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

2. 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

3. 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

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

5. 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

6. 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

7. 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

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