6-1 introduction to the second law 6-2 thermal energy reservoirs 6-3 heat engines (热机) 6-3 heat engines (热机)--steam power plant 6-3 heat engines– thermal efficiency (热效率) 6-3 heat engines (热机) – thermal efficiency 6-4 Refrigerators and heat pumps 6-5 perpetual-motion machines (永动机)