Thermal Energy Transfer

1. Thermal Energy Transfer 5/5/10

2. B1:Can I open the window to let some cool air in? B2: No! Why not, B2? How does heat move? • Temperature = average kinetic energy of all of the molecules of a substance. • Heat transfer is the exchange of kinetic energy between particles. • Energy flows from hot to cold, until equilibrium is reached

3. Conduction • Conduction = The transfer of heat through direct contact with solids • Increase in temperature  excited electrons  molecular collisions  more excited electrons  higher temperature throughout solid • The ability to transfer heat within an object is called thermal conductivity. • Good conductors: gold, silver and copper • Bad conductors: glass, wood and wool • Bad conductors = good insulators. A thermal infrared image of a coffee cup filled with a hot liquid. Notice the rings of color showing heat traveling from the hot liquid through the metal cup. You can see this in the metal spoon as well. This is a good example of conduction.

4. Convection • Convection = Transfer of heat by circulation of liquids and/or gases • Warmer areas of a liquid or gas rise to cooler areas; the cooler liquid or gas takes the place of the warmer areas • Creates a continuous circulation pattern: convection current • "Hot air rises” - convection in our atmosphere. This thermal infrared image shows hot oil boiling in a pan. The oil is transfering heat out of the pan by convection. Notice the hot (yellow) centers of rising hot oil and the cooler outlines of the sinking oil. Image courtesy of K.-P. Möllmann and M. Vollmer, University of Applied Sciences Brandenburg/Germany.

5. Convection & Weather • Associated with low pressure & unstable air • Scale • small convection cells typically produce cumulous clouds • large scale that produces low-pressure areas and hurricanes • Convection in the large weather systems is more a result of air wanting to move from high to low pressure. When this happens, the air converges at the center of the low-pressure system and must rise (since it can't go into the ground).

6. Radiation • Radiation = form of energy transport consisting of electromagnetic waves traveling at the speed of light. • Thermal radiation = Infrared radiation • No mass exchanged; no medium required. A thermal infrared image of the center of our galaxy. This heat from numerous stars and interstellar clouds traveled about 24,000 light years (about 150,000,000,000,000,000 miles!) through space by radiation to reach our infrared telescopes.

7. All objects absorb & emit radiation • Objects emit radiation when high energy electrons in a higher atomic level fall down to lower energy levels. The energy lost is emitted as electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels. • Objects that absorb heat well, often emit heat well. • Objects that reflect heat well, often emit heat poorly. • Generally dull, rough surfaces absorb radiant heat very well, therefore are easily heated by radiation. • Smooth polished surfaces are usually good reflectors that do not hold heat efficiently.

8. The Electromagnetic Spectrum