Pressure-Volume Diagrams

Discussion

math, math, math

Recall from the previous section …

ΔU = Q + W
 
Q > 0   system absorbs heat
Q < 0   system releases heat
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 = − 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 fuel tank to the point of explosion. Thank you Superman. You've saved us. [slide]

… and the rest

liquids

solids