When two bodies A and B are in thermal equilibrium

Options

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 the two bodies will be equal

Explanation

**Step-by-Step Solution:**

1. **Understanding Thermal Equilibrium**:
- Thermal equilibrium occurs when two bodies (A and B) are in contact and there is no net flow of thermal energy between them. This means that they are at the same temperature.

2. **Kinetic Energy of Molecules**:
- The statement that "the kinetic energy of all molecules are the same" is not necessarily true. While temperature is related to the average kinetic energy of the molecules, individual molecules can have different kinetic energies.

3. **Potential Energy of Molecules**:
- The potential energy of the molecules may vary depending on their positions and interactions. Therefore, the potential energy of all molecules being the same is also not a valid statement.

4. **Internal Energy**:
- Internal energy is a function of temperature. Since both bodies A and B are in thermal equilibrium, they must have the same temperature, which implies that their internal energies are also the same.

5. **Average Kinetic Energy**:
- The average kinetic energy of the molecules in both bodies A and B will be the same because they are at the same temperature. This is a key point in thermodynamics.

6. **Conclusion**:
- Based on the analysis, the correct statement when two bodies A and B are in thermal equilibrium is that the average kinetic energy of A and B is the same, and their internal energies are also the same.

**Final Answer**: The average kinetic energy of A and B is the same.

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