A gas does work during isothermal expansion. What is the source of mechanical energy so produced?

Explanation

To solve the question regarding the source of mechanical energy produced during isothermal expansion of a gas, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Isothermal Expansion**:
- Isothermal expansion refers to the process where a gas expands at a constant temperature. In this case, the change in temperature (ΔT) is zero.

2. **Apply the First Law of Thermodynamics**:
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