Fundamental Laws of Thermodynamics

Understanding the fundamental laws of thermodynamics is key in engineering. These laws explain how energy behaves, how systems reach equilibrium, and the limits of efficiency. Grasping these concepts helps engineers design better systems and optimize energy use in real-world applications.

1. First Law of Thermodynamics (Conservation of Energy)

   • Energy cannot be created or destroyed, only transformed from one form to another.
   • The change in internal energy of a system is equal to the heat added to the system minus the work done by the system.
   • This law establishes the principle of energy conservation in thermodynamic processes.

2. Second Law of Thermodynamics (Entropy)

   • In any energy transfer, the total entropy of a closed system can never decrease over time.
   • Entropy is a measure of disorder or randomness in a system; systems naturally progress towards greater disorder.
   • This law explains why certain processes are irreversible and sets limits on the efficiency of energy conversions.

3. Zeroth Law of Thermodynamics (Thermal Equilibrium)

   • If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
   • This law allows for the definition of temperature and establishes a basis for temperature measurement.
   • It underpins the concept of thermal equilibrium, which is essential for understanding heat transfer.

4. Third Law of Thermodynamics (Absolute Zero)

   • As the temperature of a system approaches absolute zero, the entropy of a perfect crystal approaches zero.
   • Absolute zero (0 Kelvin) is the theoretical point where molecular motion ceases.
   • This law implies that it is impossible to reach absolute zero in a finite number of steps.

5. Ideal Gas Law

   • The relationship between pressure (P), volume (V), temperature (T), and the number of moles (n) of an ideal gas is expressed as PV = nRT.
   • It provides a good approximation for the behavior of real gases under many conditions.
   • The law combines several gas laws (Boyle's, Charles's, and Avogadro's) into a single equation.

6. Carnot Cycle

   • The Carnot cycle is a theoretical model that defines the maximum possible efficiency of a heat engine operating between two temperature reservoirs.
   • It consists of four reversible processes: two isothermal and two adiabatic.
   • The efficiency of a Carnot engine depends only on the temperatures of the hot and cold reservoirs.

7. Heat Engines and Efficiency

   • Heat engines convert thermal energy into mechanical work, operating between a hot and a cold reservoir.
   • Efficiency is defined as the ratio of work output to heat input, often expressed as a percentage.
   • The maximum efficiency is limited by the Second Law of Thermodynamics and is achieved by idealized engines like the Carnot engine.

8. Enthalpy

   • Enthalpy (H) is a thermodynamic quantity that represents the total heat content of a system, defined as H = U + PV, where U is internal energy.
   • It is useful for analyzing processes occurring at constant pressure, such as chemical reactions and phase changes.
   • Changes in enthalpy (ΔH) indicate the heat absorbed or released during a process.

9. Gibbs Free Energy

   • Gibbs free energy (G) is a thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure.
   • The change in Gibbs free energy (ΔG) indicates the spontaneity of a process: ΔG < 0 means spontaneous, ΔG = 0 means equilibrium, and ΔG > 0 means non-spontaneous.
   • It is crucial for understanding chemical reactions and phase transitions.

10. Thermodynamic Processes (Isothermal, Adiabatic, Isobaric, Isochoric)

    • Isothermal: Occurs at constant temperature; heat is exchanged with the surroundings.
    • Adiabatic: No heat is exchanged with the surroundings; all energy changes are due to work done on or by the system.
    • Isobaric: Occurs at constant pressure; heat added or removed results in work done by the system.
    • Isochoric: Occurs at constant volume; any heat added increases the internal energy and temperature of the system.