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The Transfer of Heat

Chapter 13. The Transfer of Heat. 13.1 Convection. The process in which heat is carried from place to place by the bulk movement of a fluid . Examples: heating a pot of water, heating (or cooling) a home, ground warming neighboring air

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The Transfer of Heat

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  1. Chapter 13 The Transfer of Heat

  2. 13.1 Convection • The process in which heat is carried from place to place by the bulk movement of a fluid. • Examples: heating a pot of water, heating (or cooling) a home, ground warming neighboring air • Birds use “thermals” to soar through the air. Convection principles are being used to cool everything from athletes to automobile engines.

  3. 13.2 Conduction • The process whereby heat is transferred directly through a material, with any bulk motion of the material playing no role in the transfer. • Movement of particles causes collisions with other particles. Collisions increase KE of all particles involved, thus increasing temperature of material. • Metals are great conductors because the “sea of electrons” allows quick, easy transfer of energy.

  4. Factors Affecting Conduction • Q is proportional to the time conduction takes place. (More time = more heat flow) • Q is proportional to the temperature difference. (Larger ∆T = more heat flow) • Q is proportional to cross-sectional area. (Larger area = more heat flow) • Q is inversely proportional to length. (Greater lengths conduct less heat)

  5. Equation for Conduction of Heat • SI unit of thermal conductivity: J/(smC°) Why does a down coat keep me warm? • Q is heat conducted • t is time through material • L is material length • A is cross-sectional area • ∆T is temperature difference • k is thermal conductivity (a numerical constant describing the ability of the material itself to conduct heat due to its chemical structure)

  6. 13.3 Radiation • Radiation is the process in which energy is transferred by means of electromagnetic waves. • These waves do not require a medium. • ALL BODIES continuously radiate energy in the form of electromagnetic waves. • Objects on Earth radiate infrared waves.

  7. Interesting Tidbits • The term blackbody is used to describe an object that absorbs all the electromagnetic waves falling on it. • All objects emit and absorb electromagnetic waves simultaneously. • A good absorber is also a good emitter. Why am I so uncomfortable wearing black in the summer?

  8. Factors Affecting Radiation • Q increases with more time. • Q increases with surface area. • Q increases with the 4th power of Kelvin Temperature. (Increasing temp. dramatically increases heat transfer)

  9. The Stefan-Boltzmann Law of Radiation • σ is the Stefan-Boltzmann constant and is equal to 5.67 x 10-8 J/(sm2 K4 ) • e is the emissivity. This number will be between 0 and 1. It describes the ratio of the energy the object actually radiates to the energy it would if it were a perfect emitter. This value depends on the condition of the surface.

  10. Sample Problem • The supergiant star Betelgeuse has a surface temperature of about 2900K and emits a radiant power of approximately 4 x 1030 W. Assuming that Betelgeuse is a perfect emitter (e =1) and spherical, find its radius. Q/t is the power emitted so A is equal to … And A is equal to Rearranged Finally…

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