1. 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

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

3. 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

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

5. 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

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

7. 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

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

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

10. Atmosphere

11. 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?