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