When a gas undergoes adiabatic expansion, it gets cooles. This is because

Options

A. It is an exothermic process
B. It is an endothermic process
C. Internal energy of the system decreases
D. Ideal gas becomes a real gas

Answer

To understand why a gas cools during adiabatic expansion, we can analyze the process step by step using the principles of thermodynamics.

### Step-by-Step Solution:

1. **Understanding Adiabatic Process**:
- An adiabatic process is one in which no heat is exchanged with the surroundings. This means that the heat transfer (Q) is equal to zero.
- **Hint**: Remember that in an adiabatic process, the system is insulated from heat exchange.

2. **First Law of Thermodynamics**:
- The first law of thermodynamics states that the change in internal energy (ΔU) of a system is equal to the heat added to the system (Q) plus the work done on the system (W):
\[ \Delta U = Q + W \]
- Since Q = 0 for an adiabatic process, this simplifies to:
\[ \Delta U = W \]
- **Hint**: Keep in mind that work done on the system is considered positive, while work done by the system is negative.

3. **Work Done During Expansion**:
- During adiabatic expansion, the gas does work on the surroundings. Therefore, W is negative (since the gas is expanding).
- This means:
\[ \Delta U = W < 0 \]
- **Hint**: When a gas expands, it uses energy to do work, which leads to a decrease in internal energy.

4. **Internal Energy and Temperature**:
- The internal energy (U) of an ideal gas is directly related to its temperature. If the internal energy decreases (ΔU < 0), it implies that the temperature of the gas also decreases.
- **Hint**: Remember that for an ideal gas, a decrease in internal energy corresponds to a decrease in temperature.

5. **Conclusion**:
- Therefore, during adiabatic expansion, the gas cools down because its internal energy decreases as it does work on the surroundings without any heat being added to compensate for that energy loss.
- The correct answer to the question is that the internal energy of the system decreases (Option C).

### Final Answer:
The gas cools during adiabatic expansion because the internal energy of the system decreases.