1. Lecture 5 Heat Transfer –Conduction –Convection –Radiation Phase Changes

2. Heat Transfer The 3 mechanisms of heat transfer:

3. Conduction Conduction: transfer of heat resulting from collisions between particles. –Heat travels between two points at different temperatures. Metals = good conductors of heat. –Electrons in metals are highly mobile and transfer energy quickly.

4. Conduction Insulators: poor conductors of heat (glass, wood, air, plastic foam, wool). Hot coals are a poor conductor of heat.

5. Conduction Insulators do not prevent heat flow, they only decrease rate of flow. –Electrons are not mobile – energy transfer very gradual. –Air spaces in many insulators – air is a poor conductor. –Example: snow is a good insulator because of air- spaces.

6. Convection Convection: transfer of heat by the motion of fluid as it rises and sinks. Occurs only in fluids (liquids and gases).

7. Radiant Energy Radiant Energy: a form of energy that does not require a medium (matter) to travel through. Electromagnetic Waves: a wide spectrum of radiant energy which includes visible light. –Travels through a vacuum at 300,000 km/s (186,000 miles/s).

8. Radiant Energy Dark materials are good absorbers of radiant energy.

9. Terms to be familiar with: Thermal Energy: energy matter has due to the vibration of atoms and molecules. –Sum of the kinetic energies of all particles in a sample. Temperature: quantity that indicates how warm or cold an object is relative to some standard. –Related to average kinetic energy per particle.

10. A simulation of the motions of helium gas atoms – slowed down two trillion times from room temperature.

11. Thermal Energy Heat: the thermal energy transferred from one thing to another due to temperature difference. Second Law of Thermodynamics: heat never spontaneously flows from a cold substance to a hot substance. –Energy has a tendency to become more dispersed – greater entropy or disorder.

12. Temperature Celsius Scale – most common temperature scale used worldwide. 0°C – melting/freezing point of water. 100°C – boiling point of water.

13. Absolute Zero: lower limit of temperature at which molecules have lost all kinetic energy = - 273°C The volume of a gas changes by 1/273 of its volume at 0°C with each 1°C change in temperature.

14. Temperature Kelvin Temperature Scale: –Absolute Zero = 0 K K = °C + 273 0°C = 273 K 100°C = 373 K - 273°C = 0 K

16. Particle collisions within CERN’s Large Hadron Collider have resulted in temperatures exceeding 5 Trillion K.

17. Thermal Energy Thermal Expansion: as temperature increases, molecules move more rapidly and farther apart. –Most materials expand when heated and contract when cooled.

18. Expansion joints in a bridge allow for the structure to expand and contract without buckling.

21. Different substances expand at different rates.

23. Three Phases of Water at Earth’s Surface Liquid Solid (Ice) Gas (Vapor) Energy is captured or released upon change from one phase to another

24. Molecular Structure of Ice

26. Liquid Water H 2 O (l) Water

27. Gas (Steam) H 2 O (g) Steam

28. Water Vapor (Steam) Condenses into liquid droplets

30. Energy and Change of Phase Ice Ice + Water Water Water + Vapor Vapor Heat of Vaporization Heat of Fusion

31. Energy and Phase Changes Only one or the other can occur - not both at the same time: –Change in temperature –Change in phase