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Exploring Heat Transfer: Convection, Conduction, and Radiation

Discover the principles of convection, conduction, and radiation in this informative guide on heat transfer mechanisms and their applications. Learn about emitters of thermal radiation, the greenhouse effect, and global warming.

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Exploring Heat Transfer: Convection, Conduction, and Radiation

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  1. L 18 Thermodynamics [3] • Heat transfer • convection • conduction • radiation • emitters of radiation • seeing behind closed doors • Greenhouse effect • global warming

  2. Convection • heat is carried from place to place by the bulk movement of either liquids or gases • does not apply to solids • when water is boiled, hot liquid rises and mixes with cooler liquid, thus the heat is transferred • Hot air rises: • want heat into lower level of house (winter) • cooled air into upper levels (summer)

  3. Conduction iron is a particularly poor conductor of heat • heat is transferred directly through a material, with no bulk movement of stuff • only energy moves

  4. heat conduction Cross sectional area A L HOT COLD Heat Flow Heat Flow rate depends on A / L

  5. Thermal Conductivity • The effectiveness of a material in conducting heat is characterized by a parameter called the thermal conductivity • there are good thermal conductors (metals) and poor ones (insulators)

  6. Thermal Conductivities of Metals

  7. Grandma’s silver spoons

  8. Heat flow • HEAT  the energy that flows from one system to another because of temperature differences. • But how does it flow? Three ways: • convection  • conduction  • radiation

  9.  radiation Heat as moving light • Radiation is the heat transfer by electromagnetic waves – thermal light waves - invisible to the eyes • thermal radiation is a small part of the electromagnetic spectrum – waves are characterized by their frequency or wavelength • different colors in the visible correspond to different wavelengths from red to blue

  10. electromagnetic spectrum thermal radiation microwaves, cell phones TV radio waves x-rays visible

  11. visible electromagnetic waves: LIGHT shorter wavelength  more energy visible light thermal radiation UV radiation produces sunburn

  12. Thermal Radiation • The warmth you feel from the sun is the sun’s thermal radiation • It travels through the vacuum of space to reach earth, no material is necessary (takes 8 minutes) • you can feel its effects even though you cannot see the radiation. • you can feel the thermal radiation from a fireplace

  13. What produces thermal radiation? • all objects whose temperature is above absolute zero emit thermal radiation • The hotter the object, the more radiation it emits, the amount of radiation is ~ T4 • We all continuously emit thermal radiation • We also absorb it from objects and people around us • If we just emitted radiation we would eventually cool to absolute zero!

  14. Emission and Absorption are balanced

  15. Thermal radiation spectrum • The intensity of radiation increases with temperature • the color shifts toward the blue at higher temperatures • The UV radiation from the sun is just beyond the violet (11,000 F)

  16. sources of thermal radiation tungsten filament, can get very hot and not melt • the incandescent light bulb ( the ones that have a filament) are sources of both visible light and heat. • when electricity flows through a wire it gets hot. • it emits radiation even though you can’t see it • as it gets hotter it glows red then orange then white evacuated glass bulb

  17. Radiation emitted by hot objects • The hotter they are, the more they emit • the efficiency with which an object emits thermal radiation is characterized be a parameter called its emissivity  e • e is a number between 0 and 1 • a good emitter has an e close to 1 • a poor emitter has an e close to 0

  18. good emitters are good absorbers • an object that is a good emitter is also a good absorber of thermal radiation • a poor emitter is also a poor absorber • generally dark, dull objects are the best emitters/absorbers • shinny objects are poor emitters/absorbers, they are good reflectors of radiation • If you do not want the edges of your pie to burn, you wrap it in aluminum foil

  19. good/bad emitters-Leslie’s cube infrared radiation sensor copper cube filled with hot water this side is painted black

  20. Practical considerations • wear light clothing in summer  light clothing absorbs less sunlight • cover all body parts in winter  warm body parts (like your head) emit radiation

  21. thermal radiation • all objects that are at a temperature above absolute zero emit thermal radiation (waves) • the higher the temp, the more they emit • the color (wavelength) of the emitted waves goes from redorangeyellow blue as the temperature increases

  22. seeing behind closed doors we can “see” behind closed doors because of the heat signature left by warm objects on walls Infrared sensors can pick up temp- erature differences of 0.05 degrees C.

  23. Which one is best? • silvered • silvered and un-evacuated • evacuated • un-silvered andun-evacuated

  24. Why are the poles colder than the equatorial regions? SUN • More of the Sun’s energy per unit area falls on the equatorial regions compared to the polar regions • the earth reflects about 30% of incident solar energy • without the atmosphere the earth would be 30C cooler!

  25. http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/ The Greenhouse effect Sun’s visible light infrared radiation is trapped C O2 EARTH

  26. Effect of CO2 • the sun’s visible light can penetrate through the atmosphere to the earth’s surface where it heats it • the visible light energy is converted to thermal light energy • the thermal radiation is reflected from CO2 in the atmosphere

  27. Greenhouse effect • concentrations of CO2 have been increasing •  rise in earth’s temperature • same effect occurs in your car during the day.

  28. James Hansen • NASA Goddard Institute for Space Studies, Columbia University, NY • Born 1941 Denison, IA • PhD 1967 Univ. Iowa • MS 1965 Univ. Iowa • BA 1963 Univ. Iowa http://www.columbia.edu/~jeh1/dots_feb2007.ppt

  29. Temperature change 1880-2003

  30. 1960-2020

  31. Global warming issues • 88,800,000 sites on Google • http://www.hillsdale.edu/news/imprimis/archive/issue.asp?year=2007&month=08 • http://www.epa.gov/climatechange/ • http://ipcc-wg1.ucar.edu/wg1/wg1-report.html • Is the buildup of Greenhouse gases due to human activity (anthropogenic)? • (NRC 2001) Because of the large and still uncertain level of natural variability inherent in the climate record and the uncertainties in the time histories of the various forcing agents (and particularly aerosols), a causal linkage between the buildup of greenhouse gases in the atmosphere and the observed climate changes during the 20th century cannot be unequivocally established. • The IPCC (International Panel on Climate Change) 2/2/07: “global warming is “very likely” caused by man. • buy cars with high mpg ratings and use fluorescent lights

  32. The ozone layer- a related but different issue • ozone, O3 is a naturally occurring trace element in the atmosphere • It absorbs solar ultraviolet radiation, especially the harmful UV-B rays • it is destroyed by Cfc’s (chlorofluorocarbons) • loss affects us and environment

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