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Heat

Heat. Molecular-scale work. pp. 82–84. high T. heat. low T. Heat. Energy transferred because of a temperature difference. Q. Zero heat flow at same temperature. Put Your Hands Together!. (Activity 1) Doing work on your hands made them warmer. Adding energy raised the temperature.

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Heat

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  1. Heat Molecular-scale work pp. 82–84

  2. high T heat low T Heat • Energy transferred because of a temperature difference Q • Zero heat flow at same temperature

  3. 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.

  4. 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

  5. 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

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

  7. Heating things pp. 84–85

  8. Q mDT Specific Heat • The amount of heat required to change the temperature of a unit mass of substance. c = • c= specific heat • Q= heat added • m = mass of sample • DT= temperature change

  9. Q mDT Heat required Solve for heat input c = Q = mcDT

  10. Heating Mechanisms pp. 86–88

  11. Heating • Heat is transferred between objects by: • Conduction: collisional transfer of kinetic energy • Convection: buoyancy-driven fluid circulation • Radiation and absorption of electromagnetic waves

  12. Conduction • Conductivity varies with material • Poor conductors are insulators • Conductivities • solids > liquids > gases

  13. Convection • Warm fluids expand, becoming less dense • Circulation is driven by buoyancy forces • Much faster than conduction

  14. Convection • Convection drives the weather!

  15. Radiation • Objects are heated by absorbing radiation. • Objects are cooled by emitting radiation. • What is radiation?

  16. Question Hot applesauce takes longer to cool than the same quantity of hot water in the same pot. Why? Conduction is slower in applesauce. Convection is slower in applesauce. Radiation is slower in applesauce.

  17. Question Solids conduct heat better than gases do. So why does a coat keep you warm in winter? The coat reduces conduction. The coat reduces convection. The coat reduces radiation.

  18. Question Aluminized Mylar blankets actually can keep you pretty warm. Which heat transfer mechanism(s) do they reduce? The blanket reduces conduction. The blanket reduces convection. The blanket reduces radiation.

  19. Question A thermos bottle keeps hot things hot, and it keeps cold things cold. But how does it know? It reduces heat transfer by conduction. It reduces heat transfer by convection. It reduces heat transfer by radiation.

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