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Heat and work: similarities and differences
Short answer
Similar: both are energy in transit, boundary phenomena, path functions, not properties. Dissimilar: heat requires temperature difference, is disorganized; work uses force/pressure, is organized, and can be fully converted to heat but not the reverse.
In what respect the heat and work interactions are (i) similar and (ii) dissimilar?
(i) Similarities
- Both are transient (energy in transit) phenomena: they are recognised only when they cross the system boundary; a system never "contains" heat or work.
- Both are boundary phenomena: they occur at the boundary.
- Both are path functions: the amount depends on the process followed, so their differentials are inexact (δQ, δW), not dQ or dW.
- Both are associated with a process, not a state; neither is a property.
- Both are energy interactions, have the same unit (J or kJ), and can be positive or negative depending on direction.
(ii) Dissimilarities
| Basis | Heat | Work |
|---|---|---|
| Driving potential | Temperature difference | Force, pressure, voltage or any non-thermal potential |
| Nature | Disorganised molecular motion | Organised motion (force through a distance) |
| Conversion | Cannot be completely converted to work in a cycle (second law) | Can be completely converted into heat |
| Sign convention | Into system = +ve | By system = +ve |
| Example | Heat flow from flame to water | Expansion of gas against a piston |
Heat always flows from higher to lower temperature, whereas work can be transferred without any temperature difference.
✓ Answer checked by Claude Opus 5.5 (high effort)
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