Various engines (petrol, diesel, jet engines, etc.) are heat engines.
To understand heat engines, one must study thermodynamics, the physics branch dealing with heat and temperature.
Thermodynamics Basics
Thermodynamics: Physics branch that deals with heat and temperature.
Macroscopic Science: Focuses on big or bulk systems rather than microscopic (molecules, atoms).
System: Part of the universe under observation.
Surroundings: Everything outside the system.
Thermodynamic Equilibrium: When a system's variables (pressure, temperature, volume) remain constant over time.
Comparing Mechanics and Thermodynamics
Mechanics: Focuses on object movement; no concern with state properties (temperature, volume).
Thermodynamics: Focuses on object's state and energy transformations, particularly kinetic energy to heat energy.
Thermodynamic Variables
Internal Energy: Sum of kinetic and potential energy of molecules in a system.
State Variables: Define thermodynamic state (pressure, volume, temperature, etc.).
Intensive Variables: Do not depend on system size (e.g., pressure, temperature).
Extensive Variables: Depend on system size (e.g., volume, internal energy).
Types of Thermodynamic Processes
Isothermal Process: Temperature remains constant.
Isobaric Process: Pressure remains constant.
Isochoric Process: Volume remains constant.
Adiabatic Process: No heat flow between system and surroundings.
Equilibrium in Thermodynamics
Mechanical Equilibrium: No net force acting on the system.
Thermal Equilibrium: Same temperature with surroundings implies no heat transfer.
Chemical Equilibrium: Chemical composition remains constant.
Laws of Thermodynamics
Zeroth Law of Thermodynamics
If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
First Law of Thermodynamics
Energy cannot be created or destroyed, only transformed.
Mathematical Expression: ΔQ = ΔU + ΔW
ΔQ: Heat supplied to the system.
ΔU: Change in internal energy.
ΔW: Work done by the system.
Heat and Work
Heat (Q): Energy transfer due to temperature difference.
Work (W): Energy transfer when a force moves an object.
ΔU (Internal Energy Change): ΔU = ΔQ - ΔW
Heat and work are not state variables.
Reversibility and Irreversibility
Reversible Process: Can return to initial state without leaving changes in surroundings (hypothetical ideal).
Irreversible Process: Cannot return to initial state without leaving changes in surroundings.
Quasi-Static Process: Extremely slow process keeping the system and surroundings in equilibrium.
Heat Engines
Convert heat into work using cycles of processes.
Takes heat from a hot reservoir.
Converts part of the heat into work.
Releases remaining heat to a cold reservoir.
Efficiency (η): Work output divided by heat input; impossible to achieve 100% efficiency (Carnot's theorem).
η = W / Q₁
η = 1 - Q₂ / Q₁
Carnot Engine and Cycle
Carnot Engine: Idealized heat engine with maximum possible efficiency.
Processes in Carnot Cycle:
Isothermal Expansion: System absorbs heat at T₁.
Adiabatic Expansion: Temperature drops to T₂.
Isothermal Compression: System releases heat at T₂.
Adiabatic Compression: Temperature rises back to T₁.
Carnot Efficiency: η = 1 - T₂ / T₁ (Temperatures in Kelvin).
Refrigerators and Heat Pumps
Reverse of heat engines: Use work to transfer heat from a cold to a hot reservoir.
Coefficient of Performance (COP): Ratio of heat extracted to work input.
COP = Q₂ / W
Should always be finite, cannot be infinite.
Second Law of Thermodynamics
Kelvin-Planck Statement: Impossible for any device to operate in a cycle and convert all heat input into work.
Clausius Statement: Impossible for any process to transfer heat from a colder to a hotter body without work input.
Problems and Applications
Problems on specific heat capacity, adiabatic processes, and Carnot cycle calculations help in understanding real-world applications and validating theoretical concepts.
Conclusion
Thermodynamics is essential for understanding energy transformations and system equilibria in physical and engineering processes.