chapter 18 temperature, heat, and the first law of thermodynamics
TRANSCRIPT
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Chapter 18
Temperature, Heat, and the First Law of Thermodynamics
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Temperature
• Thermodynamics – branch of physics studying thermal energy of systems
• Temperature (T), a scalar – measure of the thermal (internal) energy of a system
• SI unit: K (Kelvin)
• Kelvin scale has a lower limit (absolute zero) and has no upper limit
William Thomson(Lord Kelvin)
(1824 - 1907)
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Kelvin scale
• Kelvin scale is defined by the temperature of the triple point of pure water
• Triple point – set of pressure and temperature values at which solid, liquid, and gas phases can coexist
• International convention:T of the triple point of water is
KT 16.2733
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The zeroth law of thermodynamics
• If two (or more) bodies in contact don’t change their internal energy with time, they are in thermal equilibrium
• 0th law of thermodynamics: if bodies are in thermal equilibrium, their temperatures are equal
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Measuring temperature
• Temperature measurement principle: if bodies A and B are each in thermal equilibrium with a third body T, then A and B are in thermal equilibrium with each other (and their temperatures are equal)
• The standard temperature for the Kelvin scale is measured by the constant-volume gas thermometer
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Constant-volume gas thermometer
ghpp 0
CpT
33 CpT
33 p
pTT
3
16.273p
pK
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Celsius and Fahrenheit scales
• Celsius scale:
• Fahrenheit scale:
Anders Cornelius Celsius
(1701 - 1744)
Gabriel DanielFahrenheit
(1686 - 1736)
15.273TTC
325
9CF TT
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Thermal expansion
• Thermal expansion: increase in size with an increase of a temperature
• Linear expansion:
• Volume expansion:
TL
L
3
TV
V
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Chapter 18Problem 19
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Temperature and heat
• Heat (Q): energy transferred between a system and its environment because of a temperature difference that exists between them
• SI Unit: Joule
• Alternative unit: calorie (cal): Jcal 1868.4 1
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Absorption of heat
• Specific heat (c): heat capacity per unit mass
• Common states (phases) of matter: solid, liquid, gas
• Heat of transformation (L): the amount of energy per unit mass transferred during a phase change (boiling, condensation, melting, freezing, etc.)
)( if TTcmTcmQ
LmQ
Q Q
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Chapter 18Problem 37
)( if TTcmTcmQ LmQ
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Heat and work
Thermodynamic cycle
sdFdW
dspA )( )(Adsp pdV
f
i
V
VpdVW
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The first law of thermodynamics
• Work and heat are path-dependent quantities
• Quantity Q – W = Eint (internal energy) is path-
independent
• 1st law of thermodynamics: the internal energy of a system increases if heat is added to the system and decreases if work is done by the system
WQEEE if int,int,int
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The first law of thermodynamics
• Adiabatic process: no heat transfer between the system and the environment
• Isochoric (constant volume) process
• Free expansion:
• Cyclical process:
WWE 0int
QQE 0int
000int E
0int WQE
WQ
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Chapter 18Problem 48
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Heat transfer mechanisms
• Thermal conduction
• Conduction rate:
• Thermal resistance:
• Conduction through a composite slab:
L
TTkA
t
QP CH
cond
2211 // kLkL
TTAP CH
cond
k
LR Thermal conductivity
21 RR
TTA CH
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Heat transfer mechanisms
• Thermal radiation
• Radiation rate:
• Stefan-Boltzmann constant:
• Absorption rate:
4ATPrad
4envabs ATP
428 /1067.5 KmW
)( 44 TTA env Ludwig Eduard
Boltzmann(1844-1906)
Josef Stefan(1835-1893)
radabsnet PPP
Emissivity
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Chapter 18Problem 52
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Heat transfer mechanisms
• Convection
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Heat transfer mechanisms
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Answers to the even-numbered problems
Chapter 18:
Problem 4
(a) 320º F; (b) -12.3º F
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Answers to the even-numbered problems
Chapter 18:
Problem 10
1.1 cm
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Answers to the even-numbered problems
Chapter 18:
Problem 22
109 g
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Answers to the even-numbered problems
Chapter 18:
Problem 44
(a) − 200 J; (b) − 293 J; (c) − 93 J
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Answers to the even-numbered problems
Chapter 18:
Problem 50
(a) 0.42 m; (b) 7.4 km