Chapter 12Chapter 12

The Laws of The Laws of ThermodynamicsThermodynamics

Work in Thermodynamic Work in Thermodynamic ProcessesProcesses

Work is an important energy Work is an important energy transfer mechanism in transfer mechanism in thermodynamic systemsthermodynamic systems

Heat is another energy transfer Heat is another energy transfer mechanismmechanism

Work in a Gas Cylinder, Work in a Gas Cylinder, cont.cont.

A force is applied A force is applied to slowly to slowly compress the gascompress the gas The compression The compression

is slow enough for is slow enough for all the system to all the system to remain essentially remain essentially in thermal in thermal equilibriumequilibrium

W = - P ΔVW = - P ΔV This is the work This is the work

done done onon the gas the gas

More about Work on a Gas More about Work on a Gas CylinderCylinder

When the gas is compressedWhen the gas is compressed ΔV is negative ΔV is negative The work done on the gas is positiveThe work done on the gas is positive

When the gas is allowed to expandWhen the gas is allowed to expand ΔV is positiveΔV is positive The work done on the gas is negativeThe work done on the gas is negative

When the volume remains constantWhen the volume remains constant No work is done on the gasNo work is done on the gas

PV DiagramsPV Diagrams Used when the Used when the

pressure and volume pressure and volume are known at each step are known at each step of the processof the process

The work done on a gas The work done on a gas that takes it from some that takes it from some initial state to some initial state to some final state is the final state is the negative of the area negative of the area under the curve on the under the curve on the PV diagramPV diagram This is true whether or This is true whether or

not the pressure stays not the pressure stays constantconstant

Additional Notes About the Additional Notes About the First LawFirst Law

The First Law is a general equation The First Law is a general equation of Conservation of Energyof Conservation of Energy

There is no practical, macroscopic, There is no practical, macroscopic, distinction between the results of distinction between the results of energy transfer by heat and by energy transfer by heat and by workwork

Q and W are related to the Q and W are related to the properties of state for a systemproperties of state for a system

Fig. 12.13, p. 374

Slide 22

TA<TB<TC

B

DAir Conditioner/Refrigerator

Fig. 12.13, p. 374

Slide 22