10-11 Temperature and Heat

1. 10-11 Temperature and Heat • Temperature & Scales • Thermometry • Thermal Expansion • Heat and Internal Energy • Heat Transfer • Heat and Temperature Change, Specific Heat Capacity

2. Homework • Ch. 10 • 13, 20, 24, 45, 50, 58, 59. • Ch.11 • 1, 3, 4, 5, 6, 9, 11, 21, 23, 54, 63, 64.

3. Temperature • A measure of the average kinetic energy per molecule of a substance. • T increases with average speed of molecules, e.g. hammered metal has increased temperature. • Higher T gas expands or has increased pressure

4. Thermometric property: a physical property that changes with temperature. Examples: pressure or volume of a gas resistance of a metal length of a piece of metal

5. Common Temperature Scales • Farenheit, water freezes at 32F • Celsius, water freezes at 0C • Kelvin, water freezes at 273K • Size of unit: 1K = 1C = (9/5)F

6. Thermal Expansion • Most materials increase in size when their temperature increases due to the increased thermal motion. • basis of much thermometry

7. Water Expansion • Water expands from 4°Cto 100°C. • However, water contracts when warmed from 0°C to 4°C (transient ice disappears as T increases to 4°C)

8. Ideal Gas • PV ~ T • Ex: V is constant: P ~ T, if kelvin temperature is doubled, pressure of gas is doubled. • Ex: P is constant: V ~ T.

9. Linear Thermal Expansion • Length of a solid depends on temperature • Frequently the change in length is proportional to the change in temperature • The constant is called the “coefficient of linear thermal expansion” (symbol a)

10. Linear Thermal Expansion DL = a LoDT Example: 100C increase in Aluminum causes a fractional increase in length of 0.0024 = 0.24% change.

11. Bi-Metallic Strips

12. 11

13. Heat • Heat is energy transferred due to temperature difference. • Symbol, Q [J] • Ex. 4186J heat needed to raise 1kg of water one degree C.

14. specific heat • c = Q/mDT [J/(kg·K)] • heat needed per kg to raise temperature by 1 degree C or K. • slope warming water = DT/Q = 1/(mc)

15. example c’s • in J/(kg-C): • aluminum 920 • copper 390 • ice 2100 • water 4186

16. Example: • A student wants to check “c” for an unknown substance. She adds 230J of heat to 0.50kg of the substance. The temperature rises 4.0K.

17. Calorimetry • literally: ‘meter’ the calories emitted by a substance as it cools. • Ex. Heated object is added to water. change in temperature of water determines specific heat of object.

18. Example Calorimetry • 2kg of “substance-A” heated to 100C. Placed in 5kg of water at 20C. After five minutes the water temp. is 25C. • heat lost by substance = heat gained water.

19. continued:

20. latent heat • L = Q/m [J/(kg)] • heat needed per kg to melt (f) or vaporize (v) a substance

21. example L’s • in J/kg: • melting (f) vaporization (v) • alcohol 100,000 850,000 • water 333,000 2,226,000

22. Example: • How much heat must be added to 0.5kg of ice at 0C to melt it? • Q = mL = (0.5kg)(333,000J/kg) • = 167,000J • same amount of heat must be removed from 0.5kg water at 0C to freeze it.

23. Heat Transfer • Conduction • Convection • Radiation

24. Conduction • Heat conduction is the transmission of heat through matter. • dense substances are usually better conductors • most metals are excellent conductors

25. conduction equation • heat current = energy/time [watts] • heat current = kADT/L • k = thermal conductivity • & DT = temperature difference, L below

26. conduction example • some conductivities in J/(m-s-C): • silver 429 copper 401 aluminum 240 • Ex: Water in aluminum pot. bottom = 101C, inside = 100C, thickness = 3mm, area = 280sq.cm. • Q/t = kA(Th-Tc)/L • = (240)(0.028)(101-100)/(0.003) • = 2,240 watts heat current

27. Convection • Convection – transfer through bulk motion of a fluid. • Natural, e.g. warm air rises, cool falls • Forced, e.g. water-cooled engine

28. Radiation • Heat transfer by emission of electromagnetic radiation, e.g. infrared. • Examples: • space heaters with the shiny reflector use radiation to heat. • If they add a fan, they use both radiation and convection

29. Greenhouse Effect • ‘dirtier’ air must be at higher temperature to radiate out as much as Earth receives • higher temperature air is associated with higher surface temperatures, thus the term ‘global warming’ • very complicated model!

30. Summary • T measured in C, K, F. Use K for gas laws. • thermometry uses thermometric properties • change in length is proportional to change in temperature for many solids • c: heat needed to raise 1kg by 1C. • L: heat needed to melt or vaporize 1kg. • Heat transfer

31. Phase Change • freeze (liquid to solid) • melt (solid to liquid) • evaporate (liquid to gas) • sublime (solid to gas) • phase changes occur at constant temperature

32. Temperature vs. Heat (ice, water, water vapor)

33. Heat and Phase Change • Latent Heat of Fusion – heat supplied to melt or the heat removed to freeze • Latent Heat of Vaporization – heat supplied to vaporize or heat removed to liquify.

34. Newton’s Law of Cooling • For a body cooling in a draft (i.e., by forced convection), the rate of heat loss is proportional to the difference in temperatures between the body and its surroundings • rate of heat-loss ~ DT

35. Real Greenhouse • covering allows sunlight to enter, which warms the ground and air inside the greenhouse. • the ‘house’ is mostly enclosed so the warm air cannot leave, thus keeping the greenhouse warm (a car in the sun does this very effectively!)

36. Solar Power Solar Constant • Describes the Solar Radiation that falls on an area above the atmosphere = 1.37 kW / m².In space, solar radiation is practically constant; on earth it varies with the time of day and year as well as with the latitude and weather. The maximum value on earth is between 0.8 and 1.0 kW / m². • see: solarserver.de