Which condition is essential for a process to be considered reversible?
**Internal energy** state function depends only on temperature for ideal gas, U = f/2 n R T, change ΔU = n C_v ΔT, first law connects heat, work, internal energy, for expansion work done by gas positive, compression work done on gas negative, heat added positive. A reversible process requires the absence of dissipative effects (e.g., friction, viscosity), allowing the system and surroundings to return to their original states. Quasi-static conditions alone are insufficient without eliminating dissipation. 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
Ref: NCERT > Physics Book > Thermodynamics > First Law of Thermodynamics Applications