PHYSICAL CHEMISTRY IN BRIEF

CHAP. 10: TRANSPORT PROCESSES [CONTENTS] 333

10.2 Heat flow—thermal conductivity.

10.2.1 Ways of heat transfer.

Heat may be transferred in three ways: by convection, conduction or radiation.
Convectionis a heat transfer driven by the transfer of mass. This type of heat transfer
occurs in single-storey heating systems in which hot water from a boiler is driven by a pump
to the radiators.
Radiationis a heat transfer driven by electromagnetic radiation (photons). This way heat
is transferred from the Sun to the Earth.
Conductionis a heat transfer without any mass transferred. In this case the molecules
endowed with more energy pass heat to those with less energy. Heat transfer through a wall
from a warm room to the colder surroundings may serve as an example.

10.2.2 Fourier’s law.

For the heat flux, equation (10.1) acquires the form

Jz=

1

S

dQ

dτ

. (10.4)

The heat flow in a system is caused by temperature differences. The driving force is a
change of temperature with location, or more accurately the derivative of temperature with
respect to location,ddTz. The general equation (10.3) becomes

1

S

dQ

dτ

=−λ

dT

dz

. (10.5)

This equation is called theFourier’s law. The quantity λis thecoefficient of thermal
conductivity, or more brieflythermal conductivity.

10.2.3 Thermal conductivity

The coefficient of thermal conductivity is the amount of heat that passes through unit surface
per unit time at unit temperature gradient. The higher the value ofλ, the faster the tem-
peratures in the system equalize, or the faster the given environment conducts heat. Thermal
conductivity is a property of material.