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Chapter 3

CHMB20 Chapter 3

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Jamie Donaldson

Chapter 3 The Second LawThe Direction of Spontaneous ChangeSpontaneous the direction of change that does not require work to bring it aboutSecond Law of Thermodynamics no process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work some energy is discarded and not converted into workDuring a spontaneous change in an isolated system the total energy is dispersed into random thermal motion of the particles in the systemThe First Law uses the internal energy to identify permissible changes while the Second Law uses the entropy to identify the spontaneous changes among those permissible changesEntropy a measure of the energy dispersed in a process acts as a signpost of spontaneous changeoThe entropy of an isolated system increase in the course of a spontaneous change S0totoThermodynamically irreversible processes are spontaneous processesodSdqT where q is the heat supplied reversiblyrevrevoEntropy change of a perfect gas when it expands isothermally from V to V S if nRlnVVfioEntropy Change of the Surroundings STqsurrsurrsurroFor any adiabatic change q0 so S 0surrsurr 23oBoltzmann Formula for the Entropy SklnW where k1381 x 10 JK and W is the number of microstates ways in which the molecules of a system can be arranged while keeping the total energy constant more disorderly distribution more microstatesEntropy is related to dispersal of energyThe more microstates or more dispersal the higher the entropyHeating will increase the number of accessible energy levels and W will increaseThe change in entropy should be inversely proportional to the temperature at which the transfer takes place high temperature higher entropyoEntropy is a state function so dqT0 around a close pathrevsurroCarnot CycleReversible isothermal expansion from A to B at T the entropy change his qT where q is the energy supplied to the system as heat from the hhhhot sourceReversible adiabatic expansion from B to C No energy leaves the system as heat so the change in entropy is zero In the course of expansion the temperature falls from T to T the temperature of the hccold sinkReversible isothermal compression from C to D at T Energy is creleased as heat to a cold sink change in entropy of the system is qT in this expression q is negativeccc
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