Final answer:
Efficiency in thermodynamics is defined as W/Qh to account for energy not converted into work but lost to the environment, with limits placed by the second law of thermodynamics.
Step-by-step explanation:
The student's question pertains to the definition of efficiency in practical terms, particularly in the context of thermodynamics, and why efficiency is defined as W/Qh.
Thermal efficiency is a way to quantify how efficiently a machine converts heat into work. It is mathematically expressed as Eff = (Qh - Qc) / Qh, where Qh is the heat supplied to the system, Qc is the heat discharged to the environment, and W (W = Qh - Qc) is the work done by the system.
The definition of efficiency as W/Qh is chosen because it accounts for energy that is not converted into work but is instead lost to the environment. This loss is inevitable due to the second law of thermodynamics, which places limits on the conversion efficiency of heat into work.
An efficiency of 1, or 100%, would only be possible if there were no heat transfer to the environment (Qc = 0).