# Equation #15: Zeroth Law of Thermodynamics

In thermodynamics, being in thermal equilibrium with another body is nothing but a way of saying that the two bodies are at same temperature (definition of temperature).

Basically, this law states that if two systems, A and C are at the same temperature, and another system B and C are at the same temperature then A and B are at the same temperature as well.

Mathematically,

if T_{A} = T_{C} and T_{B} = T_{C}

then T_{A} = T_{B}

It seems so obvious that most of us take it for granted. In fact for a long time it wasn’t questioned, even after the first and second law were established (The First law of thermodynamics is a statement of the conservation of energy, second law deals with entropy). However, both of them can’t be proved unless you take the Zeroth law as an axiom. Since it was established later but is more basic hence the name ‘zeroth’.

For more equations in Physics, see Famous equations