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Chapter 16 Heat Transfer

Chapter 16 Heat Transfer. Heat Transfer. Heat always flows from high temperature objects to low temperature objects. Heat flow stops when temperatures equal. Various ways by which heat may flow. Heat flows from child into air. 75º. 98º. 32º. Heat flows from child and air into

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Chapter 16 Heat Transfer

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  1. Chapter 16Heat Transfer Physics 1 (Garcia) SJSU

  2. Heat Transfer Heat always flows from high temperature objects to low temperature objects. Heat flow stops when temperatures equal. Various ways by which heat may flow. Heat flows from child into air 75º 98º 32º Heat flows from child and air into the ice cream Physics 1 (Garcia) SJSU

  3. Conduction Conduction is heat flow by direct contact. Some materials are good thermal conductors, others are insulators. Tile floor feels colder than wood floor 98º 98º 75º 75º Wood is an insulator Tile is a conductor Physics 1 (Garcia) SJSU

  4. Demo: Torch the Money Wrap a dollar bill tightly around a copper pipe. Put it into a flame. Physics 1 (Garcia) SJSU

  5. Air is a Poor Conductor Because air is such a poor conductor, some pizza ovens don’t have a door. Can safely put your hand in an oven. Metal is good conductor so you need oven mitt to touch it safely (cloth is a poor conductor). Physics 1 (Garcia) SJSU

  6. Demo: Boiling Ice Water Water and glass are relatively poor conductors of heat. Can boil water at the top of a test tube with ice at the bottom of the tube. Steel wool prevents ice from floating Physics 1 (Garcia) SJSU

  7. Convection Heat transfer in a fluid often occurs mostly by convection. Buoyancy causes warm air to rise, which carries thermal energy directly by its motion. Physics 1 (Garcia) SJSU

  8. Demo: Candle Very HOT Rising hot air above a candle carries most of the heat generated by the burning flame. Warm Shadows reveal rising air currents of hot air. Physics 1 (Garcia) SJSU

  9. Demo: Convection & Buoyancy Candle goes out while in free fall because in weightless state hot air does not rise so fresh air cannot reach the candle’s wick. Physics 1 (Garcia) SJSU

  10. Convection Oven Convection oven has a fan to enhance the circulation of the air, increasing the transfer of heat. Physics 1 (Garcia) SJSU

  11. Demo: Candle in a Tube Candle stays light until the partition in the tube is removed. Why? Physics 1 (Garcia) SJSU

  12. Fiberglass Insulation Air is a poor thermal conductor but easily transfers heat by convection. Fiberglass insulation is mostly air, with the fibers disrupting the convection flow. Physics 1 (Garcia) SJSU

  13. Radiation Light has many different wavelengths, most of which are not visible to the eye. All light carries energy, thus transfers heat. Heat Lamp Physics 1 (Garcia) SJSU

  14. Emission of Radiant Energy 75º All objects radiate light; higher the temperature the higher the frequency. At room temperature the radiated light is at frequencies too low for our eyes to see. Special cameras are sensitive to this infrared radiation. 98º Attics in this house were kept warm for growing marijuana. 70 Physics 1 (Garcia) SJSU

  15. Reflection of Radiant Energy White objects reflect light, black objects don’t. Hole in a box with white interior looks black because almost none of the light entering the hole reflects back out. White tubes look black inside. Physics 1 (Garcia) SJSU

  16. Controlling Heat Transfer Thermos bottle eliminates conduction and convection by having double-walled sides with vacuum. Silvered interior walls minimize heat transfer by radiation. Physics 1 (Garcia) SJSU

  17. Greenhouse Effect Glass is transparent to sunlight (short-wavelength). Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat. Physics 1 (Garcia) SJSU

  18. Earth’s Greenhouse Effect Earth’s atmosphere acts as a greenhouse, trapping solar energy. Most of the trapping is due to carbon dioxide and water vapor, which is why they’re called “greenhouse gasses.” Physics 1 (Garcia) SJSU

  19. Check Yourself What does it mean to say that the greenhouse effect is like a one-way valve? Can this build up energy forever? Physics 1 (Garcia) SJSU

  20. Global Temperature Variations Temperatures increased from 1910 to 1940. Temperatures then cooled for 40 years until they started rising again in the 1980’s. Physics 1 (Garcia) SJSU

  21. Greenhouse Carbon Dioxide Over past 1000 years temperatures nearly constant until CO2 emissions increased starting with the industrial revolution. Physics 1 (Garcia) SJSU Industrial revolution begins

  22. Cars & Carbon Dioxide One gallon of gasoline has about 5.2 lb of carbon. A 5-lb bag of charcoal (which is pure carbon) holds about 100 briquettes. At 26 miles per gallon, that’s 0.2 lb of carbon per mile, or about four to five charcoal briquettes per mile. Physics 1 (Garcia) SJSU

  23. Anthropogenic* Global Warming Rising temperatures are due to human production of greenhouse gases. Temperature *Caused by humans Physics 1 (Garcia) SJSU

  24. Consequences of Global Warming Melting of Polar Ice Caps Species extinctions Weather modifications Physics 1 (Garcia) SJSU

  25. Solutions to Global Warming Many simple, small changes together could make a significant difference. Efficient Lighting Efficient Appliances Building Insulation Physics 1 (Garcia) SJSU

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