Transcript Ch. 11 Heat

11 Heat
• Homework:
• 1, 3, 4, 5, 6, 9, 11, 21, 23, 54, 63, 64.
1
Heat
• Heat is energy transferred due
to temperature difference.
• Symbol, Q [J]
• Ex. 4186J heat needed to raise 1kg of
water one degree C.
2
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)
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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.
Q
230 J
J
c

 115
mDT (0.5kg)( 4.0 K )
kg  K
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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.
QA  Qw
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continued:
QA  Qw
mAcA DTA  mwcwDTw
(2kg)(c A )(100C  25C )  (5kg)(cw )(25C  20C )
(150kg  C )(c A )  (25kg  C )(cw )
1
4186
J
( c A )  (c w ) 
 698
6
6
kg  C
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latent heat
• L = Q/m [J/(kg)]
• heat needed per kg to melt (f) or
vaporize (v) a substance
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example L’s
• in J/kg:
•
melting (f)
• alcohol 100,000
• water
333,000
vaporization (v)
850,000
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
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 = 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)
21
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.
22
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
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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
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