First Law of Thermodynamics Physics 202 Professor Lee Carkner Lecture 13.
Heat Transfer Physics 202 Professor Lee Carkner Lecture 12.
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Transcript of Heat Transfer Physics 202 Professor Lee Carkner Lecture 12.
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Heat Transfer
Physics 202Professor Lee
CarknerLecture 12
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PAL #11 First Law Final temperature of 20 g, 0 C ice cube dropped into 300 g of hot tea
at 90 C. Add up all heats (Q = cm T and Q = Lm) Heat 1: melt ice
Q1 = (333000)(0.02) = 6660 J Heat 2: warm up now melted ice cube
Q2 = (4190)(0.02)(Tf-0) Heat 3: cool down tea
Q3 = (4190)(0.3)(Tf-90) Step 4: add up heat
Q1 + Q2 + Q3 = 0
6660 + 83.8Tf + 1257Tf –113130 = 0
1340.6Tf = 106470
Tf = 79.4 C
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Heat Transfer
What is moving? In mechanics energy can be transferred
through a particle (e.g. a bullet) or a wave (e.g. a sound wave)
Heat can also be transferred by radiation both a particle and a wave
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Conduction
The end in the fire experiences a large vibration of the molecules of the metal
This is called conduction The movement of heat from a high
temperature region to a low temperature region through another material
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Conductive Heat Transfer The rate at which heat is transferred by
conduction is given byH = Q/t = kA (TH - TC)/L
Where: Q is heat and t is time A is the cross sectional area of the material (in
the direction of heat transfer) T is the temperature (hot or cold)
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Thermal Conductivities
For Al, k=235 for Cu, k=428 (W/ m K)
Materials with low k are good thermal insulators
We use foam to insulate our houses Down filled winter coats trap air for
insulation
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Composite Slabs
H = Q/t = A (TH - TC)/ (L/k) Where (L/k) is the sum of the ratios
of the thickness and thermal conductivity of each layer of the slab
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Radiation
This is how you are warmed by sunlight The power (in Watts) that is emitted by
an object depends on its temperature (T), its area (A) and it emissivity ()
Pr = AT4
Where is the Stefan-Boltzmann constant = 5.6703 X 10-8 W/m2 K4
T must be in absolute units (Kelvin)
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Absorption of Radiation
Pa =ATenv4
Where Tenv is the temperature of the environment
Pn = Pa -Pr = A(Tenv4 - T4)
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Emissivity What is the value of emissivity?
Objects with an emissivity of 1 are called blackbody emitters or absorbers
Every object whose temperature is above 0 K emits thermal radiation People emit thermal radiation at infrared
wavelengths and thus can be detected at night with IR goggles
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Convection
The hot air is thus lighter and rises If the hot air cools as it rises it will
eventually fall back down to be re-heated and rise again
Examples: baseboard heating, boiling water, Earth’s atmosphere
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Convection Rate Factors
Fluidity
Energy exchange with environment
How rapidly will the material lose heat?
A small temperature difference may result
in not enough density difference to move
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Next Time
Test #2 For Monday January 9:
Read: 19.1-19.7 Do Webassign homework
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Water condenses out of the air onto a cold piece of metal. Due to this condensation, the temperature of the air around the metal,
A) IncreasesB) DecreasesC) Stays the sameD) Fluctuates unpredictablyE) It depends on the temperature of the
metal
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Ten joules of heat are added to a cylinder of gas causing the piston at the top to rise. How much work does the piston do?
A) 0 JoulesB) 5 JoulesC) 10 JoulesD) -10 joulesE) You cannot tell from the
information given
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Which of the processes in the diagram produces the least work?
A) 1B) 2C) 3D) 4E) All are the same
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Which of the processes in the diagram involves the least change in internal energy?
A) 1B) 2C) 3D) 4E) All are the same
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Which of the processes in the diagram must have net heat output?
A) 1B) 2C) 3D) 4E) 3 and 4
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Which of the processes in the diagram must have net heat input
A) 1B) 2C) 3D) 4E) 1 and 2