Published on April 7, 2008
Heat Transfer: Heat Transfer Chapter 22 Heat Transfer: Heat Transfer Always from warmer to cooler objects Three methods: Conduction Convection Radiation Conduction: Conduction Occurs within the same material and between different materials that are in thermal contact. Heat up one end of a metal rod and the other end eventually gets hotter. Conduction: Conduction When two objects of different temperatures touch, heat is transferred between the two objects by conduction. Conduction: Conduction Caused by the collisions between atoms and molecules and the actions of loosely bound electrons. These collisions transfer energy to other atoms and molecules in the material. Heat Conductors: Heat Conductors Materials that conduct heat well are know as heat conductors. Metals are the best conductors. Insulators: Insulators Materials that do not conduct heat well are know as insulators. Wood, wool, straw, paper, cork, and Styrofoam are good insulators. Insulators: Insulators Liquids and gases, in general, are very good insulators. Air is a very good insulator. Porous materials with many small air spaces are good insulators. (Fur, fluffing feathers, fiberglass insulation, snow) Insulators: Insulators Insulate home to keep heat in; not to keep cold out. Does not prevent heat from getting through it. Reduces the rate at which heat penetrates. Question: Question When you step out of a shower, you place one foot on a rug and the other foot on a tile floor. Which is the better conductor and which is the better insulator? Answer: Answer Tile is the better conductor: the heat readily flows from your foot into the tile. Rug is the better insulator: the heat does not easily flow from your foot to the rug (your foot generally stays warm). Question : Question Why can you stick your hand into a hot oven for several seconds without harm, whereas you’d never touch the metal surfaces inside of the oven for even a second? Answer: Answer Air is a poor conductor, so the rate of heat flow from the hot air to your relatively cool hand is low. When you touch the metal parts, metal is a good conductor of heat and very much heat is conducted to your hand in a short time. Slide14: Heat Conduction Slide15: Table of Thermal Conductivities Slide16: Convection: Convection When heat transfer occurs by currents in a fluid. Convection occurs in all fluids. Remember that a fluid is anything that flows, including liquid or gas. Air in contact with a hot stove rises and warms the area above. Convection: Convection When the fluid is heated, it expands, becomes less dense, and rises. Cooler fluid then moves to the bottom and the process continues. Slide20: Natural Convection Wind: Wind Convection currents stirring the atmosphere help produce winds. Shore Ocean Breeze - day Land Breeze - night Slide22: Moderate Temperatures Near Large Bodies of Water Why Rising Air Cools: Why Rising Air Cools Rising warm air expands because atmospheric pressure is lower at higher altitudes. When air expands air molecules are receding. Collisions involving receding air molecules result in lower rebound speeds. Lower rebound speeds result in lower kinetic energy – lower temperature. Heat Radiation: Heat Radiation How the Sun’s heat is transmitted to the Earth. Radiant energy - energy transmitted by electromagnetic waves (a fancy word for all kinds of light: visible, infrared, UV…). Electromagnetic Waves: Electromagnetic Waves Radio – longest wavelength Microwaves Infrared radiation Visible Light Ultraviolet Radiation X-rays Gamma rays – shortest wavelength Slide26: Energy High energy Low energy Electromagnetic Waves: Electromagnetic Waves All objects continually emit radiant energy in a mixture of wavelengths. Heat radiant energy is transmitted as infrared radiation (“night vision” goggles!). Heat Radiant Energy: Heat Radiant Energy Lower temperature – longer wavelength in infrared Higher temperature – shorter wavelength in infrared Objects at even higher temperatures also emit visible light Heat Radiant Energy: Heat Radiant Energy 500 ºC – red hot – red glow Higher Temperature – yellow glow (the Sun!) 1200 ºC – white hot – white glow (stars hotter than our Sun) Absorption: Absorption Absorption and Reflection are opposite processes. Good absorber reflects very little radiant energy including light – appears black. Perfect absorber reflects no radiant energy. Reflectors: Reflectors Good reflectors are poor absorbers. Light colored objects reflect more light and heat than dark colored objects. Emission of Radiant Energy: Emission of Radiant Energy Good absorbers of radiant energy are also good emitters. Poor absorbers of radiant energy are also poor emitters. Absorber or Emitter of Radiant Energy: Absorber or Emitter of Radiant Energy Whether an object is an emitter or absorber of radiant energy depends on its temperature and its surroundings. Hotter – emitter Colder – absorber Newton’s Law of Cooling: Newton’s Law of Cooling The rate of cooling of an object whether by conduction, convection, or radiation is approximately proportional to the temperature difference (∆T) between the object and its surroundings. Greenhouse Effect: Greenhouse Effect Short wavelength radiation from the Sun is transmitted through the glass. Longer wavelength reradiated energy is not transmitted out through the glass and is trapped inside the greenhouse. Greenhouse Effect and Cars: Greenhouse Effect and Cars Hot Sun radiates short wavelength waves that easily pass through air and glass. Car’s interior absorbs much of the Sun’s radiant energy. Greenhouse Effect and Cars: Greenhouse Effect and Cars Car’s interior is at a lower temperature and emits longer wavelength waves that can’t pass through glass. The radiant energy stays inside the car, which makes the car’s interior even hotter. Greenhouse Effect: Greenhouse Effect Helps keep the Earth’s temperature at a level to support life. Without it Earth would be at a temperature of -18ºC. Over the last 500,000 years the average temp has fluctuated between 19ºC and 27ºC. Greenhouse Effect: Greenhouse Effect Remember Venus!