PX154 - E6a - introduction to entropy

[YF 20.7]

• eg: from here: $[hot|cold]\to [warm|warm]$ is irreversible
• the system becomes more random:
  • initially sorted into hot and cold regions
  • energy "mixes", meaning it is more disordered/random
  • the system has gained entropy
• eg: a reversible isotherm
  
  $T=constant$
  $\mathrm{\Delta }U=0$
  • from $A\to B:Q$ is added to do work
  • at $V_2:$ molecules are able to move around in a larger volume, so, system is more disordered $⟹$ higher entropy
  • returning to $A$, gas is exactly the same as it was initially, so, for $A\to B\to A:\mathrm{\Delta }S=0$
  • $S$ is a function of state $⟹\mathrm{\Delta }{S}_{A\to B}$ doesn't depend on the path taken