Chapter 11 Energy in Thermal Processes

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Chapter 11Energy in Thermal Processes
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Vocabulary, 3 Kinds of Energy

• Internal Energy U Energy of microscopic motion
  and inter-molucular forces
• WorkW -FDx -PDV is work done by expansion
  (next chapter)
• HeatQ Energy transfer from microscopic contact

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Temperature and Specific Heat

• Add energy -gt T rises

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Example 11.1
Bobby Joe drinks a 130 calorie can of soda. If
the efficiency for turning energy into work is
20, how many 4 meter floors must Bobby Joe
ascend in order to work off the soda and maintain
her 55 kg mass?
Nfloors 50.4
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Example 11.2
Aluminum has a specific heat of .0924 cal/gºC. If
110 g of hot water at 90 ºC is added to an
aluminum cup of mass 50 g which is originally at
a temperature of 23 ºC, what is the final
temperature of the equilibrated water/cup combo?
T 87.3 ºC
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Phase Changes and Latent Heat

• T does not rise when phases change (at constant
  P)
• Examples solid -gt liquid (fusion), liquid -gt
  vapor (vaporization)
• Latent heat energy required to change phases

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Example 11.3
1.0 liters of water is heated from 12 ºC to 100
ºC, then boiled away. a) How much energy is
required to bring the water to boiling? b) How
much extra energy is required to vaporize the
water? c) If electricity costs 75 per MW-hrs,
what was the cost of boiling the water?
a) Q 8.8x104 cal 3.68x105 J
b) Q 5.4x105 cal 2.26x106 J
c) 5.5
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Example 11.4
Consider Bobby Joe from the previous example. If
the 80 of the 130 kcals from her soda went into
heat which was taken from her body from
radiation, how much water was perspired to
maintain her normal body temperature? (Assume a
latent heat of vaporization of 540 cal/g even
though T 37 ºC)
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Three Kinds of Heat Transer

• Conduction
• Shake your neighbor - pass it down
• Examples Heating a skillet, losing heat through
  the walls
• Convection
• Move hot region to a different location
• Examples Hot-water heating for buildings
  Circulating air Unstable
  atmospheres
• Radiation
• Light is emitted from hot object
• Examples Stars, Incandescent bulbs

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Conduction

• Power depends on area A, thickness Dx,
  temperature difference Dt and conductivity of
  material

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Example 11.5
A copper pot of radius 12 cm and thickness 5 mm
sits on a burner and boils water. The temperature
of the burner is 115 ºC while the temperature of
the inside of the pot is 100 ºC. What mass of
water is boiled away every minute? DATA kCu
397 W/mºC
m1.43 kg
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Conductivities and R-values

• Conductivity (k)
• Property of Material
• SI units are W/(m ºC)

• R-Value
• Property of material and thickness Dx.
• Measures resistance to heat
• Useful for comparing insulation products
• Quoted values are in AWFUL units

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Conducitivities and R-values
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What makes a good heat conductor?

• Free electrons (metals)
• Easy transport of sound (lattice vibrations)
• Stiff is good
• Low Density is good
• Pure crystal structure

Diamond is perfect!
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R-values for layers
Consider a layered system, e.g. glass-air-glass
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Example 11.6
Consider three panes of glass, each of thickness
5 mm.The panes trap two 2.5 cm layers of air in a
large glass door. How much power leaks through a
2.0 m2 glass door if the temperature outside is
-40 ºC and the temperature inside is 20 ºC? DATA
kglass 0.84 WmºC, kair 0.0234 Wm ºC
P 55.7 W
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Convection

• If warm air blows across the room, it is
  convection
• If there is no wind, it is conduction
• Can be instigated by turbulence or instabilities

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Why are windows triple paned?
To stop convection!
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Transfer of heat by radiation

• All objects emit light if T gt 0
• Colder objects emit longer wavelengths (red or
  infra-red)
• Hotter objects emit shorter wavelengths(blue or
  ultraviolet)
• Stefans Law give power of emitted radiation

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Example 11.7
If the temperature of the Sun fell 5, and the
radius shrank 10, what would be the percentage
change of the Suns power output?
- 34
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Example 11.8
DATA The sun radiates 3.74x1026 W
Distance from Sun to Earth 1.5x1011 m
Radius of Earth 6.36x106 m

• What is the intensity (power/m2) of sunlight when
  it reaches Earth?
• How much power is absorbed by Earth in sunlight?
  (assume that none of the sunlight is reflected)
• What average temperature would allow Earth to
  radiate an amount of power equal to the amount of
  sun power absorbed?

a) 1323 W/m2 b) 1.68x1017 W c) T 276 ºK
3 ºC 37 ºF
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What is neglected in estimate?

• Earth is not at one single temperature
• Some of Suns energy is reflected
• Emissivity lower at Earths thermal wavelengths
  than at Suns wavelengths
• Radioactive decays inside Earth
• Hot underground (less so in Canada)
• Most of Jupiters radiation

NOTE Venus has a surface T of 900 C
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Greenhouse Gases

• Sun is much hotter than Earth so sunlight has
  much shorter wavelengths than light radiated by
  Earth (infrared)
• Emissivity of Earth depends on wavelength
• CO2 in Earths atmosphere reflects in the
  infrared
• Barely affects incoming sunlight
• Reduces emissivity, e, of re-radiated heat

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Global warming

• Tearth has risen 1 ºF
• consistent with greenhouse effect
• Other gases, e.g. S02, could cool Earth