Transcript Chapter 11 Energy in Thermal Processes

Chapter 11
Energy in Thermal Processes
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
• Latenet heat (L): the amount of energy per unit
mass transferred during a phase change (boiling,
condensation, melting, freezing, etc.)
Q
Q
Absorption of heat
Q
Q
Absorption of heat
Absorption of heat
Chapter 11
Problem 17
A 100-g aluminum calorimeter contains 250 g of water. The two substances are
in thermal equilibrium at 10°C. Two metallic blocks are placed in the water. One
is a 50-g piece of copper at 80°C. The other sample has a mass of 70 g and is
originally at a temperature of 100°C. The entire system stabilizes at a final
temperature of 20°C. Determine the specific heat of the unknown second
sample.
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 rod:
Pcond
ATh  Tc 

L1 / k1  L2 / k 2
ATh  Tc 

R1  R2
Heat transfer mechanisms
Chapter 11
Problem 36
A box with a total surface area of 1.20 m2 and a wall thickness of 4.00 cm is
made of an insulating material. A 10.0-W electric heater inside the box
maintains the inside temperature at 15.0°C above the outside temperature. Find
the thermal conductivity k of the insulating material.
Heat transfer mechanisms
• Thermal radiation
• Radiation rate:
Emissivity
Prad  eAT
• Stefan-Boltzmann constant:
• Absorption rate:
4
  5.67 10 8W / m 2  K 4
Pabs  eAT
4
env
Pnet  Pabs  Prad
 eA(T
4
env
Josef Stefan
(1835-1893)
T )
4
Chapter 11
Problem 46
At high noon, the Sun delivers 1.00 kW to each square meter of a blacktop
road. If the hot asphalt loses energy only by radiation, what is its equilibrium
temperature?
Heat transfer mechanisms
• Convection
Heat transfer mechanisms
Questions?
Answers to the even-numbered problems
Chapter 11
Problem 4
0.152 mm
Answers to the even-numbered problems
Chapter 11
Problem 12
467 pellets
Answers to the even-numbered problems
Chapter 11
Problem 16
1.7 kg
Answers to the even-numbered problems
Chapter 11
Problem 20
49 kJ
Answers to the even-numbered problems
Chapter 11
Problem 40
7.2 × 10-2 W/(m °C)
Answers to the even-numbered problems
Chapter 11
Problem 50
14.1 h