1. Chapter 10 Heat Transfer and Change of Phase

2. Conduction
Heat transfer through material by atoms & electrons colliding inside the object being heated
Solids whose atoms or molecules have loosely held electrons are good conductors of heat (ie. Metals)
Poor conductors of heat are called insulators
Wood is a good insulator even when its red hot
Air is a very poor conductor of heat.

3. Conduction cont.
Good insulating properties are largely due to air spaces between molecules
Snow is a poor conductor
Insulation does not stop heat transfer, only slows it down

4. Convection
Heat transfer by motion of a fluid (liquid or gas) by currents
Figure As the fluid is heated the molecules move faster, spread out and become less dense. The denser, cooler fluid migrates to the bottom.

5. Convection currents stir the atmosphere and produce winds

6. Radiation Heat transfer through radiant energy (electromagnetic waves)
For example, from the sun to the earth through mostly empty space
Good absorbers of radiant energy are also good emitters
A dark object that absorbs plenty of radiant energy must emit a lot as well
Figure When the containers are filled with hot (or cold) water, the darker one cools (or warms) faster
Every surface, hot or cold, both absorbs and emits radiant energy.

7. Heat transfer occurs whenever matter changes phase – adding thermal energy changes phase from solids to liquids to gasses to plasma

9. Melting Substance changing from a solid to a liquid
When a solid is heated the heat is absorbed and the molecules vibrate more violently and the attractive forces between the molecules can no longer hold them together
During melting added thermal energy increases the potential energy of the molecules breaking the bonds that hold them together, not the kinetic energy – temp. does not increase

10. Evaporation
A change of phase from liquid to gas at the surface of the liquid
Molecules at the surface gain kinetic energy by being bumped
As water is heated, some molecules gain more energy than others; ones with most energy break free leaving molecules with less energy then dropping the temperature of the liquid
Perspiration to cool our body temperature

11. Vaporization or Boiling
Evaporation within a liquid
Bubbles of vapor form in the liquid and rise to the surface, where they escape
At the boiling point, any added thermal energy is used to increase the potential energy of the molecules, not the kinetic energy. Therefore, the temperature does not change during boiling

12. Sublimation A change of phase from solid to gas
Ex. Snow, ice cubes, etc.

13. Condensation Change of phase from gas to liquid
When gas molecules near the surface of a liquid are attracted to the liquid, they strike the surface with increased kinetic energy and become part of the liquid resulting in a rise in temperature
Steam burns

14. Why do you feel warmer in the moist shower area than outside the shower?

15. Why do they say “its not the heat, it’s the humidity” to compare the climate in Arizona to the climate in Wisconsin?

16. Freezing Liquid changes to solid
Liquid freezes when its molecules vibrate about fixed positions and form a solid

17. Energy needed for changes in phase
Whenever a substance changes phase, a transfer of energy occurs
Heat of fusion – the amount of energy needed to change any substance from solid to liquid (and vice versa)
Heat of vaporization – the amount of energy required to change any substance from liquid to gas (and vice versa)

18. http://mutuslab. cs. uwindsor