Thermodynamics Overview

Thermodynamics

• Thermodynamics is the branch of physics that studies heat and energy transformations.
• It focuses on macroscopic properties like pressure, volume, temperature, internal energy, entropy, and enthalpy.
• A thermodynamic system is a collection of atoms or molecules confined within boundaries.
• Surroundings are the area outside the thermodynamic system.
• An open system exchanges both energy and matter with its surroundings.
• A closed system only exchanges energy with its surroundings.
• An isolated system exchanges neither energy nor matter.
• An adiabatic wall does not allow energy exchange.
• A diathermic wall allows energy exchange.
• Thermodynamic state variables describe the state of a system (e.g., pressure, temperature, volume).
• The equation of state relates state variables for a given system (e.g., PV = μRT for an ideal gas).
  • Extensive variables change with the system size (e.g., internal energy, volume).
  • Intensive variables remain constant as the system size changes (e.g., temperature, pressure).

Thermal Equilibrium

• Two systems are in thermal equilibrium if their temperatures are the same.
• In thermal equilibrium, macroscopic properties like pressure, volume, and temperature remain constant.

Zeroth Law of Thermodynamics

• If two systems are each in thermal equilibrium with a third, then the first two systems are in thermal equilibrium with each other.
• This law defines temperature as a common property among all systems in equilibrium.

Heat

• Heat is energy transferred between a system and its surroundings due to a temperature difference.
• Positive heat indicates energy transfer to the system, negative heat indicates energy transfer from the system.

Work

• Work is done when a force acts on a system causing displacement in the direction of the force.
• In thermodynamics, work is often related to volume changes in a gas.
• Sign convention: Work done by the system is positive; work done on the system is negative.

Internal Energy

• Internal energy (U) is the sum of kinetic and potential energies of the constituent particles in a system.

First Law of Thermodynamics

• The first law states that heat added to a system is equal to the increase in internal energy plus the work done by the system.
  • 𝑑𝑄 = 𝑑𝑈 + 𝑃𝑑𝑉
• This law is a statement of energy conservation.

Specific Heat Capacities

• Specific heat capacity measures the amount of heat required to change the temperature of a unit mass of a substance by one degree.
  • 𝐶𝑣 is the specific heat at constant volume.
  • 𝐶𝑝 is the specific heat at constant pressure.
  • 𝐶𝑝 = 𝐶𝑣 + 𝑅 (Mayer's relation)

Thermodynamic Processes

• A thermodynamic process is any change in the thermodynamic properties of a system.
• Quasi-static processes occur infinitesimally slowly, allowing the system to remain in equilibrium at each step.
• Isothermal processes maintain constant temperature.
• Adiabatic processes occur with no heat exchange between the system and its surroundings.
• Isochoric processes maintain constant volume.
• Isobaric processes maintain constant pressure.

Reversible Process

• A reversible process can reverse direction without any change to the system.

Irreversible Process

• An irreversible process cannot be reversed to return both the system and surroundings to their original states.

Carnot Engine

• A Carnot engine is an idealized engine that operates on a cyclic process with maximum thermodynamic efficiency.
• It has four steps: isothermal expansions, adiabatic expansions, isothermal contractions, and adiabatic contractions.
• The efficiency of a Carnot engine depends solely on the temperatures of the source (higher) and the sink (lower).