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): Jcal 1868.4 1
Absorption of heat
• Specific heat (c): heat capacity per unit mass
• Common states (phases) of matter: solid, liquid, gas
• Latenet heat (L): the amount of energy per unit mass transferred during a phase change (boiling, condensation, melting, freezing, etc.)
)( if TTcmTcmQ
LmQ
Q Q
Absorption of heat
Q Q
Absorption of heat
Absorption of heat
Chapter 11Problem 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:
• Thermal resistance:
• Conduction through a composite rod:
L
TTkA
t
Q chcond
P
2211 // kLkL
TTA chcond
P
k
LR Thermal conductivity
21 RR
TTA ch
Heat transfer mechanisms
Chapter 11Problem 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:
• Stefan-Boltzmann constant:
• Absorption rate:
4eATrad P
4envabs eATP
428 /1067.5 KmW
)( 44 TTeA env Josef Stefan(1835-1893)
radabsnet PPP
Emissivity
Chapter 11Problem 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
Top Related