PlanetPhysics/Thermodynamics an Introduction and Definitions

Thermodynamics is the science of the flow of heat. It applies to macroscopic systems in equilibrium and how to go from one equilibrium state to another. It is entirely empirical and summed up into four laws and basic mathematics.

Zeroth law of thermodynamics: defines temperature $T$

First law of thermodynamics: defines energy $U$

Second law of Thermodynamics: defines entropy $S$

Third Law of Thermodynamics: gives numerical value to entropy $S$

These laws are UNIVERSALLY VALID and cannot be circumvented.

Definitions used in Thermodynamics:

• System : The part of the Universe that we choose to study
• Surroundings : The rest of the Universe
• boundary: The surface dividing the System from the Surroundings
• Homogeneous : A single phase is in the system
• Hetrogeneous : Different phases are in the system

Examples of systems:

• A person
• Hot coffee in a thermos
• glass of ice water
• volume of 4 liters of air in a room

whatever is left over is the surroundings. Between the system and the surroundings is the boundary.

Examples of boundaries:

• Real like the outside of a person's skin
• The inner wall of the thermos
• An imaginary boundary surrounding the 4 liters of air

Systems can be:

• Open : mass and Energy can transfer between the System and the Surroundings
• Closed : Energy can transfer between the System and the Surroundings, but not mass
• Isolated : Neither Mass nor Energy can transfer between the System and the Surroundings

Describing Systems requires:

• A few macroscopic properties: p, T, V, n, m, etc.
• Knowledge if System is Homogeneous or Hetrogeneous
• Knowledge if System is in Equilibrium State
• Knowledge of the number of components

Two classes of Properties:

• Extensive : Depend on the size of the system (n,m,V,...)
• Intensive : Independent of the size of the system (T, p, ${\bar {V}}={\frac {V}{n}}$,...)

A system is in equilibrium if the properties that describe the system, such as $P$, $T$, $V$, etc. do not change in time or space. A gas in a container needs to be the same $P$, $T$, $V$ to be in equilibrium.

References

This is a derivative work from [1] a Creative Commons Attribution-Noncommercial-Share Alike 3.0 work

[1] MIT OpenCourseWare, 5.60 Thermodynamics and Kinetics: Thermodynamics and Kinetics, Spring 2008