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Heat and Temperature

Heat and Temperature. PHYS 1090 Unit 4. Put Your Hands Together!. (Activity 1) Doing work on your hands made them warmer. Adding energy raised the temperature. Work had the same effect as heat. Mechanical Equivalent of Heat. James Joule’s life-long obsession.

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Heat and Temperature

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  1. Heat and Temperature PHYS 1090 Unit 4

  2. Put Your Hands Together! (Activity 1) • Doing work on your hands made them warmer. • Adding energy raised the temperature. • Work had the same effect as heat.

  3. Mechanical Equivalent of Heat James Joule’s life-long obsession No difference between adding heat to a system and doing work on it. Source: Griffith, The Physics of Everyday Phenomena

  4. Heat Units • Joule • Calorie (cal): amount of heat needed to raise 1 gram of water 1 degree C (or K) = 4.184 J. • British Thermal Unit (BTU): amount of heat needed to raise 1 pound of water 1 degree F = 1054.35 J

  5. q mDT Specific Heat • The amount of heat required to change the temperature of a unit mass of substance. Cp = • C= specific heat • q= heat added • m = mass of sample • DT= temperature change

  6. Another Heat Unit • U.S. Food Calorie: Cal = 1000 cal • Food energy values are often presented in kJ in other countries

  7. First Law of Thermodynamics DE = q + w where DE = change in internal energy q = heat input w = work input

  8. Kinetic-Molecular Theory • Everything is made of molecules. • The molecules are constantly moving in random directions. • (Absolute) temperature is proportional to molecular translational kinetic energy. • Molecules colliding with objects they contact causes pressure.

  9. Absolute Zero • When molecules have zero kinetic energy • Absolute zero = −273.15 °C • Kelvin temperature = °C + 273.15 • Absolute zero = 0 K • 0 °C= 273.15 K

  10. Liquid N2 Boil (Activity 2) • More boiled away with cold water • More heat transferred from cold water • Because there was more cold than hot water

  11. Warm and Cool Colors (Activity 3) • Dye dispersed faster in warm water. • Why? • Water molecules were moving faster in the warm water.

  12. Phase Change (Activity 4) • Melting ice temperature was constant even though heat was being added • Specific heat is infinite? • Boiling water temperature was also constant • Boiling temperature  100°C?

  13. Sensing Latent Heats (Activity 5) • Evaporation is a cooling process. • Condensation is a warming process.

  14. Phase Changes • Potential energies: Gas condense evaporate Liquid sublime deposit freeze melt Solid • During a phase change, potential energy, not kinetic energy (temperature) changes. • Heating or cooling a changing phase does not change its temperature!

  15. 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 100 °C • Not all heat transfer is expressed as a temperature change.

  16. Heats of Phase Changes • To melt 1 kg water at 0 °C: 335 kJ • To boil 1 kg water at 100 °C: 2,255 kJ

  17. steam Water boils Liquid water Ice melts ice Heating Curve for Water 2,255 kJ/kg 335 kJ/kg

  18. Heat Transfer Mechanisms • Heat transfers between objects by: • Conduction: collisional transfer of kinetic energy • Convection: buoyancy-driven fluid circulation • Radiation and absorption of electromagnetic waves

  19. Conduction (Activity 6) • Thermal energy moves through different materials at different speeds. • Conductivity varies with material; solids > liquids > gases

  20. Convection • Hot water stayed on top, cold stayed on the bottom • Hot water moved to the top, cold to the bottom (with mixing)

  21. Radiation (Activity 8) • Coil heated your hand from afar. • Heating less intense farther away. • Aluminum blocked heat.

  22. Radiation • Power output increases as T4. • Objects are heated by absorbing radiation. • Objects are cooled by emitting radiation.

  23. Entropy (Activity 9) • Dice moved in either direction with equal probability • Began highly localized • Localized  Dispersed • Inevitable and irreversible

  24. Model of Diffusion • Particles randomly move from high concentration to low concentration • Individual flow either way • Net flow high → low • Equilibrium: uniformity • Energy flows in the same way • Transfer in collisions • Heat flow: high temp → low temp

  25. Entropy • A measure of “disorder” • Related to the number of equivalent ways to arrange a system Low entropy High entropy

  26. Overall Summary • Particles and energy tend to become spread out uniformly. • Total entropy increases in all processes that actually occur.

  27. Drinking Bird • Evaporation from the bird’s head cooled the vapor inside. • This reduced the head pressure to less than in the bottom bulb. • The higher pressure in the bottom pushed the fluid uphill, making the bird top-heavy.

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