1. Heat Transfer  How does the energy move from a hotter to a colder object?  Three mechanisms  Conduction  Convection  Radiation

2.  Conductors are substances that conduct thermal energy very well.  Insulators are substances that do not conduct thermal energy very well.

3. Conduction  Stir your hot soup with a metal spoon  Pretty soon you need a pot holder because the end of the spoon you are holding gets hot  This is heat transfer by conduction  Energy travels up the spoon from the end in the hot soup to the end in your hand

4. Conduction  We sense the movement of energy by the increasing temperature  This means the atoms and molecules have higher average kinetic energy. (increasing temperature.)  Primarily occurs by the movement of electrons in the material  The more easily the electrons can move, the better the conduction

5. Conduction  Metals have some electrons that are very loosely bound to the atoms in the material  These electrons can move easily and can rapidly pick up additional kinetic energy  Metals are good conductors  Wood and plastic don’t have loosely bound electrons, so they are poor conductors

6. Conduction

7.  Air is a poor thermal conductor  If you stand in the sun on a cold winter day and are shielded from the wind, you stay pretty warm  Snow is a poor conductor, while water is better  Makes igloos a useful as a house

8. Convection  A phenomenon in fluids  Instead of having energy moved by successive collisions of electrons, atoms and molecules, the fluid itself is set into motion called a current  These moving fluid currents are convection

9. Convection

10.  When the radiator heats the air, it becomes less dense and rises  Cool air moves in to replace the air that rose  This generates the air flow  So radiators don’t need a fan to stir the air and to distribute heat throughout a room  The rising air cools until its density matches that of the surrounding air

11. Convection  We take advantage of the cooling that occurs during an expansion  We make refrigerators and air conditioners operate by forcing gas under pressure through a small hole and expanding it into an empty space

12. Convection  Explains why breezes come from the ocean in the day and from the land at night

13. Radiation  Energy carried by electromagnetic waves  Study waves later in detail  Light, microwaves, radio waves, x-rays  Wavelength is related to vibration frequency

14. Radiation

15.  Every object is emitting electromagnetic waves regardless of temperature  Things we can see from their own radiation are very hot to have energy emitted in the visible region of the spectrum  Most things emit primarily in the infrared  Night vision goggles, etc.

16. Radiation  Things also absorb radiation  If they didn’t, they would run out of energy to emit  Good emitters are also good absorbers  Equilibrium established between emission and absorption  When something can’t equilibrate, it gets hotter or colder

17. Radiation  Interior of a car on a sunny day  Sunlight comes in as visible light  Seats and interior are much cooler so they radiate in the infrared instead of visible  Glass in the windows blocks infrared so energy can’t get out  Car interior heats up!

18. Radiation  A good absorber reflects very little energy  Think about dark pavement  A poor absorber reflects a lot of energy  Think about snow that doesn’t melt in sunshine even though 1400 watts/meter 2 are hitting it

19. Radiation  At night, objects receive no input energy from the sun  But, they are warmer than outer space, so they continue to radiate energy  Thus, they cool off  Can we make ice in the desert without a refrigerator?

20. Newton’s Law of Cooling  Rate of cooling of an object is proportional to the temperature difference between an object and its surroundings  Works both ways, cooling and heating  Rate of heating also depends on the temperature difference