320 likes | 332 Views
Understand the principles of heat flow through convection, conduction, and radiation. Learn about thermal conductivity, thermal radiation, and practical applications of heat transfer. Explore how objects emit and absorb thermal radiation.
E N D
L 18 Thermodynamics [3] • Heat transfer • convection • conduction • radiation • emitters of radiation • seeing behind closed doors • Greenhouse effect • global warming
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
Convection • heat is carried from place to place by the bulk movement of either liquids or gases • Convection does NOT occur in 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)
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
heat conduction Cross sectional area A L HOT COLD Heat Flow Heat Flow rate depends on A / L
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)
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
electromagnetic spectrum thermal radiation microwaves, cell phones TV radio waves x-rays visible
visible electromagnetic waves: LIGHT shorter wavelength more energy visible light thermal radiation UV radiation produces sunburn
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
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!
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)
sources of thermal radiation tungsten filament, has a very high melting point, 3400 C • 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
e = 0 e = 0.5 e = 1 not anemitter pooremitter goodemitter perfectemitter 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
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. The aluminum foil reflects the heat rather than absorbing it.
good/bad emitters-Leslie’s cube infrared radiation sensor copper cube filled with hot water this side is painted black
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
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 redorangeyellow blue as the temperature increases
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.
Which one is best? • silvered • silvered and un-evacuated • evacuated • un-silvered andun-evacuated
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!
http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/ The Greenhouse effect Sun’s visible light infrared radiation is trapped C O2, H2O EARTH
Effect of greenhouse gases:H2O, CO2, CH4, . . . • 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 the greenhouse gases in the atmosphere
Greenhouse effect and climate change • concentrations of CO2 have been increasing • rise in earth’s temperature • same effect occurs in your car during the day.
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
Global warming issues • 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 • What is the effect of human activity (anthropogenic) on the buildup of greenhouse gases? • (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. • What you can do: convert to fluorescent lights
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