Applied Physics > Thermal Physics > Classical Thermodynamics

Classical Thermodynamics is a fundamental subfield within Thermal Physics, which itself is a vital area of Applied Physics. The study of Classical Thermodynamics focuses on the principles governing the transfer and conversion of energy, specifically in forms of heat and work, within physical systems. This area of physics provides a macroscopic perspective, dealing with large-scale observables rather than microscopic states.

Key Concepts:

1. Systems and Surroundings:
   Classical Thermodynamics studies different types of systems—isolated, closed, and open—and their interactions with their surroundings. An isolated system exchanges neither energy nor matter with its surroundings, a closed system exchanges energy but not matter, and an open system exchanges both energy and matter.

2. State Variables:
   Key variables in thermodynamics include temperature \( T \), pressure \( P \), volume \( V \), and internal energy \( U \). These variables define the state of a system and can be experimentally measured.

3. Thermodynamic Processes:
   Processes describe how a system changes from one state to another. Common processes include isothermal (constant temperature), adiabatic (no heat exchange), isobaric (constant pressure), and isochoric (constant volume) processes.

4. Laws of Thermodynamics:
   The field is grounded in four fundamental laws:

   • Zeroth Law: If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This principle allows us to define temperature.
   • First Law (Law of Energy Conservation): \( \Delta U = Q - W \), where \( \Delta U \) is the change in internal energy, \( Q \) is the heat added to the system, and \( W \) is the work done by the system.
   • Second Law: This states that the entropy \( S \) of an isolated system never decreases. In other words, for any spontaneous process, the total entropy of the system and its surroundings always increases.
   • Third Law: As the temperature of a system approaches absolute zero (\( T \rightarrow 0 \, \text{K} \)), the entropy of a perfect crystalline structure approaches a minimum value, typically taken to be zero.

5. Entropy \( S \):
   Entropy is a measure of the disorder or randomness in a system. It quantifies the number of microscopic configurations that correspond to a thermodynamic system’s macroscopic state. The change in entropy \( \Delta S \) for a reversible process is given by

\[ \Delta S = \int \frac{dQ_{\text{rev}}}{T} \]

where \( dQ_{\text{rev}} \) is the infinitesimal amount of heat added reversibly at temperature \( T \).

6. Enthalpy \( H \):
   Enthalpy is a state function defined as \( H = U + PV \), where \( P \) is the pressure and \( V \) is the volume. It is particularly useful in processes occurring at constant pressure where the heat exchange \( Q \) is equal to the change in enthalpy (\( \Delta H \)).

7. Gibbs Free Energy \( G \) and Helmholtz Free Energy \( F \):
   These are potentials that predict the direction of processes and the equilibrium state under certain conditions. Gibbs Free Energy is given by \( G = H - TS \), and it indicates the amount of reversible work obtainable from a system at constant \( T \) and \( P \). Helmholtz Free Energy is given by \( F = U - TS \), useful for constant \( T \) and \( V \) conditions.

Through these principles and laws, Classical Thermodynamics provides essential insights into the behavior of physical systems, guiding the design and analysis of engines, refrigerators, and numerous industrial processes. It also builds a bridge to other disciplines such as Statistical Mechanics and Quantum Thermodynamics, where a more detailed, microscopic understanding of thermodynamic phenomena is developed.