Second Law of Thermodynamics Engines and Refrigerators Physics 3: Charm and Electron 08 Second Law of Thermodynamics Engines and Refrigerators Thermodynamics Meeting 4

Heat Engine Any device that transforms heat partly into work or mechanical energy *working substance – matter inside the engine which undergoes inflow and outflow of heat, expansion and compression, and sometimes phase change

how it works Working substance absorbs heat from the hot reservoir Performs some mechanical work Discards the remaining energy in the form of heat into the cold reservoir *cyclic process Young

thermal efficiency e = W/QH = (|QH|-|QC|)/|QH| = 1 –(|QC|/QH|) Young

Physics 3: Charm and Electron 08 Refrigerator takes heat from a cold place and gives of heat to a warmer place requires a net input of mechanical work working substance – refrigerant fluid cold reservoir – inside of refrigerator hot reservoir – outside of refrigerator Thermodynamics Meeting 4

how it works fluid absorbs heat from the cold reservoir work is done on the fluid energy from heat transfer and work done is discarded into the hot reservoir *cyclic process Young

the fluid in the evaporator coil is colder than the inside of the ref, so it absorbs heat the compressor takes in fluid and compresses it adiabatically (work is done on the fluid) fluid is delivered to condenser at high pressure, fluid temperature is higher than that of surrounding air, fluid gives of heat and condenses fluid expands adiabatically (expansion valve) into the evaporator and cools considerably Young

coefficient of performance K = |QC|/|W| = |QC|/(|QH|-|QC|)