Zeroth law of thermodynamics is a fundamental principle in thermal physics that establishes the basis for the concept of temperature and thermal equilibrium. It states that if two thermodynamic systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. In engineering terms, this law allows us to define temperature as a measurable property that is consistent across systems, for instance, it justifies the use of a thermometer to compare the temperatures of different objects without direct contact between them. Without this law, temperature scales and heat transfer analyses would lack a foundational consistency.
This law was formulated after the first and second laws of thermodynamics had already been established, which is why it is numbered "zeroth," to indicate its precedence in logical order despite its later recognition. From an engineering perspective, the zeroth law is crucial in applications like designing heat exchangers, calibrating sensors, or modeling multi-component systems in process control, where assuming transitive equilibrium simplifies calculations and ensures reliable predictions of energy flow. In practical terms, thermal equilibrium implies no net heat transfer between systems when they are in contact, meaning their temperatures are equal.
