Chapter 16 Heat Transfer

Transcription

1 Chapter 16 Heat Transfer Conduction Convection Radiation John Suchocki demonstrates the low conductivity of red-hot coals with his bare feet. 1

2 Three different ways of heat transmission Heat transfers from warmer to cooler things. If several objects with different temperatures are in contact, those that are warm become cooler and those that are cool become warmer. They tend to reach a common temperature. This equalizing of temperature occurs in three ways: Conduction Convection Radiation 2

3 Conduction The tile floor feels colder than the wooden floor, even though both floor materials are the same temperature. This is because title is a better conductor of heat than wood, and so heat is more readily conducted out of the foot touching the tile. Hold one end of an iron nail in a flame. It will quickly become too hot to hold. The heat enters the metal nail at the end that is kept in the flame and the heat is transmitted along the nail s whole length. The transmission of heat in this manner is called conduction. Heat conduction occurs by electron and atomic collisions. How well a solid object conducts heat depends on the bonding within its atomic or molecular structure. Good heat conductors: metals Poor heat conductors (insulators): wool, wood, straw, paper and most liquids and gases, e.g. air 3

4 Example: wood is a good insulator CAUTION: Don t try this on your own; even experienced firewalkers sometimes receive bad burns when conditions aren t just right. Because wood is a good insulator (poor conductor) it is used for handles of cookware. Wood is a good insulator even when it is red hot. The principal factor in firewalking is the low conductivity of wood - even red-hot wood. Although its temperature is high, relatively little heat is conducted to the feet. Similarly, air is a very poor heat conductor. This is why little heat is conducted by air when you put your hand briefly in a hot pizza oven. 4

5 Example: insulation slows the transfer of heat Heat is transmitted from a higher to a lower temperature. Insulation slows the transfer of heat. In desert regions that are hot in the daytime and cold at nighttime, the walls of houses are often made of mud. Why is it important that the mud walls be thick? To increase insulation. Keeps the house warm at night by slowing the flow of heat from inside to outside. Keeps the house cool at daytime by slowing the flow of heat from outside to inside. 5

6 Convection (a) Convection currents in air. (b) Convection currents in liquid. Liquids and gases (fluids) transmit heat mainly by convection. Convection is heat transfer due to the actual motion of the fluid itself. Difference between convection and conduction In conduction, heat is transferred by successive collisions of electrons and atoms. Convection involves the motion of mass-the overall motion of a fluid. 6

7 Radiation Energy from the sun passes through space and the Earth s atmosphere by radiation. Radiation: electromagnetic radiation, including visible light. Radiant energy: The energy radiated is called radiant energy. Radiant energy is in the form of electromagnetic waves. Wavelength: the types of radiant energy are listed in order of wavelength, from longest to shortest. Spectrum of visible light An infrared image of a boy and his dog 7

8 Let s do some examples A good heat conductor is a A) Poor insulator. B) Good insulator. C) Neither a poor nor a good insulator. You feel warmer on a rug than on a tile floor because the rug A) Is usually warmer than the tile. B) Is a better insulator than tile. C) For the same mass has more internal energy than tile. D) All of these. E) None of these. 8

9 Homework 8 (03/08/04) Chapter 16, P. 321, Exercises: 4, 5, 13. 9