Adding Heat Raises energy

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. 3. 4 MJ.

Raising Temperature by adding heat § 12.7

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

Board Work Show that DT = q/(mc)

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?

Latent Heats of phase changes § 12.8

Solid Water (Ice) Source: M. Chaplin, Water Structure and Behavior. www.lsbu.ac.uk/water/ice1h.html

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

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.

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

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)!

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

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

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

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.