--- title: "Heat and work: similarities and differences" subject: "Fundamental of Thermodynamics & Heat Transfer" chapter: 2 key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:heat-work-similar-dissimilar" source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/heat-work-similar-dissimilar site: BCT Engineering --- # Heat and work: similarities and differences Fundamental of Thermodynamics & Heat Transfer · Chapter 2: Energy and Energy Transfer - Asked in: 2074 Bhadra, 4 marks; 2069 Poush, 4 marks; 2068 Bhadra, 4 marks (asked 3 times) - Video: [Thermodynamics: What do HEAT and WORK really mean? | Basics of Thermodynamics](https://www.youtube.com/watch?v=bM4ykIumlss) — Parth G - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/heat-work-similar-dissimilar - Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-2 ## Question In what respect the heat and work interactions are (i) similar and (ii) dissimilar? ## 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. ## Answer #### (i) Similarities 1. Both are **transient (energy in transit)** phenomena: they are recognised only when they cross the system boundary; a system never "contains" heat or work. 2. Both are **boundary phenomena**: they occur at the boundary. 3. Both are **path functions**: the amount depends on the process followed, so their differentials are inexact (δQ, δW), not dQ or dW. 4. Both are associated with a **process**, not a state; neither is a property. 5. 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a similarity between heat and work? A. Both are properties of the system B. Both are path functions with inexact differentials C. Both need a temperature difference D. Both are stored in the system Answer: B. Both depend on the process, so their differentials are δQ and δW, not exact. (Key point: Similarity) 2. Heat transfer is driven by a temperature difference. Work transfer is driven by A. a temperature difference too B. only a pressure difference C. any non-thermal potential (force, pressure, voltage) D. nothing Answer: C. Work can be transferred without any temperature difference, by force, pressure or voltage potentials. (Key point: Driving potential) 3. Which statement about conversion is correct? A. Heat can be fully converted to work in a cycle B. Neither can be converted C. Heat is fully convertible to work; work is not to heat D. Work can be fully converted to heat, but heat cannot be fully converted to work in a cycle Answer: D. By the second law, heat cannot be completely converted to work in a cycle, while work can fully become heat. (Key point: Conversion) 4. The standard sign convention is A. heat into system +ve, work by system +ve B. heat out +ve, work in +ve C. both +ve into system D. both +ve out of system Answer: A. Heat added to the system and work done by the system are taken as positive. 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