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11 Heat

11 Heat. Homework: 1, 3, 4, 5, 6, 9, 11, 21, 23, 54, 63, 64. Heat. Heat is energy transferred due to temperature difference. Symbol, Q [J] Ex. 4186J heat needed to raise 1kg of water one degree C. specific heat. c = Q/m D T [J/(kg·K)]

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11 Heat

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  1. 11 Heat • Homework: • 1, 3, 4, 5, 6, 9, 11, 21, 23, 54, 63, 64.

  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. 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)

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

  5. 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.

  6. 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.

  7. 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.

  8. continued:

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

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

  11. 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.

  12. Heat Transfer • Conduction • Convection • Radiation

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

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

  15. 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

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

  17. 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

  18. 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!

  19. 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

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

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

  22. 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.

  23. 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

  24. 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!)

  25. 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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