1. Adding Heat
Raises energy

2. Board Work
The 2004 Tour de France Alpe d’Huez time trial stage was a climb with a finish 1200 m higher than the start. The winner, Lance Armstrong, had a mass (with gear) of 84 kg.
Assuming friction is negligible, how much work did he do on the climb?
Muscle is about 20% efficient. How much energy did he expend?
How much became thermal energy?
(84 kg)\(9.8 N/kg)(1200 m) = 987,840 J, about 1 MJ.
1 MJ = 0.20 * 5 MJ MJ.

3. Raising Temperature
by adding heat
§ 12.7

4. Specific Heat (Capacity)
Heat needed to change the temperature of a unit amount of a substance.
c = q
mDT
q = heat input
m = mass of sample
Dt = temperature change
Units: J/(kg K) or cal/(kg K) or J/(mol K)…
Intensive

5. Board Work
Show that DT = q/(mc)

6. Board Work
Lance Armstrong’s mass was 75 kg. Assume c = 4184 J/(kg K). If he were a closed system, what would have been his change in temperature?

7. Latent Heats
of phase changes
§ 12.8

8. Solid Water (Ice)
Source: M. Chaplin, Water Structure and Behavior.

9. Solids Strong connections between atoms
rigidity and elasticity
Atoms vibrate about fixed positions

10. Solid < Liquid < Gas
Phase Changes
Potential energies:
Solid < Liquid < Gas
During a phase change, potential energy, not kinetic energy (temperature) changes.
Heating or cooling a changing phase does not change its temperature!
Demo solid ice + hot water. Final temperature is NOT midway between initial temperatures; it is lower. Some energy was used to raise potential energy of the system.

11. Evaporation of a Liquid
More energetic jostling = higher temperature
An especially fast molecule at the surface may detach.

12. Evaporation Evaporating molecules carry away energy KE PE
Remaining liquid cools (KE decreases)
Adding heat is how we convert liquid water to gas (steam, vapor)!

13. Melting of a Solid Solid structure maximizes favorable interactions
Higher energy allows more movement
Molecules slow as potential energy increases

14. Heating Curve for Water
steam
Water boils
Liquid water
Ice melts
ice

15. Latent heat
Potential energy of phase change (energy required to change the phase of 1 kg of substance)
Water’s latent heat of fusion (melting):
335,000 J/kg
Water’s latent heat of vaporization:
2,255,000 J/kg

16. Whiteboard Work
During the Alpe d’Huez climb, how much sweat would Lance have needed to evaporate to keep his body temperature constant?
The heat q needed to vaporize a mass m of water is
q = m (2.255  106 J/kg).
Solve for mass m and substitute in the values.