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14 Heat

14 Heat. Homework: Problems: 3, 5, 13, 21, 33, 47, 49. Internal Energy Heat Capacity & Specific Heat Phase Transitions Thermal Conduction. Heat. Heat is energy transferred due to temperature difference. Symbol, Q [J] Ex. 4186J heat needed to raise 1kg of water one degree C.

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14 Heat

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  1. 14 Heat • Homework: • Problems: 3, 5, 13, 21, 33, 47, 49. • Internal Energy • Heat Capacity & Specific Heat • Phase Transitions • Thermal Conduction

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

  3. example c’s • in J/(kg·°C) • aluminum 920 • copper 390 • ice 2100 • water 4186

  4. specific heat • c = Q/mDT [J/(kg·K)] • heat to raise 1kg by 1 degree °C or K. • slope warming curve = DT/Q = 1/(mc) • Q = mcDT

  5. Calorimetry • Measure heat lost/gained:

  6. 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.

  7. continued:

  8. Phase Transitions: Latent Heat • L = Q/m [J/(kg)] • heat needed to melt (f) or vaporize (v) 1kg

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

  10. 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.

  11. Heat Transfer • Conduction • Convection • Radiation

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

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

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

  15. Heat transfer • 2m x 1m window, 4mm thick, single pane glass. • Assume temp. difference = 5°C • Q/t = kA(DT)/L = (0.84)(2)(5)/0.004 • About 2,000 watts

  16. R-Factors and Thermal Resistance

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

  18. Radiation • Heat transfer by 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

  19. Summary • Definition of Internal Energy • Heat Capacity • Specific Heat • Phase Transitions • Latent Heat • Phase Diagrams • Energy Transport by Conduction, Convection, and Radiation

  20. 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.

  21. 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!

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

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

  24. 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.

  25. 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

  26. 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!)

  27. 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

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