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Group Work The 2004 Tour de France’s Alpe d’Huez time trial was a climb with its finish 1200 m higher than the start. The winner, Lance Armstrong, and his gear had a combined mass of 84 kg . The work Lance had to do was ( 84 kg )( 9.8 N/kg )( 1200 m ) = about 10 6 J .

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## Group Work

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**Group Work**• The 2004 Tour de France’s Alpe d’Huez time trial was a climb with its finish 1200 m higher than the start. The winner, Lance Armstrong, and his gear had a combined mass of 84 kg. The work Lance had to do was (84 kg)(9.8 N/kg)(1200 m)= about106 J. Muscle is about 20% efficient, so Lance had to deplete 5 times as much potential energy as the work he produced. How much energy did he consume? How much of the energy he consumed was not converted to work?**Heat and Phase Changes**Our interaction with the world of the small**Announcements**• Questions on drill?**What’s the Point?**• What happens to all our energy? • How does energy convert in freezing, melting, evaporation, and condensation?**Objectives**• Define and differentiate heat and temperature. • Explain the role of heat in conservation of energy. • Describe the energy transfer of phase changes.**CPS Question**• Transfer of force. • Transfer of momentum. • Transfer of mass. • Transfer of energy. What is work?**Mechanical Equivalent of Heat**James Joule’s life-long obsession Identical effects of adding heat to a system and doing work on it. Source: Griffith, The Physics of Everyday Phenomena**Heat and work**• Doing work on an object changes its energy. • Heat transfer is another way to change an object’s energy! • Work heat: easy • Heat work: more difficult**CPS Question**• Its speed increases. • Its height increases. • Its temperature increases. • Its energy does not change. When heat is added to an object, how does the object’s energy change?**Temperature and Energy**• Averagetranslational molecular kinetic energy is 1/2 kBT per mode of motion. • kB = 1.3806610–23 J/K (Boltzmann constant) • Individual molecules can have higher or lower kinetic energies than average.**Terms**• Temperature is proportional to average molecular translational kinetic energy. • Internal energy (U) is total molecular kinetic + potential energy. • Heat is transferred molecular energy.**When a red-hot piece of iron is dropped into a bucket of**water, the water CPS Question becomes hotter. increases its temperature. increases its internal energy. receives heat from the iron. all of the above.**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) • Intensive**Heat Capacity**• Heat needed to change the temperature of a particular sample. C = q/DT = mc • Units: J/K • Extensive**Group Work**• Show that the temperature change DT of an object to which an amount of heat q is added is given by the formula DT = q/(mc) where m is the object’s mass and c is its specific heat. Hint: What is the formula for specific heat?**Group Work**• The specific heat of Lance Armstrong is about the same as the specific heat of water: 4184 J/(kg °C). If all the non-work energy he converted in the Alpe d’Huez climb stayed in his 75-kg body, how much would his body temperature have risen?**Heat Units**• Joule • Calorie (cal): heat needed to raise 1 gram of water 1 degree C (or K) = 4.184 J. • British Thermal Unit (BTU): heat needed to raise 1 pound of water 1 degree F = 1054.35 J**Another Heat Unit**• U.S. Food Calorie: Cal = 1000 cal • Food energy values are often presented in kJ in other countries**Quick Group Work**• How many Calories (food calories) did Lance burn in the Alpe d’Huez climb?**Phase Changes**• Melting, boiling, freezing, condensing… • Added or removed heat changes the substance’s potential rather than kinetic energy • Water freezes at 0 °C, boils at 100 °C (well, at 92 °C in Laramie) • Not all heat transfer is expressed as a temperature change.**Molecular simulations**• Water phase changes**Evaporation of a Liquid**• More energetic jostling = higher temperature • An especially fast molecule at the surface may detach!**Evaporation of a Liquid**• More energetic jostling = higher temperature • An especially fast molecule at the surface may detach!**KE PE**Evaporation • Evaporating molecules carry away energy • Remaining liquid cools (KE decreases)**CPS Question**When are the molecules’ potential energies highest? • When they are together in the liquid. • When they are separated in the 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!**steam**Water boils Liquid water Ice melts ice Heating Curve for Water**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**Group 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.**Reading for Next Time**Thermodynamics Big ideas: Why most processes are irreversible Entropy

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