Question

When two bodies $A$ and $B$ are in them equilibrium

• A. the kinetic energies of all the molecules of A and B will be equal
• B. the potential energies of all the molecules of A and B will be equal
• C. the internal energies of the two bodies will be equal
• D. the average kinetic energy of the molecules of two bodies will be equal

Answer 1

Answer: (C)

When bodies are in thermal equilibrium, the temperature remains constant. Since, internal energy is a function of temperature $(\Delta T)$ ,
$\left(\Delta U=\frac{1}{2}f\Delta T\right)$ ,
hence for isothermal change $\Delta T=0$ ,
therefore $\Delta U=0$
Hence, internal energies of the two bodies will be equal.