First Law Sign Convention

Question

In first law ΔU = Q - W, W is taken positive when?

RankGuru · Accepted Answer

1. **Energy Conservation**: The First Law states $\Delta U = Q_{in} - W_{out}$, indicating that energy increases by heat input and decreases by work output.
2. **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).
3. **Effect on U**: If a system does work ($W > 0$), it consumes its internal energy to perform that work, hence the minus sign in $\Delta U = Q - W$.
4. **Note**: In chemistry, the convention often used is $\Delta U = Q + W$, where work done *on* the system is positive.

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First Law Sign Convention

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