chapter 15 heat capacity thermal expansion chapter 16 heat
TRANSCRIPT
3-Oct-10
Chapter 15Heat Capacity
Thermal ExpansionChapter 16
Heat Transfer
Lecture 17
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Internal EnergyInternal energy of an
object depends on:• Temperature• Mass• Material
1 kg
Temperature InternalEnergy
300 K 120,000 J
200 K 80,000 J
100 K 40,000 J
0 K 0 Joules
Iron
Temperature InternalEnergy
300 K 120 J
200 K 80 J
100 K 40 J
0 K 0 Joules
Iron
1 gram
Temperature InternalEnergy
300 K 1,200,000 J
200 K 800,000 J
100 K 400,000 J
0 K 0 Joules
Water
1 kg
1000 grams
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Specific Heat CapacitySpecific heat capacity is the amount of heat
energy required to raise the temperature of one unit mass of a material by one degree.
SI Unit: J/(kg•K) or J/(kg•°C)Other Units: cal/(g •°C)
Heat energy needed to raise temperature of material by ∆T is:
(Specific heat cap. of material)•(mass)•∆T
Some specific heat capacity values.Specific heats of gases are complicated.
Example• How much heat energy is needed to raise the
temperature of 2 kg of copper (s.h.c. = 387 J/kg-K) from 10° to 30°C?
• Q = (s.h.c.)•m•∆T = (387 J/kg-K)(2kg)(20K)= 1.55 x 104 J
• How long would it take for a 1000 W heater to do this? Power = (Energy provided)/∆t
∆t = (Energy needed)/(Power) = 15500J/(1000 J/s)= 15.5s
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Check Yourself
Why does a piece of watermelon stay cool for a longer time than sandwiches do when both are removed from a cooler on a hot day?
Why is it that the climate in the desert is so hot during the day yet so cold at night?
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Thermal Expansion
Due to increased molecular motion, most materials expand as temperature increases.
Sidewalk buckles and cracks due to expansion on a hot summer day
Space allows for expansion
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Demo: Expansion of a Ring
Metal ball barely fits past the metal ring.
Not surprising that heated ball won’t pass through cold ring.
Will cold ball pass through heated (expanded) ring?
Coefficients of Linear Expansion
Thermal Expansion DifferencesA bimetallic strip has two metals of different coefficients of thermal expansion, A and B in the figure. It will bend when heated or cooled.
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Demo: Heat, Cool, Break
COOL (quickly)
HEAT
Glass expands when heated. If hot glass is cooled quickly, exterior cools before the interior. Exterior contracts faster than the interior, cracking the glass.Pyrex glass expands much less than regular glass.
GLASS
Cracks form
Thermal ExpansionThe amount of thermal expansion of length L is:
∆L = (expansion coefficient)•L•∆TAn area gets linear expansion in both directions.
Holes expand as well:
You have a (glass) jar and you can’t get the metal lid off. What should you do:
a) ask your friendb) run the jar & lid under cold water
c) run the jar & lid under hot water
You have a (glass) jar and you can’t get the metal lid off. What should you do:
a) ask your friendb) run the jar & lid under cold waterc) run the jar & lid under hot water
Because the metal has a substantially higher coefficient of thermal expansion than the glass, heating them will make both of them bigger, but the metal will be ‘more bigger’.
Water Density vs. Temperature
This explains why lakes freeze from the top.
Heat Transfer(Flow of Heat Energy)
Three Methods•Conduction - Thermal kinetic energy passed
from particle-to-particle along a length ofmaterial.
•Convection - Thermal energy carried bymoving fluid.
•Radiation - Thermal energy carried byelectromagnetic waves.
Heat Transfer: ConductionHeat conduction can be visualized as occurring through molecular collisions. Thermal kinetic energy is passed along as “hotter” particles collide with “colder” ones.
L
Q
Cross-sectional area
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Conduction
Conduction is heat flow by direct contact.
Some materials are good thermal conductors, others are insulators.
98º
75º
98º
75ºWood is an
insulatorTile is a
conductor
Tile floor feels colder than wood floor
ConductionExperimentally, it is found that the amount of heat Q that flows through a piece of material:
• Increases proportionally to the cross-sectional area A
• Increases proportionally to the temperature difference ∆T from one end to the other
• Increases steadily with time t
• Decreases with the length L of the piece
• Depends on the “thermal conductivity” of the material. More conductive → more heat flows
Thermal Conductivity
Some typical thermal conductivities:
Substances with high thermal conductivities are good conductors of heat; those with low thermal conductivities are good insulators.
Vacuum has a thermal conductivity = 0.
Convection
Convection is flow of fluid due to difference in temperatures, such as warm air rising. Fluid “carries” heat with it as it moves.
“Natural” convection: Warm fluid will rise because it is less dense then cold fluid.
Heat Transfer: ConvectionConvection occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced (fans); both these examples are natural convection.
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Convection
Heat transfer in a fluid often occurs mostly by convection.
Buoyancy causes warm air to rise, which carries thermal energy directly by its motion.
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Convection OvenConvection oven has a fan to enhance the circulation of the air, increasing the transfer of heat.
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Fiberglass Insulation
Air is a poor thermal conductor but easily transfers heat by convection.
Fiberglass insulation is mostly air, with the fibers disrupting the convection flow.
RadiationAll objects give off energy in the form of radiation, as electromagnetic waves – infrared, visible light, ultraviolet – which, unlike conduction and convection, can transport heat through a vacuum.
Objects that are hot enough will glow visibly –first red, then yellow, white, and blue as temperature increases. Objects at body temperature radiate in the infrared, and can be seen with night vision binoculars.
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RadiationRadiation has many
different wavelengths, most of which are not visible to the eye.
All radiation carries energy, and thus transfers heat. Heat Lamp
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Emission of Radiant Energy
All objects radiate; higher the temperature, the higher the frequency.
At room temperature, the radiated light is at frequencies too low for our eyes to see.
Special cameras are sensitive to this infrared radiation.
Attics in this house were kept warm for growing marijuana.
70
98º
75º
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Reflection of Radiant EnergyWhite and “silver” objects reflect light, black objects
and “holes” don’t.
Hole in a box with white interior looks black because almost none of the light entering the hole reflects back out.
White tubes look black inside.
Black objects are also the best emitters of radiation. White objects emit less radiation, and perfectly reflective objects don’t emit at all. (Space blanket.)
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Controlling Heat TransferThermos bottle eliminates conduction
and convection by having double-walled sides with vacuum.
Silvered interior walls minimize heat transfer by radiation.
RadiationIf you are in sunlight, Sun’s radiation will warm you. The intensity of solar radiation is 1000 W/m2. In general, you will not be perfectly perpendicular to the Sun’s rays, and will absorb energy at a rate that depends on your angle to the sun’s rays.
SeasonsThis angle effect is also responsible for the seasons.
3-Oct-10 Physics 1 (Garcia) SJSU
Greenhouse EffectGlass is transparent to sunlight (short-wavelength).
Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat.
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Earth’s Greenhouse Effect
Earth’s atmosphere acts as a greenhouse, trapping solar energy.
Most of the trapping is due to carbon dioxide and water vapor, which is why they’re called “greenhouse gasses.”
Key Points of Lecture 17Key Points of Lecture 17
Before Friday, read Hewitt Chap. 16.
Homework Assignment #12 is due before 11:00 PM on Friday, Oct. 8.
• Specific Heat Capacity• Thermal Expansion• Transfer of Heat by Conduction• Transfer of Heat by Convection• Transfer of Heat by Radiation