!100%!Work!?!internal!energy?!Easy.! Second Law of Thermodynamics Pick!up!a!couple!of!rocks. Rub!them!together!vigorously,! work!W!is!done!against!fricBon Throw!them!back!into!the!sea. The!warm!rocks!cool,!and!energy!is! transferred!to!the!ocean!by!heaBng. What!about!the!other!way? 100%!internal!energy!!?!work?! Some!internal!energy!!?!work?! W Hot!reservoir!Th Cold!reservoir!Tc Qc No. Yes.117 A!heat!engine!is!any!closedJcycle!device! that!transfers!energy!Qh!from!a!hot! reservoir,!does!useful!work!W!and! exhausts!energy!Qc!to!a!cold!reservoir Heat engines ClosedJcycle!means!the!system!returns! every!now!and!then!to!its!iniBal! condiBons,!only!to!repeat!the!process. This!means!that,!over!a!complete! cycle,!?E!=!0!!because!the!system! returns!to!its!original!temperature So!FLOT!becomes: work!done!per!cycle! !is!W!=!Qh!–!Qc! Cold!reservoir!Tc Qc Hot!reservoir!Th Qh W 118 A!refrigerator!transfers!energy!from!a!cold! object!–!the!air!inside!the!fridge!–!to!a!hoRer! object!the!air!outside. It!doesn’t!happen!spontaneously!–!it!has!to!be! assisted,!driven. Work!W!is!done!to!transfer!energy!Qc!from!the! cold!to!the!hot!reservoir Refrigerators The!closedJcycle!condiBon! applies!here!too. ie,!?E!=!0 So!FLT!becomes: energy!transferred!per!cycle!is Qh!=!W!+!Qc! Cold!reservoir!Tc Qc Hot!reservoir!Th Qh W 119 Can!we!build!a!perfect!refrigerator? Refrigerators So!a!real!fridge!always!needs!work!input No. Because!then!W!=!0!and!Qh!=!Qc! spontaneously,!breaking!SLOT W!=!0 Hot!reservoir!Th Cold!reservoir!Tc Qh Qc Not$ allowed Cold!reservoir!Tc Qc Hot!reservoir!Th Qh W 120
Perfect Heat Engines? Why!not? Consider!a!perfect!engine!driving!a!real! refrigerator.! Its!energy!transfer!diagram!must!look!like:! Can!we!build!a!perfect!heat!engine? W Hot!reservoir!Th Qh Hot!reservoir Th Cold!reservoir!Tc Perfect engine Real refrigerator W!=!Qh Qh Qc + Qh Qc Qc Qc Not$ allowed ?? 121 Efficiency No!engine!is!perfect Define!efficiency!as!what!you!get!out! compared!to!what!you!put!in!(pay!for)…