Why is the Zeroth Law fundamental to the measurement of temperature?
**Isobaric process** constant pressure, work W = P ΔV = P(V₂ - V₁) = n R ΔT, for expansion ΔV positive W positive, for compression negative, heat Q = n C_p ΔT, ΔU = n C_v ΔT. Isothermal process constant temperature ΔU=0, work W = n R T ln(V₂/V₁) = n R T ln(P₁/P₂), Q = W, heat absorbed equals work done. The Zeroth Law establishes that two systems in thermal equilibrium with a third have the same temperature, providing the basis for a consistent temperature scale and thermometer calibration. Using first law ΔU = Q - W, W = ∫ P dV, isobaric W
Ref: NCERT > Physics Book > Thermodynamics > Isobaric and Isothermal Processes Work Calculation