Final answer:
The output work of a heat engine cycle described by points A-B-D-A on a p-V diagram is done during the expansion phases between A to B and B to D. The work output is represented by the area enclosed by the cycle path on the diagram and is related to the first law of thermodynamics, involving the difference between heat added and removed from the system.
The correct answer is 3).
Step-by-step explanation:
The student's question is related to the specific step in a heat engine cycle where output work is removed. For a typical heat engine operating in a cycle described as points A-B-C-D on a p-V diagram (pressure-volume diagram), the output work is done by the system, during the expansion of the gas.
When the engine follows the path ABDA, the output work is conducted from point A to point B (isothermal expansion) and from B to D (adiabatic expansion). The path from D back to A is typically a compression, which doesn't contribute to the output work. The work output is less for path ABDA compared to ABCDA because the area under the curve on a p-V diagram, which represents work, is smaller for the former path since it bypasses the isobaric expansion from B to C.
The net work output is calculated using the areas enclosed by the paths on the p-V diagram. This work is represented by the area within the cycle path A-B-D-A and is the difference between the heat added to the system and the heat removed from the system, according to the first law of thermodynamics. The work output is considered at point 2, which corresponds to the path of expansion where gas does work on the surroundings. In contrast, point 1 would typically represent the work done on the system during compression.