Know that there are many solids that conduct thermal energy better than thermal insulators but do so less well than good thermal conductors

2.3.1 Conduction

Objective

Understand that many solids conduct thermal energy better than thermal insulators, but not as well as good thermal conductors.

What is Thermal Conduction?

Thermal conduction is the transfer of kinetic energy between neighbouring particles of a material without any overall movement of the material itself. Energy moves from the hotter region to the cooler region through direct molecular collisions.

How Conduction Occurs

In a solid, particles are closely packed. When one part of the solid is heated, its particles vibrate more vigorously and pass kinetic energy to adjacent particles. This process continues, creating a flow of heat.

Mathematical Description

The rate of heat transfer by conduction through a uniform slab is given by Fourier’s law:

\$ Q = \frac{k A \Delta T}{d} \$

where

• \$Q\$ = heat transferred per unit time (W)
• \$k\$ = thermal conductivity of the material (W·m⁻¹·K⁻¹)
• \$A\$ = cross‑sectional area through which heat flows (m²)
• \$\Delta T\$ = temperature difference between the two faces (K)
• \$d\$ = thickness of the material (m)

Factors Influencing Conduction

1. Thermal conductivity (\$k\$) – intrinsic property of the material.
2. Cross‑sectional area (\$A\$) – larger area allows more heat to pass.
3. Thickness (\$d\$) – thicker sections reduce the rate of heat flow.
4. Temperature gradient (\$\Delta T\$) – a larger difference drives faster conduction.

Types of Solids

Solids can be grouped according to their ability to conduct heat:

• Good thermal conductors – metals such as copper, aluminium, silver.
• Moderate conductors – many non‑metallic solids (e.g., glass, ceramics, some polymers) that conduct better than insulators but worse than metals.
• Thermal insulators – materials with very low \$k\$, such as wood, rubber, polystyrene foam.

Comparative Table of Thermal Conductivity

Why Some Solids Are Only Moderate Conductors

In non‑metallic solids, heat is mainly transferred by lattice vibrations (phonons). The presence of irregularities, grain boundaries, and less free electrons reduces the efficiency of energy transfer compared with metals, where free electrons carry heat very rapidly.

Practical Implications

• Cooking utensils: metal pans heat quickly (good conductors), while handles are often made of wood or plastic (insulators) to reduce heat transfer to the hand.
• Building construction: walls may contain bricks (moderate conductors) with insulation layers (insulators) to control heat loss.
• Electronics: heat sinks are made of aluminium or copper to draw heat away from components, while surrounding casings may use polymers for protection.

Key Points to Remember

• All solids conduct heat to some extent; the rate depends on thermal conductivity.
• Metals are the best conductors because of free electrons.
• Many non‑metallic solids conduct better than typical insulators but far less efficiently than metals.
• Understanding \$k\$ helps predict how quickly a material will transfer heat in real‑world situations.

Check Your Understanding

1. Explain why a glass window lets heat pass more readily than a wooden door.
2. Given a copper rod (k = 400 W·m⁻¹·K⁻¹) and a glass rod (k = 1.2 W·m⁻¹·K⁻¹) of the same dimensions, which one will transfer heat faster? By what factor?
3. Identify a situation where a material with moderate conductivity is preferred over a good conductor.