Efficiency Of Heat Engine 1 Q Sink Q Source — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Efficiency of Heat Engine (η = 1 - Q sink/Q source) Applicable to all cyclic heat engines, regardless of whether they are reversible or irreversible. Define thermal efficiency as Work Output / Heat Input ( = W / Q source ). Apply the First Law of Thermodynamics for a complete cycle: U = 0, so Q net = W. Calculate net heat: Q net = Q source - Q sink . Substitute W: = (Q source - Q sink ) / Q source . Simplify the fraction: = 1 - Q sink / Q source . Q source must be greater than 0. Q sink must be less than or equal to Q source for a real engine. Values for Q are typically magnitudes (positive). Confusing this formula with the Carnot efficiency (1 - T L/T H); while similar, this formula applies to all engines using heat values, not just reversible ones using temperatures. Assuming efficiency can reach 100% (requires Q sink = 0, which violates the Second Law). Mixing up Q source and Q sink resulting in negative efficiency.