Ch. 4 - closed systems

closed systems and boundary work

closed system

nothing but energy can cross the boundary of a closed system

mass CANNOT be transfered

ways energy crosses the boundaries of a closed system

• heat

• work

heat

energy transfer due to a temperature difference between the system and its surroundings

work

energy transfer across a system NOT due to a temperature difference between a system and its surroundings

the shape of a closed system

NOT fixed

it can change during a process

boundary work

• only exists when the shape of the system changes

• when it grows or decreases in size

  • another form of mechanical work

if the shape of the system doesn’t change

there is NO boundary work

general boundary work equation

∫ P • dv

from V1 to V2

when the system expands it will

• be producing work

• work will be going out of system

• the boundary work will be a positive number

when the system contracts it will

• receive work

• work will be going into the system

• the boundary work will be a negative number

ideal gas, constant temperature equation for work

Wb = mRT ln │ V2 / V1 │

same for specific volume because mass cancels

constant pressure process equation for work

Wb = P ( V2 - V1 )

same for specific volume

polyatropic process boundary work equation

Wb = (P2 V2 - P1 V1 ) / 1-n

polyatropic pressure equation

P1 V1^n = P2 V2^n

n =

polyatropic constant

if not an ideal gas or incompressible substance

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