


PhasesPhases A region of uniformity in a system i.e a region of A region of uniformity in a system i.e a region of

uniform (homogeneous) chemical composition uniform (homogeneous) chemical composition and uniform physical properties – separated by and uniform physical properties – separated by definite physical boundarydefinite physical boundary

A system containing liquid and vapour has two A system containing liquid and vapour has two regions of uniformity. In the vapour phase the regions of uniformity. In the vapour phase the density is uniform throughout. In the liquid phase, density is uniform throughout. In the liquid phase, the density is uniform throughout but has a value the density is uniform throughout but has a value different from that in vap.different from that in vap.

E.g A system containing CClE.g A system containing CCl44, H, H22O and air has 3 O and air has 3 phases.phases.

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