Whiteboard Work 1.The 2004 Tour de France’s Alpe d’Huez time trial was a climb with its finish...
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Transcript of Whiteboard Work 1.The 2004 Tour de France’s Alpe d’Huez time trial was a climb with its finish...
![Page 1: Whiteboard Work 1.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,](https://reader030.fdocuments.net/reader030/viewer/2022032523/56649d885503460f94a6dc75/html5/thumbnails/1.jpg)
Whiteboard Work
1. 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 mgh =
(84 kg)(9.8 N/kg)(1200 m) = about 106 J.
a. 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?
b. How much of the energy he consumed was not converted to work?
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Heat and Phase Changes
Our interaction with the world of the small
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What’s the Point?
• What happens to all our energy?
• How does energy convert in freezing, melting, evaporation, and condensation?
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Objectives
• Define and differentiate heat and temperature.
• Explain the role of heat in conservation of energy.
• Describe the energy transfer of phase changes.
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Mechanical Equivalent of Heat
James Joule’s life-long obsession
Identical effects of adding heat and doing work
Source: Griffith, The Physics of Everyday Phenomena
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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
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Temperature and Energy
• Average translational 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.
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Terms
• Temperature is proportional to average molecular translational kinetic energy.
• Internal energy (U) is total molecular kinetic + potential energy.
• Heat is molecular energy transferred from high to low temperature.
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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
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When a red-hot piece of iron is dropped into a bucket of water,
Poll Question
A. the water becomes hotter.
B. the water’s temperature increases .
C. the water’s internal energy increases .
D. the water receives heat from the iron.
E. all of the above.
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Specific Heat (Capacity)
• Heat needed to change the temperature of a unit amount of a substance.
– q = heat input– m = mass of sample– T = temperature change
• Units: J/(kg K) or J/(mol K)
• Intensive
c =q
mT
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Using c
The temperature change T of an object to which an amount of heat q is added is
T = q/(mc)
where m is the object’s mass and c is its specific heat.
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Whiteboard Work
2. 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, by how many degrees would his body temperature have risen?
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Another Heat Unit
• U.S. Food Calorie: Cal = 1000 cal
• Food energy values are often presented in kJ in other countries
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Quick Whiteboard Work
3. How many Calories (food calories) did Lance burn in the Alpe d’Huez climb?
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Phase Changes
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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, about 92 °C in Laramie)
• Not all heat transfer is expressed as a temperature change.
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ice
Liquid water
steam
Water temperature with heating
-100
-50
0
50
100
150
200
0.0E+00 1.0E+06 2.0E+06 3.0E+06 4.0E+06
heat input (J/kg)
temperature (C)
Heating Curve for Water
Water boils
Ice melts
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Think Question
When are the molecules’ intermolecular potential energies highest?
A. When they are together in the liquid.
B. When they are separated in the gas.
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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!
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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
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Whiteboard Work
4. 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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Evaporation of a Liquid
• More energetic jostling = higher temperature
• An especially fast molecule at the surface may detach!
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Evaporation of a Liquid
• More energetic jostling = higher temperature
• An especially fast molecule at the surface may detach!
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Evaporation
• Evaporating molecules carry away energy
• KE PE
• Remaining liquid cools (KE decreases)
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Reading for Next Time
• Thermodynamics
• Big ideas:– Why most processes are irreversible– Entropy