1. Concept:During an expansion, a gas uses its own internal energy to push outward against external pressure. If it wants to maintain a constant temperature, it must replenish that lost energy.
2. Formula:$$\Delta U = 0 \implies Q = W$$
3. Solution:- An expansion strictly means work is done BY the gas (\(W > 0\)).
- Since the temperature is rigidly constant (isothermal), internal energy cannot change (\(\Delta U = 0\)).
- Applying the First Law: \(Q = 0 + W\). Since \(W\) is positive, \(Q\) must be positive.
- A positive \(Q\) indicates that the gas actively absorbs heat from the thermal reservoir to compensate for the work it is doing.
4. Why other options are incorrect:If it released heat or absorbed nothing, its internal energy would plummet during expansion, causing a massive drop in temperature, violating the isothermal constraint.
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