10.4 Controlling Heat

1. 10.4 Controlling Heat pp. 383 - 385 Mr. Richter

2. Agenda • Warm Up • Review HW • Business: • Science Fair Posters • What if there’s a snow day? • Introduction to Heat Transfer • Notes: • Conduction • Conductors and Insulators • Sweating • Convection • Radiation

3. Objectives: We Will Be Able To… • Explain how heat is transferred through conduction, convection and radiation. • Explain and identify conductors and insulators.

4. “Warm”-Up: • When it is a cold night, sometimes we use an extra blanket on the bed. Why does this make us feel warmer? Does our body just turn up the thermostat? What happens? • Discuss at your table. Then write a 2-3 sentence explanation in your notes about what you think is happening.

5. Conduction

6. Conduction • Heat transfer by conduction is the transfer of thermal energy by direct contact between particles of matter. • Basically, by touching atoms together. • Warming your hands on a hot coffee mug. • Touching a cold car door handle. • Energetic molecules excite their neighbors, who excite their neighbors, and so on… • It’s like spreading the news of a party.

7. Conductors and Insulators • Different materials allow the flow of thermal energy better than others. • Most materials can be divided up into conductors and insulators.

8. Conductors • Thermal conductors are materials that allow atoms and molecules to easily transmit their kinetic energy to other atoms and molecules. • Allow thermal energy to flow easily. • These tend to be flexible solids like metals: • gold, silver, copper • atoms packed closely together • can bend and vibrate easily • Good party people!

9. Insulators • Thermal insulators do not allow atoms and molecules to easily transmit vibrations. • Inhibit (prevents or partially prevents) the flow of thermal energy. • They tend to be either • very rigid solids • no flexibility, no vibrations • hard rubber, wood, concrete • gases • molecules are too far apart to transmit vibrations • air, helium, etc. • vacuums (no atoms at all)

10. Air: The Perfect(ish) Insulation • Humans use air as an insulator because it’s cheap and easy to use. All you have to do is trap it. • Puffy jackets and down comforters keep air trapped in the layers, making it difficult for heat to pass. • Thermoses keep a layer of air between the inner and outer layers, trapping the heat (or cold) inside.

11. Sweatingand Evaporation • When we sweat and produce moisture, we obviously do not boil off the water we exude. Sweating ≠ boiling! • The average temperature of each particle of water is the same as the temperature on our skin (because of conduction!), but: • Some particles have a much higher level of energy. High enough to fly away! • The remaining particles have a lower average kinetic energy than they did before, cooling us off. Neat!

12. Convection

13. Convection • Heat transfer by convection is the transfer of thermal energy through a fluid (liquid or gas). • Convection happens when warm fluids switch places with colder fluids somewhere else.

14. Convection • There are two types of convection: natural and forced. • Natural convection occurs when the hotter fluid expands and becomes less dense than the colder fluid, allowing them to change places. • Forced convection occurs when a hotter fluid is forced (pumped, fanned, etc.) to a colder location.

15. Radiation

16. Radiation • Thermal radiation is the heat transfer in the form of electromagnetic waves, including light. • x-rays, microwaves, the sun, etc. • Thermal radiation is the only form of heat transfer that can travel through a vacuum. • No molecules need to touch or change places.

17. Radiation • All objects emit (give off) and absorb thermal radiation. • Because they have moving electrons. • The more reflective the object (lighter color, shinier), the less radiation it will absorb. • The less reflective (darker) the object, the more thermal radiation it will absorb. • This is why you feel warmer in dark clothes on a sunny day.

18. Wrap-Up: Did we meet our objectives? • Explain how heat is transferred through conduction, convection and radiation. • Explain and identify conductors and insulators.

19. Homework • p. 389 #36 - 39