New Symbol

1. New Symbol • b – “beta” • How “expandable” a material is. • Depends on the material. Look it up in the book, or wikipedia. • b = 3a (usually)

2. Volume Expansion

3. Problem • b for aluminum is .0000643 • A block of Al is 1m by 2m by 3m at 20 C. It is heated to 80 C. What is the new volume?

4. Applications of Thermal Expansion – Bimetallic Strip • Thermostats • Use a bimetallic strip • Two metals expand differently • Since they have different coefficients of expansion

5. Question (New subject) • If two objects (touching) are not the same temperature, what will happen?

6. Heat Transfer • Heat flows naturally from high temperature to low temperature. • There are 3 ways this happens. • KNOW THE 3 WAYS!!! of heat transfer.

7. New Symbol • HEAT: Q • Q is for “heat” • Thermal energy. Joules. • NOT THE SAME AS TEMPERATURE. • Which has more heat: • 1.) A cup of very hot coffee • 2.) An iceberg

8. New Symbol • Which has more heat: • 2.) An iceberg • The BIG iceberg has more heat even though it has a lower temperature.

9. New Symbol • Teacher example: • 2 parking lots. • 1 with 10 Fast cars, 10 gal gas each. • 1 with 1000 slow cars, 2 gal gas each. • Who has higher speed or temp, • Who has higher energy or heat?

10. Heat Flow • Heat Flow: Q/t • Q/t is for “heat flow” • Movement of Thermal Energy. In Joules/Second or Watts.

11. Conduction • Objects touching allows transfer of heat energy through collisions of the molecules. • Faster moving molecules beat on the slower moving ones and speed them up. • Heat flows from a warm object to a cool object that is in direct contact with it.

12. CONDUCTION • = heat transfer by molecules vibrating and colliding. • Matter does not move places, but vibrates around a fixed position.

13. Conduction example • Molecules vibrate in the heating element. • Molecules in the coil vibrate more than molecules in the pot. • They collide.

14. Conduction example • Molecules vibrate. • Molecules in the coil vibrate more than molecules in the pot. • They collide. • Molecules in the bottom of the pot begin vibrating more. • They collide with other molecules in the pot.

15. Conduction example • Molecules vibrate. • Molecules in the coil vibrate more than molecules in the pot. • They collide. • Molecules in the pot begin vibrating more. • They collide with other molecules in the pot. • Eventually, everything is vibrating the same amount. • The temp. has equalized. All heat is transferred by vibrating molecules hitting each other.

16. Conductors • What are some good conductors?

17. Conductors • What are some good conductors? • Metals. (Note: Good heat conductors usually are also good electrical conductors. I wonder if the flow of electrons have anything to do with heat flow?) • What are some bad conductors?

18. Conductors • What are some good conductors? • Metals. • What are some bad conductors? • (Also called insulators) • Air, wood, glass, rubber. • Many insulators work by trapping pockets of stationary air. • Nothing is a bad conductor, LOL (means vacuums)

19. What will make heat flow faster?

20. What will make heat flow faster? • Higher Temp difference. • Better conductor. • Wider area to go through. • What will make it go slower?

21. What will make heat flow faster? • Higher Temp difference. • Better conductor. • Wider area to go through. • What will make it go slower? • Longer distance to travel.

22. Heat Flow Equation • Q/t = kAΔT/L • Q = Heat t= time Q/t = Heat Flow • k = conductivity (unique for each material) in Joules/(sec*meter*Celsius) • A = cross sectional area • ΔT = temp difference between areas. • L = distance heat needs to travel. • Note: A and T on top = faster • L on bottom = slower

23. Copper has a conductivity of 390 J/(s*m*C) Problem • A copper wire 18 cm long, and 1 mm in diameter passes through the wall of house. • It is 29 Celsius indoors,-17 outdoors. • How much heat per second will pass through the wire? • Q/t = kAΔT/L

24. Convection • 2nd method of heat transfer. • Heat is transferred by moving matter. Matter actually moves to a new location. • Usually a liquid or a gas. • Think of weather clouds moving and bringing in “coldness”.

25. Convection • Start with a heat source. • Air near the source heats up. • It expands, causing it to rise.

26. Convection • The hot air rises and creates a vacuum. • Cool air gets drawn in to take it’s place. • After hot air rises, it cools down and starts to sink. • So the air moves up and down, causing heat transfer. • (Sounds like a lava lamp.)

27. Convection • By that time, the air that took its place has heated up and is rising. • We get a cycle. • Just like on the lava lamp.

28. Convection Lawrence Kansas City 10 cooler 10 deg hotter farm land buildings and concrete

29. Convection

30. Demo now. • Use a gravity mat to demonstrate convection and weight of air.

31. Convection • What are some examples of convection?

32. Convection • What are some examples of convection?

33. Convection • What are some examples of convection? • Boiling water • The earth’s mantle • Heating and air conditioning systems.

34. Radiation • Last type of heat transfer. • Transfer of heat through a vacuum by electromagnetic energy motion (energy moving as radiation). • Electromagnetic waves

35. Radiation • Warm material gives off light. • Cool material absorbs light. • Energy flows from hot material to cool material.

36. Example • Guess a famous example of a radiation source.

37. Example • We get heat from the sun. • Even though there’s no air in space to conduct or convect it to us.

38. Radiation Equation • Q/t = AeσT4 • A = surface area of object • e = “emissivity of the material”a “table value” between 0 & 1 • σ = The “Stephan-Boltzmann” Constant • σ = 5.67  10-8 W/m2.K4. • T = Temp. (must be in Kelvin)

39. A common Radiator. • A lightbulb gives off 60 Watts of heat. It is made of Tungsten (e = .15). If it is 2700 K, what is the surface area of the filament?