Thermodynamics

Definition and Scope

Thermodynamics is a branch of physics that studies the transformation of heat energy into other forms of energy and vice versa. It provides a framework for understanding how energy is transferred and converted in physical systems.

• Key Focus: The study of energy, heat, work, and the laws governing their interactions.

• Applications: Engines, refrigerators, biological systems, and more.

Important Terms in Thermodynamics

• System: The part of the universe under study. It can be as small as a gas in a cylinder or as large as a planet.

• Surroundings: Everything outside the system that can interact with it.

• Thermodynamic Variables: Quantities that describe the state of a system, such as pressure (P), volume (V), and temperature (T).

• Thermodynamic Equilibrium: A state in which all macroscopic flows of matter and energy have ceased within the system, and the properties are uniform throughout.

• Thermal Equilibrium: A special case of thermodynamic equilibrium where there is no net flow of heat between parts of the system or between the system and its surroundings (i.e., temperature is uniform and unchanging).

• Internal Energy (U): The total energy contained within a system, including both kinetic and potential energies of its particles.

  • Expressed as:

  • Where is potential energy and is kinetic energy.

Thermodynamic Laws

Zeroth Law of Thermodynamics

The Zeroth Law of Thermodynamics establishes the concept of temperature and thermal equilibrium.

• Statement: If two systems, A and B, are each in thermal equilibrium with a third system, C, then A and B are in thermal equilibrium with each other.

• Implication: This law allows the definition of temperature as a property that is the same for all systems in thermal equilibrium.

Example: If a thermometer (system C) is in equilibrium with a cup of water (system A) and with a block of metal (system B), then the water and the metal are at the same temperature.

Work in Thermodynamics

Work Done During Expansion

When a gas expands, it can do work on its surroundings. The work done by the gas during a small displacement can be calculated analytically.

• Infinitesimal Work:

  • For a small displacement , the work done by the gas is:

  • Where is force, is area, is pressure, and is the infinitesimal change in volume.

• Total Work Done:

  • When the volume changes from to , the total work done is:

Example: If a gas expands isothermally (at constant temperature) and pressure is constant, the work done is .