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.