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Science and Society

Require Insulation?
an you spend too much for home to insulation, meaning when the cost of benefits, advantages, and disadvantages of
C insulation? Should local governments installing insulation exceeds any returns requiring maximum insulation in all new
require maximum insulation in new homes in energy saved. Write a script designed homes.
to save energy? Research economic limits to inform a city planning board about the

Wooden and metal parts of your desk have the same tem-
perature, but the metal parts will feel cooler if you touch them.
Metal is a better conductor of heat than wood and feels cooler

because it conducts heat from your finger faster. This is the

same reason that a wood or tile floor feels cold to your bare

feet. You use an insulating rug to slow the conduction of heat
from your feet.

CONCEPTS Applied

Touch Temperature
Objects that have been in a room at constant temperature
for some time should all have the same temperature. Touch
metal, plastic, and wooden parts of a desk or chair to sense
their temperature. Explain your findings.
FIGURE 4.14 Fiberglass insulation is rated in terms of the
R-value, a ratio of the conductivity of the material to its thickness.

Convection
TABLE 4.3 Convection is the transfer of heat by a large-scale displace-

Rate of conduction of materials* ment of groups of molecules with relatively higher kinetic
energy. In conduction, increased kinetic energy is passed from
Silver 0.97 molecule to molecule. In convection, molecules with higher
Copper 0.92 kinetic energy are moved from one place to another place.
Aluminum 0.50 Conduction happens primarily in solids, but convection hap-

Iron 0.11 pens only in liquids and gases, where fluid motion can carry
Lead 0.08 molecules with higher kinetic energy over a distance. When
Concrete 4.0 × 10 –3 molecules gain energy, they move more rapidly and push
Better Conductor Better Insulator Tile 1.6 × 10 –3 expansion as the region of heated molecules pushes outward

more vigorously against their surroundings. The result is an
–3
2.5 × 10
Glass
and increases the volume. Since the same amount of matter
–3
1.5 × 10
Brick
now occupies a larger volume, the overall density has been
Water
–4
3.0 × 10
Wood 1.3 × 10 –3 decreased (Figure 4.15).

In fluids, expansion sets the stage for convection. Warm,
Cotton 1.8 × 10 –4

less dense fluid is pushed upward by the cooler, more dense
Styrofoam 1.0 × 10 –4 fluid around it. In general, cooler air is more dense; it sinks

Glass wool 9.0 × 10 –5 and flows downhill. Cold air, being more dense, fl ows out

Air 6.0 × 10 –5 near the bottom of an open refrigerator. You can feel the cold,
Vacuum 0 dense air pouring from the bottom of a refrigerator to your
toes on the floor. On the other hand, you hold your hands

over a heater because the warm, less dense air is pushed
*Based on temperature difference of 1°C per cm. Values are cal/s through a square
centimeter of the material. upward. In a room, warm air is pushed upward from a heater.
4-13 CHAPTER 4 Heat and Temperature 97