- 2067 Mangsir · 4 marks
Work and heat transfer; sign conventions
Short answer
Work is energy crossing boundary due to force; heat is energy from temperature difference. Standard sign convention: positive for heat added and work done by the system. First law for closed system: ΔU = Q − W.
Define work transfer and heat transfer. Also mention their sign conventions used in the analysis of thermodynamic problems.
Work transfer
Work is the energy interaction between a system and its surroundings that crosses the boundary when a system moves under the action of a force; formally, it is done when the sole external effect on the surroundings could be the raising of a weight. Examples: displacement work P dV, shaft work, electrical work.
Heat transfer
Heat is the energy that crosses the boundary of a system because of a temperature difference between the system and the surroundings. It flows from the higher to the lower temperature by conduction, convection or radiation.
Both are transient, boundary phenomena and path functions (δQ, δW).
Sign conventions (standard)
| Quantity | Positive (+) | Negative (−) |
|---|---|---|
| Heat Q | Heat added to the system | Heat rejected from the system |
| Work W | Work done by the system on surroundings | Work done on the system |
Q in (+) W out (+)
---------------> [SYSTEM] --------------->
Q out (-) <------ <------ W in (-)
With this convention the first law for a closed system is Q − W = ΔE, i.e. ΔU = Q − W for a stationary system.
✓ Answer checked by Claude Opus 5.5 (high effort)
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