15-May-15 Physics 1 (Garcia) SJSU Chapter 16 Heat Transfer

15-May-15 Physics 1 (Garcia) SJSU Heat Transfer Heat always flows from high temperature objects to low temperature objects. Heat flow stops when temperatures equal. Various ways by which heat may flow. 98º 32º 75º Heat flows from child and air into the ice cream Heat flows from child into air

15-May-15 Physics 1 (Garcia) SJSU Conduction Conduction is heat flow by direct contact. Some materials are good thermal conductors, others are insulators. 98º 75º 98º 75º Wood is an insulator Tile is a conductor Tile floor feels colder than wood floor

15-May-15 Physics 1 (Garcia) SJSU Demo: Torch the Money Wrap a dollar bill tightly around a copper pipe. Put it into a flame.

15-May-15 Physics 1 (Garcia) SJSU Air is a Poor Conductor Can safely put your hand in an oven. Metal is good conductor so you need oven mitt to touch it safely (cloth is a poor conductor). Because air is such a poor conductor, some pizza ovens don’t have a door.

15-May-15 Physics 1 (Garcia) SJSU Demo: Boiling Ice Water Water and glass are relatively poor conductors of heat. Can boil water at the top of a test tube with ice at the bottom of the tube. Steel wool prevents ice from floating

15-May-15 Physics 1 (Garcia) SJSU Convection Heat transfer in a fluid often occurs mostly by convection. Buoyancy causes warm air to rise, which carries thermal energy directly by its motion.

15-May-15 Physics 1 (Garcia) SJSU Demo: Candle Very HOT Warm Shadows reveal rising air currents of hot air. Rising hot air above a candle carries most of the heat generated by the burning flame.

15-May-15 Physics 1 (Garcia) SJSU Demo: Convection & Buoyancy Candle goes out while in free fall because in weightless state hot air does not rise so fresh air cannot reach the candle’s wick.

15-May-15 Physics 1 (Garcia) SJSU Convection Oven Convection oven has a fan to enhance the circulation of the air, increasing the transfer of heat.

15-May-15 Physics 1 (Garcia) SJSU Demo: Candle in a Tube Candle stays light until the partition in the tube is removed. Why?

15-May-15 Physics 1 (Garcia) SJSU Fiberglass Insulation Air is a poor thermal conductor but easily transfers heat by convection. Fiberglass insulation is mostly air, with the fibers disrupting the convection flow.

15-May-15 Physics 1 (Garcia) SJSU Radiation Light has many different wavelengths, most of which are not visible to the eye. All light carries energy, thus transfers heat. Heat Lamp

15-May-15 Physics 1 (Garcia) SJSU Emission of Radiant Energy All objects radiate light; higher the temperature the higher the frequency. At room temperature the radiated light is at frequencies too low for our eyes to see. Special cameras are sensitive to this infrared radiation. Attics in this house were kept warm for growing marijuana º 75º

15-May-15 Physics 1 (Garcia) SJSU Reflection of Radiant Energy White objects reflect light, black objects don’t. Hole in a box with white interior looks black because almost none of the light entering the hole reflects back out. White tubes look black inside.

15-May-15 Physics 1 (Garcia) SJSU Controlling Heat Transfer Thermos bottle eliminates conduction and convection by having double- walled sides with vacuum. Silvered interior walls minimize heat transfer by radiation.

15-May-15 Physics 1 (Garcia) SJSU Greenhouse Effect Glass is transparent to sunlight (short-wavelength). Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat.

15-May-15 Physics 1 (Garcia) SJSU Earth’s Greenhouse Effect Earth’s atmosphere acts as a greenhouse, trapping solar energy. Most of the trapping is due to carbon dioxide and water vapor, which is why they’re called “greenhouse gasses.”

15-May-15 Physics 1 (Garcia) SJSU Check Yourself What does it mean to say that the greenhouse effect is like a one-way valve? Can this build up energy forever?

15-May-15 Physics 1 (Garcia) SJSU Global Temperature Variations Temperatures increased from 1910 to Temperatures then cooled for 40 years until they started rising again in the 1980’s.

15-May-15 Physics 1 (Garcia) SJSU Greenhouse Carbon Dioxide Over past 1000 years temperatures nearly constant until CO2 emissions increased starting with the industrial revolution. Industrial revolution begins

15-May-15 Physics 1 (Garcia) SJSU Cars & Carbon Dioxide One gallon of gasoline has about 5.2 lb of carbon. A 5-lb bag of charcoal (which is pure carbon) holds about 100 briquettes. At 26 miles per gallon, that’s 0.2 lb of carbon per mile, or about four to five charcoal briquettes per mile.

15-May-15 Physics 1 (Garcia) SJSU Anthropogenic * Global Warming Rising temperatures are due to human production of greenhouse gases. * Caused by humans Temperature

15-May-15 Physics 1 (Garcia) SJSU Consequences of Global Warming Weather modifications Species extinctions Melting of Polar Ice Caps

15-May-15 Physics 1 (Garcia) SJSU Solutions to Global Warming Many simple, small changes together could make a significant difference. Efficient Appliances Building Insulation Efficient Lighting