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Heat Transfer

Heat Transfer. Chapter 22. Conduction. Conduction – energy transfer from particle to particle within certain materials, or from one material to another when the two are in direct contact Conductors – materials that conduct heat well

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Heat Transfer

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  1. Heat Transfer Chapter 22

  2. Conduction • Conduction – energy transfer from particle to particle within certain materials, or from one material to another when the two are in direct contact • Conductors – materials that conduct heat well • Metals are the best conductors (silver, copper, aluminum, and iron) • Materials composed if atoms with “loose” outer electrons are good conductors of heat • Insulators – materials which delay the transfer of heat (wood, wool, straw, paper, cork, and Styrofoam) • Cold is simply the absence of heat, only heat is transferred through a conductor or insulator

  3. Convection • Convection – a means of heat transfer by movement of the heated substance itself, such as by currents in a fluid • Convection occurs in all fluids, whether liquid or gas • Convection is occurring all around you, the atmosphere, the ocean, Earth, the sun! • As warm air rises, it expands and cools • When the air has been cooled, it will sink back down again and warm up …

  4. Convection

  5. Radiation • Radiation – Energy transmitted by electromagnetic waves (i.e. the sun) • Radiant Energy – any energy, including heat, that is transmitted by radiation • All objects continually emit radiant energy in a mixture of wavelengths • The lower the temperature, the longer the wavelength

  6. Three Mechanisms of Heat Transfer

  7. Absorption of Radiant Energy • Absorption and reflection are opposite processes, a good absorber of radiant energy will reflect only a little amount of radiant energy • A perfect absorber will reflect no radiant energy and appear black • Good reflectors are poor absorbers of radiant energy

  8. Emission of Radiant Energy • Good absorbers are also good emitters; poor absorbers are poor emitters • If a good absorber was not also a good emitter, then black objects would remain warmer than lighter colored objects and never come to thermal equilibrium with them • Each object is emitting as much energy as it is absorbing

  9. Absorption and Emission of Radiant Energy

  10. Newton’s Law of Cooling • The rate of cooling of an object depends on how much hotter the object is than the surroundings The rate of cooling of an object – whether by conduction, convection, or radiation – is approximately proportional to the temperature difference ΔT between the object and its surroundings Rate of cooling ~ ΔT • Newton’s law of cooling also holds for heating

  11. Global Warming • Greenhouse Effect – The warming effect whose cause is that short-wavelength radiant energy from the sun can enter the atmosphere and be absorbed by Earth more easily than long-wavelength energy from Earth can leave • Earth absorbs the energy from the sun through the atmosphere • As the atmosphere gets thicker from carbon dioxide, it will not allow as much energy to escape into space • Terrestrial Radiation – Energy that Earth radiates

  12. The Greenhouse Effect

  13. Assignment Due Tuesday 1/22 • Read Chapter 22 (pg. 325-336) • Do Chapter 22 #21-30 (pg. 338)

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