In a reversible isothermal process, the change in internal energy is :

Options

A. Zero
B. Positive
C. Negative
D. Unpredictable

Answer

To solve the question regarding the change in internal energy during a reversible isothermal process, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Internal Energy**: Internal energy (ΔU) is a measure of the total energy contained within a system, which includes kinetic and potential energy of the particles in the system.

2. **Identify the Process**: The question specifies that the process is isothermal. An isothermal process is one in which the temperature of the system remains constant throughout the process.

3. **Relate Temperature to Internal Energy**: The internal energy of an ideal gas is a function of temperature. This means that if the temperature of the system does not change, the internal energy also does not change.

4. **Apply the Concept**: Since the process is isothermal (constant temperature), the internal energy of the system remains constant. Therefore, the change in internal energy (ΔU) can be expressed mathematically as:
\[ ΔU = U_{\text{final}} - U_{\text{initial}} = 0 \]

5. **Conclusion**: Thus, in a reversible isothermal process, the change in internal energy is zero.

### Final Answer:
The change in internal energy (ΔU) in a reversible isothermal process is **zero**.

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