Understanding the Second Law
The Second Law of Thermodynamics introduces the concept of entropy and establishes the direction of thermodynamic processes. It states that the entropy of an isolated system never decreases over time, and heat flows naturally from hot to cold objects.
ΔS ≥ Q/T
ΔS
Change in Entropy
Q
Heat Transferred
T
Absolute Temperature
This law explains why perpetual motion machines are impossible and why heat engines have maximum theoretical efficiencies. It also defines the arrow of time in thermodynamic processes.
Concept Explorer
Interactive Heat Engine Simulation
Heat Engine Visualization
Adjust parameters to see efficiency changes
Heat Engine Cycles
Carnot Cycle
The most efficient theoretical heat engine cycle consisting of two isothermal and two adiabatic processes:
η = 1 - (Tc/Th)
Otto Cycle
Idealized cycle for spark-ignition internal combustion engines with constant volume heat addition:
η = 1 - (1/rγ-1)
Diesel Cycle
Idealized cycle for compression-ignition engines with constant pressure heat addition:
η = 1 - (1/rγ-1) × [(ργ-1)/(γ(ρ-1))]
Rankine Cycle
Cycle used in steam power plants with phase change between liquid and vapor:
η = (Wturbine - Wpump)/Qin
Thermodynamic Cycle Diagram
Real-World Applications
Power Plants
Thermal power plants use the Rankine cycle to convert heat from fossil fuels or nuclear reactions into electricity. The Second Law determines their maximum theoretical efficiency.
Car Engines
Internal combustion engines operate on either Otto or Diesel cycles. Their efficiency is fundamentally limited by the temperature difference between combustion and the environment.
Refrigerators
Refrigeration systems operate as heat pumps, moving heat from cold to hot environments. The Second Law explains why work input is required for this process.