1 1 Quiz Question • A travel mug usually comes with a lid. This keeps your coffee from splashing out and keeps it hot for a longer time. Why? A. Heat loss due to conduction is greatly reduced. B. Heat loss due to convection is greatly reduced. C. Heat loss due to radiation is greatly reduced. D. All of the above is true. 2 Convection • Transfer of heat by macroscopic movement of relatively large portions of fluids (liquids and gases). Parts of fluid are moving from heat sources and carrying energy with it. • Examples: Fan mixing warm and cold air (forced convection), rising of hot air due to its smaller density, boiling water, hot water radiator,… 3 Natural Convection Very hot, low- density air is buoyed upward, carrying thermal energy with it. Inversion layer. Air near ground is more dense than air higher up; no convection currents to lift pollutants. Air above warmer ground rises. Convection: Examples
2 4 Atmospheric Convection • The relatively high temperatures near the equator means that the air there is always rising, with a corresponding high cloud, low rainfall zone. • This is called the intertropical convergence zone (ITCZ). You can see the layer of high cloud quite clearly in satellite photographs. 5 Quiz Question • What’s the main heat loss mechanism for tea or coffee in a closed paper cup inside your office? A. Conduction B. Convection C. Radiation D. Murphy’s Law 6 Radiation • In empty space, collisions or mass motions are impossible hence there is no conduction or convection. Radiation is the only way to transfer energy and heat. The heat is transferred from the sun to the earth mostly by visible and infrared light.
3 7 Topic 3: Convention and Radiation • Electromagnetic spectrum • Thermal radiation • Light emission and absorption • Blackbody thermometer • Emissivity • Sun’s radiation • Detailed home heating problem 8 Electromagnetic spectrum ; Thermal radiation; Emission; Absorption; Blackbody; Emissivity; Sun’s radiation; Home heating • Light model : an electromagnetic wave 9 Electromagnetic spectrum ; Thermal radiation; Emission; Absorption; Blackbody; Emissivity; Sun’s radiation; Home heating • All electromagnetic waves have the same properties. They are characterized by different frequencies and wavelengths .
4 10 Electromagnetic spectrum ; Thermal radiation; Emission; Absorption; Blackbody; Emissivity; Sun’s radiation; Home heating • Electromagnetic spectrum is a full range of light wavelength • We can only see a narrow range: Visible light.
You've reached the end of your free preview.
Want to read all 11 pages?