Basic Considerations in Thermodynamics

Summary

The lecture introduces foundational thermodynamics terms and explains two ways to describe a system’s state.

System, surroundings, and universe

• A system is the part of the world where the process being studied occurs. It has boundaries or walls.
• The surroundings are everything outside the system.
• The system together with its surroundings makes up the universe.

Properties used to describe a system

The lecture identifies three variables: volume (V), the number of particles or moles (N), and a third property represented by either entropy (S) or energy (U).

Entropy is described in terms of the arrangement of particles, while energy is described in terms of their activity or motion. The lecturer briefly connects the words’ Greek roots to “turning/changing” for entropy and “work” for energy.

Two thermodynamic representations

In the energy representation, energy is the dependent property:

[ U = U(S,V,N) ]

Entropy, volume, and particle number are treated as independent variables.

In the entropy representation, entropy is the dependent property:

[ S = S(U,V,N) ]

Energy, volume, and particle number are treated as independent variables.

The lecturer says these descriptions are equivalent, but the class will use the energy representation because it is more straightforward.

Equilibrium and thermodynamic problems

An equilibrium state is one in which there is no net change. The lecture points to entropy, volume, particle number, and energy as quantities whose changes may be considered when discussing equilibrium; further types of equilibrium are left for later.

A system may begin in an equilibrium state, be disturbed by an outside influence, and move into a nonequilibrium state. It then changes toward another equilibrium state. A central thermodynamic problem is determining the system’s properties—energy, entropy, volume, and particle number—after it reaches that new state.

Speaker and source

One lecturer or instructor speaks throughout. No other speakers or external sources are identified.