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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)] - PowerPoint PPT Presentation
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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.
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• c = Q/mT [J/(kg·K)]• heat needed per kg to raise
temperature by 1 degree C or K. • slope warming water = T/Q = 1/(mc)
specific heat
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example c’s
• in J/(kg-C):
• aluminum 920
• copper 390
• ice 2100
• water 4186
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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.
Kkg
J
Kkg
J
Tm
Qc
115
)0.4)(5.0(
230
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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.
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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.
wA QQ
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continued:
wwwAAA TcmTcm
)2025)()(5()25100)()(2( CCckgCCckg wA
wA QQ
))(25())(150( wA cCkgcCkg
Ckg
Jcc wA
6986
4186)(
6
1)(
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• L = Q/m [J/(kg)]
• heat needed per kg to melt (f) or vaporize (v) a substance
latent heat
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example L’s
• in J/kg:
• melting (f) vaporization (v)
• alcohol 100,000 850,000
• water 333,000 2,226,000
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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.
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Heat Transfer
• Conduction
• Convection
• Radiation
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Conduction
• Heat conduction is the transmission of heat through matter.
• dense substances are usually better conductors
• most metals are excellent conductors
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conduction equation
• heat current = energy/time [watts]
• heat current = kAT/L
• k = thermal conductivity
• & T = temperature difference, L below
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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
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Convection
• Convection – transfer through bulk motion of a fluid.
• Natural, e.g. warm air rises, cool falls
• Forced, e.g. water-cooled engine
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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
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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!
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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
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Phase Change
• freeze (liquid to solid)
• melt (solid to liquid)
• evaporate (liquid to gas)
• sublime (solid to gas)
• phase changes occur at constant temperature
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Temperature vs. Heat (ice, water, water vapor)
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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.
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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 ~ T
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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!)
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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