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Chapter 18
Temperature, Heat, and the First Law of
Thermodynamics
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)
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
T3  273.16 K
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
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
Constant-volume gas thermometer
p  p0  gh
T  Cp
T3  Cp3
p
T  T3
p3
p
 273.16 K 
p3
Celsius and Fahrenheit scales
• Celsius scale:
TC  T  273.15
• Fahrenheit scale:
9
TF  TC  32
5
Anders Cornelius
Celsius
(1701 - 1744)
Gabriel Daniel
Fahrenheit
(1686 - 1736)
Thermal expansion
• Thermal expansion: increase in size with an
increase of a temperature
• Linear expansion:
• Volume expansion:
L
  T
L
V
  T
V
  3
Chapter 18
Problem 19
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):
1 cal  4.1868 J
Absorption of heat
Q  cmT  cm(T f  Ti )
• Specific heat (c): heat capacity per unit mass
• Common states (phases) of matter: solid, liquid, gas
Q  Lm
• Heat of transformation (L): the amount of energy per
unit mass transferred during a phase change (boiling,
condensation, melting, freezing, etc.)
Q
Q
Chapter 18
Problem 37
Q  cmT  cm(T f  Ti )
Q  Lm
Heat and work
 
dW  F  ds  ( pA)  ds  p( Ads)  pdV
Vf
W   pdV
Vi
Thermodynamic cycle
The first law of thermodynamics
• Work and heat are path-dependent quantities
• Quantity Q – W = Eint (internal energy) is pathindependent
• 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
Eint  Eint, f  Eint,i  Q  W
The first law of thermodynamics
• Adiabatic process: no heat transfer between the
system and the environment
Eint  0  W  W
• Isochoric (constant volume) process
Eint  Q  0  Q
• Free expansion:
Eint  0  0  0
• Cyclical process:
Eint  Q  W  0
Q W
Chapter 18
Problem 48
Heat transfer mechanisms
• Thermal conduction
• Conduction rate:
Pcond
• Thermal resistance:
TH  TC
Q
  kA
t
L
L
R
k
Thermal conductivity
• Conduction through a composite slab:
Pcond
ATH  TC 

L1 / k1  L2 / k 2
ATH  TC 

R1  R2
Heat transfer mechanisms
• Thermal radiation
• Radiation rate:
Emissivity
Prad  AT
• Stefan-Boltzmann constant:
• Absorption rate:
4
  5.67 10 8W / m 2  K 4
Pabs  AT
4
env
Pnet  Pabs  Prad
 A(T
4
env
Josef Stefan
(1835-1893)
Ludwig Eduard
Boltzmann
(1844-1906)
T )
4
Chapter 18
Problem 52
Heat transfer mechanisms
• Convection
Heat transfer mechanisms
Answers to the even-numbered problems
Chapter 18:
Problem 4
(a) 320º F;
(b) -12.3º F
Answers to the even-numbered problems
Chapter 18:
Problem 10
1.1 cm
Answers to the even-numbered problems
Chapter 18:
Problem 22
109 g
Answers to the even-numbered problems
Chapter 18:
Problem 44
(a) − 200 J;
(b) − 293 J;
(c) − 93 J
Answers to the even-numbered problems
Chapter 18:
Problem 50
(a) 0.42 m;
(b) 7.4 km