1 / 15

Ch. 22

Ch. 22. Heat Transfer. Heat & Conduction.

stamos
Télécharger la présentation

Ch. 22

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Ch. 22 Heat Transfer

  2. Heat & Conduction • Heat is always transferred from warmer objects to cooler objects. Cold is merely the absence of heat. When objects of different temperatures are near each other, those that are warm become cooler and those cooler become warmer. This will occur until all of the objects are the same temperature. • This can be done in 3 ways; conduction, convection & radiation.

  3. Heat & Conduction • Conduction – Heat one end of a metal & it will eventually become too hot to hold, even at the other end. Heat has been transferred by conduction. Materials that conduct heat well are conductors (e.g. metals): Ag > Cu > Al > Fe. • Atoms at the heated end vibrate & send “loose” electrons into neighboring atoms. This causes a transfer of energy (heat).

  4. Insulators • Touch a piece of wood & a piece of metal. The metal feels colder because it is transferring heat from your hand. Wood is poorer at doing this & is known as an insulator. Other examples are wool, straw, cork & styrofoam among others. • Liquids & gases are generally good insulators (e.g. air). • Even snow is a good insulator. It prevents heat from escaping too rapidly (think igloos).

  5. Convection • Movement of a hotter substance to another location involves convection. Convection is where heating occurs by currents in a fluid (e.g. the hotter air rising in an oven). The heated air is less dense & rises (floats) to the top.

  6. Convection • Wind – Convection currents stirring the atmosphere produce winds. This is due to uneven absorption of heat from the sun for various types of surfaces (e.g. ocean vs. beach). • Expanding air cools. Compressed air warms. In expanding air, KE is reduced & cooling occurs.

  7. Radiation • Radiation is how the sun’s energy gets transferred to earth. • Radiant energy – any energy, including heat, that is transmitted by radiation. They come in the form of electromagnetic waves: e.g. radio waves, microwaves, IR radiation, visible light, UV radiation, X-rays & gamma rays.

  8. Radiation • Radio waves have the longest wavelengths with gamma waves having the shortest. • Low temperature objects emit long waves whereas high temp. objects emit waves of shorter length.

  9. Absorption of Radiant Energy • A good absorber of radiant energy reflects very little of it back. We often see good absorbers appearing black (i.e. little or no reflection occurs). E.g. the pupils of our eyeballs absorb light, allowing us to see. These appear black.

  10. Absorption of Radiant Energy • Good reflectors are poor absorbers of light & heat. Lighter colored clothing is worn during the summer to keep people cooler.

  11. Emission of Radiant Energy • Good absorbers of radiant energy are also good emitters. Poor absorbers are poor emitters. For example, the earth is a net absorber during the day & an emitter at night.

  12. Newton’s Law of Cooling • Rate of cooling ~ change in T; this is known as Newton’s Law of Cooling. • But… cooling rate & cooling time are not the same. • E.g. Hot cup of tea vs. warm cup of tea; It takes extra time for the hot cup of tea to cool to the same temp. as the warm cup of tea.

  13. Global Warming & the Greenhouse Effect • Greenhouse effect – e.g. a car in hot summer with all of its windows rolled up. • High temp. objects radiate at short wavelengths. Low temp. objects radiate at long wavelengths. • Transparency of materials is important (e.g. glass & air). Glass is transparent to visible light, but opaque to IR light.

  14. Global Warming & the Greenhouse Effect • So the sun’s rays enter a car as short wavelengths & easily penetrate the windows of the car. Heat is absorbed, but any attempt to reradiate occurs at longer wavelengths. These bounce off the “opaque” windows & energy is retained in the interior of the car.

  15. Global Warming & the Greenhouse Effect • The earth is similar. It is transparent to solar radiation. Earth’s surface absorbs this energy & reradiates at longer wavelengths (called terrestrialradiation). Carbon dioxide & water tend to absorb & reradiate this energy. • Burning fossil fuels can increase earth’s temp. & the overall greenhouseeffect.

More Related