Page 274 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 22

Heat Transfer

CONDUCTION

The three mechanisms by which heat can be transferred from one place to another are conduction, convection,
and radiation.
In conduction, heat is carried by means of collisions between rapidly moving molecules at the hot end of a
body of matter and the slower molecules at the cold end. Some of the kinetic energy of the fast molecules passes
to the slow molecules, and the result of successive collisions is a flow of heat through the body of matter. Solids,
liquids, and gases all conduct heat. Conduction is poorest in gases because their molecules are relatively far apart
and so interact less frequently than in solids and liquids. Metals are the best conductors of heat because some of
their electrons are able to move about relatively freely and can travel past many atoms between collisions.
The rate at which heat is conducted through a slab of a particular material is proportional to the area A of the
slab and to the temperature difference T between its sides and inversely proportional to the slab’s thickness d
(Fig. 22-1). The amount of heat Q that flows through the slab in the time t is given by
Q k A T
Rate of heat conduction = =
t d
where k, the thermal conductivity of the material, is a measure of its ability to conduct heat. In SI, the correct
unit of k is the W/(m· C), but the kcal/(m·s· C) is also often used. In the British system, the usual unit of k is the
◦
◦
2
Btu/(ft ·h· F/in.) since A is customarily expressed in square feet and d in inches.
◦

T 2
T 1 Q
A

d
− T = ∆T
T 1 2
Fig. 22-1

SOLVED PROBLEM 22.1
◦
The thermal conductivities of brick and pine wood are, respectively, 0.6 and 0.13 W/(m· C). What
thickness of brick has the same insulating ability as 5 cm of pine?
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