Pressure-Volume Diagrams

Discussion

math, math, math

Recall from the previous section…

ΔU = Q + W

Q > 0 system absorbs heat from the environment
Q < 0 system releases heat to the environment
W > 0 work done on the system by the environment
W < 0 work done by the system on the environment

A system can be described by three thermodynamic variables. — pressure, volume, and temperature. Well, maybe it's only two variables. With everything tied together by the ideal gas law, one variable can always be described as dependent on the other two.

 





P =  nRT
V
PV = nRT  ⇒  V =  nRT
P
  T =  PV
nR

Temperature is the slave of pressure and volume on a pressure-volume graph (PV graph).

Function of State

ΔU =  3  nRΔT
2

Function of Path: Work

W = ∫ F · ds = ∫ P dV

W = − area on PV graph

Function of Path: Heat

Q = ΔU + W = ncΔT

cP =  specific heat at constant pressure
cV =  specific heat at constant temperature

curves

Superman illustrates adiabatic cooling brought about by the rapid expansion of a gas, thus preventing the evil General Zod from heating the truck's fuel tank to the point of explosion. Thank you Superman. You've saved us.

… and the rest

liquids

solids