Thermodynamics
What to master
Properties and state (ideal gas law, compressibility, saturated and superheated tables, quality), the first law for closed systems (Q - W = delta U) and open systems (steady-flow energy equation with enthalpy), boundary work, specific heats and the relation cp - cv = R, polytropic processes, the second law and entropy, Carnot efficiency, Rankine and Brayton cycles with their efficiency-improving modifications (reheat, regeneration, superheat), refrigeration and heat pumps with COP, psychrometrics (dry-bulb, wet-bulb, humidity ratio, dew point), combustion basics, and heat transfer by conduction (Fourier), convection (Newton's law of cooling) and radiation (Stefan-Boltzmann).
Traps
- Sign convention: work done by the system is positive in most FE texts -- check the convention implied by the answer choices.
- Carnot and other thermal efficiencies require absolute temperature (kelvin or rankine).
- COP can legitimately exceed 1; efficiency cannot.
- Enthalpy, not internal energy, appears in steady-flow devices such as turbines and compressors.
- Mixing mass flow rate with molar flow rate, or forgetting to convert to a per-unit-mass basis.
Worked example
A heat engine operates between 800 K and 300 K. The Carnot limit is eta = 1 - Tc/Th = 1 - 300/800 = 0.625, or 62.5%. Using 527 C and 27 C in the same formula would give a nonsense negative result.
Rule of thumb: sketch the system boundary and label every energy crossing before writing the first law -- most thermodynamics errors are bookkeeping errors, not property errors.