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Conduction, Convection+Radiation

Conduction, Convection+Radiation. Question. Touch base of chair, and desk top, which feels colder? A) Base B) Same C) Desk. Both must be the same temperature (room temperature), but metal feels colder because it conducts heat better/faster. 10. Heat Transfer: Conduction.

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Conduction, Convection+Radiation

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  1. Conduction, Convection+Radiation

  2. Question Touch base of chair, and desk top, which feels colder? A) Base B) Same C) Desk Both must be the same temperature (room temperature), but metal feels colder because it conducts heat better/faster. 10

  3. Heat Transfer: Conduction • Hot molecules have more KE than cold molecules • High-speed molecules on left collide with low-speed molecules on right • energy transferred to lower-speed molecules • heat transfers from hot to cold • I = rate of heat transfer = Q/t [J/s] • I = k A (TH-TC)/L • Q/t = k A T/x • k = “thermal conductivity” • Units: J/s-m-C • good thermal conductors…high k • good thermal insulators … low k • R = L/(Ak) = thermal resistance L = Dx TC Cold TH Hot Area A demos 13

  4. Question If you want to maximize the amount of Heat flow which substance should you choose? 1. Aluminum 2. Copper 3. Steel 15

  5. Example: Boiling water • A steel pot with an area of 0.05m2 and thickness of 0.006m is placed on top of a burner so that its bottom surface remains at 1500C. How much water boils away each minute?

  6. Question If you want to limit the amount of Heat flow (Insulate) which substance should you choose? 1. Air 2. Wood 3. Glass 15

  7. How does an oven work?

  8. heater Heat Transfer Convection • Air heats at bottom • Thermal expansion…density gets smaller • Lower density air rises • Archimedes: low density floats on high density • Cooler air pushed down • Cycle continues with net result of circulation of air • Practical aspects • heater ducts on floor • A/C ducts on ceiling • stove heats water from bottom • “riding the thermals” demos 27

  9. Heat Transfer: Radiation Surroundings at T0 T Hot stove • All things radiate and absorb electromagnetic energy • Iemit = eAT4 • Iabsorb = eAT04 • Inet = Iemit - Iabsorb = eA(T4 - T04) • if T > T0, object cools down • if T < T0, object heats up HW 38

  10. Summary • Conduction - contact • Convection - fluid motion • Radiation – Electromagnetic waves 50

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