--- title: "Thermodynamic work vs mechanical work" subject: "Fundamental of Thermodynamics & Heat Transfer" chapter: 2 key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:thermodynamic-work-vs-mechanical-work" source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/thermodynamic-work-vs-mechanical-work site: BCT Engineering --- # Thermodynamic work vs mechanical work Fundamental of Thermodynamics & Heat Transfer · Chapter 2: Energy and Energy Transfer - Asked in: 2070 Magh, 4 marks - 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/thermodynamic-work-vs-mechanical-work - Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-2 ## Question Explain the concept of thermodynamic work and differentiate it with mechanical work. ## Short answer Thermodynamic work is any interaction whose sole effect outside the system could be raising a weight; it is a path function and includes all forms of energy transfer except heat. Mechanical work is force × displacement in direction of force, only one form. Thermodynamic work depends on system boundary choice. ## Answer #### Thermodynamic work Work is done by a system on its surroundings if the **sole effect external to the system could be reduced to the raising of a weight**. It is an interaction at the boundary, energy in transit, and a path function. Examples: gas expansion against a piston, electrical work done by a battery, shaft work of a turbine, stirring work. It does not need a visible force and displacement. #### Mechanical work Work is done when a force moves its point of application in the direction of the force: W = ∫ F · dx. It is only one form of work. | Basis | Thermodynamic work | Mechanical work | |---|---|---| | Definition | Effect at the boundary that could be reduced to raising a weight | Force × displacement in the direction of force | | Scope | Includes mechanical, electrical, magnetic, chemical, etc. | Only force and displacement | | Dependence on system | Depends on the choice of system boundary | Depends only on force and displacement | | Typical expression | δW = P dV, δW = V I dt (electrical) | δW = F dx | | Example | Current through a resistor if the battery is the system | Pushing a box on the floor | For example, electrical work is done on an electric motor that drives a pump, even though no force or displacement can be seen at its electrical terminals. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Thermodynamic work is done when the sole external effect could be reduced to A. raising a weight B. heating a body C. a temperature change D. a mass flow Answer: A. This is the thermodynamic definition; no visible force and displacement is required. (Key point: Thermodynamic work definition) 2. Which statement about scope is correct? A. Mechanical work includes electrical and magnetic work B. Both have the same scope C. Thermodynamic work includes mechanical, electrical, magnetic and other forms D. Thermodynamic work is only F dx Answer: C. Mechanical work is just one form of thermodynamic work. (Key point: Scope) 3. Thermodynamic work depends on A. only force and displacement B. the choice of system boundary C. the colour of the system D. nothing Answer: B. Whether a current counts as work depends on where the boundary is drawn, e.g. battery as system. (Key point: Dependence on boundary) 4. An electric motor driving a pump does electrical work on the motor system. This shows that thermodynamic work A. needs a visible displacement B. is always heat C. needs temperature difference D. does not need a visible force and displacement at the boundary Answer: D. Electrical terminals show no force or displacement, yet energy crosses as work. 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