Thermal Equilibrium Transitivity – Name of the Law If two bodies are each in thermal equilibrium with a third body, then they are in thermal equilibrium with each other. This fundamental statement is known as the Zeroth law of thermodynamics.

Introduction / Context:
The Zeroth law provides the basis for temperature measurement. It formalizes the concept that thermal equilibrium is transitive, enabling the use of thermometers as intermediaries between systems.

Given Data / Assumptions:

• Three systems (A, B, C) considered for equilibrium relationships.
• Thermal contact without work exchange.
• Steady conditions with no net heat flow at equilibrium.

Concept / Approach:

The Zeroth law states: If A is in thermal equilibrium with C, and B is in thermal equilibrium with C, then A and B are in thermal equilibrium. This allows a single parameter (temperature) to label thermal states consistently and justifies thermometer calibration against fixed points.

Step-by-Step Solution:

Verification / Alternative check:

Practical thermometry relies on this law: a thermometer (C) equilibrates with a system (A); the reading then characterizes any other system (B) that gives the same reading when contacted with the same thermometer.

Why Other Options Are Wrong:

The First law concerns energy conservation; the Second law concerns heat/work directionality and entropy; Kelvin-Planck is a second-law statement about engines; the Clausius inequality quantifies irreversibility.

Common Pitfalls:

Assuming the Zeroth law is less important because of its name; in fact it underpins the very definition of temperature scales.

Final Answer:

Zeroth law of thermodynamics