Why is the concept of thermal equilibrium essential to the Zeroth Law of Thermodynamics?
**Heat transfer** occurs via conduction, convection, radiation, work via volume change W=∫ P dV, electrical work, etc., first law distinguishes, internal energy includes kinetic and potential of molecules, for ideal gas only kinetic, U = f/2 n R T. The Zeroth Law relies on thermal equilibrium, where no net heat flows between systems in contact, establishing that systems sharing this property with a third system have a common property—temperature—thus defining temperature measurement. Using first law ΔU = Q - W, W = ∫ P dV, isobaric W = P ΔV, isothermal W = n R T ln(V₂/V₁), adiabatic P V^γ = const and η =
Ref: NCERT > Physics Book > Thermodynamics > Heat Transfer Work Distinction and Internal Energy Change