First Law Sign Convention
Easyphysics
In first law ΔU = Q - W, W is taken positive when?
Select the correct option:
Solution
Incorrect! Answer:
Work done by system
- Energy Conservation: The First Law states ΔU=Qin−Wout, indicating that energy increases by heat input and decreases by work output.
- Standard Physics Convention:
- Q is positive when heat is added to the system.
- W is positive when work is done by the system on its surroundings (expansion).
- Effect on U: If a system does work (W>0), it consumes its internal energy to perform that work, hence the minus sign in ΔU=Q−W.
- Note: In chemistry, the convention often used is ΔU=Q+W, where work done on the system is positive.
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About This Question
- Subject
- physics
- Chapter
- thermodynamics
- Topic
- first law sign convention
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
Work done by system
- Energy Conservation: The First Law states ΔU=Qin−Wout, indicating that energy increases by heat input and decreases by work output.
- Standard Physics Convention:
- Q is positive when heat is added to the system.
- W is positive when work is done by the system on its surroundings (expansion).
- Effect on U: If a system does work (W>0), it consumes its internal energy to perform that work, hence the minus sign in ΔU=Q−W.
- Note: In chemistry, the convention often used is ΔU=Q+W, where work done on the system is positive.
This easy difficulty physics question is from the chapter thermodynamics, covering the topic of first law sign convention. It appeared in the 2025 exam.
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