First Law of Thermodynamics Physics 3: Charm and Electron 08 First Law of Thermodynamics Q = U + W Heat added to a system = increase in internal energy + external work done by the system. Q: pos (heat flows into the system) neg (out of the system) W: pos (system loses energy through work) neg (gains energy) U: pos (increase in internal energy) neg (decrease) Thermodynamics Meeting 4

Physics 3: Charm and Electron 08 human metabolism Q = U + W Q<0 heat flows out of the body Thermodynamics Meeting 4

gas in cylinder with movable piston Physics 3: Charm and Electron 08 gas in cylinder with movable piston Q = U + W W≠0 heat gained results to less increase in temperature Thermodynamics Meeting 4

Physics 3: Charm and Electron 08 popcorn in a pot Q = U + W Q>0 system does work to lift the lid of the pot Thermodynamics Meeting 4

Physics 3: Charm and Electron 08 isochoric Q = U + W Example: heating an airtight can filled with air on a hot stove W=0 Q = U Thermodynamics Meeting 4

Physics 3: Charm and Electron 08 free expansion Q = U + W Q=0 W=0 U=0 insulated Thermodynamics Meeting 4

Physics 3: Charm and Electron 08 adiabatic Q = U + W 0 = U + W Q=0 compression: W<0 expansion: W>0  increase in U  decrease in U Thermodynamics Meeting 4

Adiabatic Expansion Adiabatic expansion of air Air quickly leaking out of a balloon Blowing on your hand

Adiabatic Compression Adiabatic compression of air Rapidly pumping up a bicycle tire

Atmosphere

Sample Problem 150 J of heat enters a system and it performs 100 J of work into its surroundings. By how much does its internal energy increases?