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

2. 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

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

4. 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

5. Conduction

6. Conduction 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

7. 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

8. Convection

9. Convection
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

10. 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

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

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

13. Radiation

14. Radiation
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.

15. 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

16. 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!

17. 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/meter2 are hitting it

18. 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?

19. 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