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Adding Heat

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?

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Adding Heat

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  1. Adding Heat Raises energy

  2. Board Work The 2004 Tour de France Alped’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?

  3. Raising Temperature by adding heat § 12.7

  4. c = q mDT Specific Heat (Capacity) • Heat needed to change the temperature of a unit amount of a substance. • 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. www.lsbu.ac.uk/water/ice1h.html

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

  10. 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!

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

  12. KE PE Evaporation • Evaporating molecules carry away energy • Remaining liquid cools (KE decreases)

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

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

  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.

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