The Clausius inequality is a way to express the Second Law of Thermodynamics:
It says that in any cyclic process (a process that eventually returns to its starting point), the total amount of heat transferred divided by the temperature is never positive.
In simpler terms:
- If the process is reversible (ideal, no losses), this value is exactly zero.
- If the process is irreversible (real-world, with friction or other losses), this value is less than zero.
The Clausius inequality essentially tells us that you cannot convert all heat into work without some loss, and it helps define entropy, which measures the “disorder” or irreversibility in a system.