Thermodynamics and Energy Technology
What this domain tests
Thermodynamics and Energy Technology tests energy conversion in closed and open systems: the first law, gas processes, engine and refrigeration cycles, steam, and the three heat-transfer modes.
Core rules
- First law for a closed system: Q − W = ΔU. For a steady-flow open system, enthalpy terms replace internal energy. Fix a sign convention and state it.
- Ideal gas: PV = mRT, with T in kelvin. R for air is about 0.287 kJ/(kg K).
- Processes: isobaric (constant pressure), isochoric (constant volume, no boundary work), isothermal (T constant, PV constant), adiabatic (no heat transfer, PV^γ constant).
- Second law: no engine can exceed the Carnot efficiency between the same reservoirs, η_Carnot = 1 − T_cold/T_hot, with temperatures in kelvin.
- Refrigeration: COP = useful cooling/work input; a heat pump COP is cooling COP + 1.
- Heat transfer: conduction follows Fourier's law, convection Newton's law of cooling, radiation the Stefan–Boltzmann law; radiation needs no medium.
Worked example
A heat engine receives 1,000 kJ from a 600 K source per cycle, rejects heat to 300 K, and produces 400 kJ of work. Its efficiency is 400/1,000 = 40%. The Carnot limit is 1 − 300/600 = 50%. The engine is possible but not reversible, because 40% < 50%. The rejected heat is 600 kJ by the first law.
Traps
Putting °C into the Carnot or gas equation; confusing gauge and absolute pressure; using efficiency formulas for a refrigerator; forgetting that adiabatic does not mean isothermal; ignoring heat loss in a real cycle; mixing kJ and J.