Exam-style question
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When does expansion work occur in a thermodynamic system?.
- A.When a gas is compressed by external pressure.
- B.When a gas expands against an external pressure.
- C.When heat is added to the gas.
- D.When the gas cools down.
Model answer
What a good answer should say
- When a gas expands against an external pressure.
Explanation
Why this works
Expansion work occurs when a gas expands against an external pressure, doing work on the surroundings. The key difference is that this process involves the gas performing work, while compression work involves work being done on the gas.
Expansion work applies in situations like engine cycles where gases expand to push pistons. Therefore, the correct option is that expansion work occurs when a gas expands against an external pressure.
Common mistake
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