Physics · Thermodynamics · NEET
The First and Second Laws were stated and numbered first. Later, in 1931, scientists saw that this idea about thermal equilibrium is even more basic and must be assumed before the First and Second Laws make sense. Since it comes logically before the First Law, it was named the Zeroth Law instead of renumbering the older laws.
It defines temperature. When two systems are in thermal equilibrium, there must be one physical quantity that has the same value in both. We give that quantity the name temperature (T). So the Zeroth Law is the reason temperature exists as a well-defined property.
If A is in equilibrium with C, then T of A equals T of C (T_A = T_C). If B is in equilibrium with C, then T_B = T_C. From these two, T_A = T_B. So A and B are in equilibrium. This chain only works if a single quantity, temperature, is equal whenever two bodies are in thermal equilibrium.
Thermal equilibrium is a state: two bodies in thermal contact stop exchanging heat, and their large-scale variables (P, V) stop changing. The Zeroth Law is a rule about equilibrium: it links three bodies and says equilibrium is transitive. Equilibrium is the condition; the Zeroth Law is the logical statement built on it.
No. Heat flowing from higher to lower temperature is a consequence that comes after we already have temperature. The Zeroth Law is the earlier step that lets us define temperature in the first place. First the Zeroth Law gives us temperature, then temperature tells us the direction of heat flow.
System C acts like a reference or a thermometer. A and B never touch directly at first; they only touch C through a conducting (diathermic) wall. C carries the comparison between A and B. This is exactly how a real thermometer works: it reaches equilibrium with each body and lets us compare them.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other.
Temperature. It is the quantity that has the same value for two systems that are in thermal equilibrium.
It was recognised and named the Zeroth Law around 1931, long after the First and Second Laws were already numbered, because it is logically more basic than them.
A conducting or diathermic wall, which allows heat to flow. An adiabatic (insulating) wall blocks heat, so no equilibrium is reached through it.
Yes, mainly as a statement or assertion-reason question. You should know the exact statement, that it defines temperature, and why heat flow is a separate idea.
