11.3 Temperature Difference and Heat Transfer 317
Because of the energy transfer from the heating element, the molecules of the container
in the region near the heating element are more energetic than those molecules farther away. The
more energetic molecules share or transfer some of their energy to the neighboring
regions, and the neighboring regions do the same thing, until the energy transfer eventually
reaches the handles and the lid of the container. The energy is transported from the high-
temperature region to the low-temperature region by molecular activity. The rate of heat trans-
fer by conduction is given by Fourier’s law, which states that the rate of heat transfer through
a material is proportional to the temperature difference, normal areaA, through which heat
transfer occurs, and the type of material involved. The law also states that the heat transfer
rate is inversely proportional to the material thickness over which the temperature difference
exists. For example, referring to Figure 11.7, we can write the Fourier’s law for a single-pane glass
window as(11.12)
where
qheat transfer rate (in W or Btu /h)
kthermal conductivity
Across-sectional area normal to heat flow ( m
2
or ft
2
)T 1 T 2 temperature difference across the material ofLthickness (C or F)
Lmaterial thickness ( m or ft)
a
W
m#°C
or
Btu
h#ft#°F
b
qkA
T 1 T 2
L
■Figure 11.7
Conduction heat transfer through
a glass window.Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀