How Does Heat Travel Through Materials?

• Conduction: heat passes between neighbouring atoms
• Convection: a flowing fluid carries heat
• Radiation: heat travels as waves

What you will learn

• Name the three ways heat travels and what sets each apart.
• Explain why metal conducts heat well while foam insulates.
• Understand how insulating materials slow the flow of heat.

Before you start

No prior knowledge is needed. You only need to know that heat goes from hot to cold.

How do the three ways of heat transfer differ?

Each way uses its own mechanism. According to a physics lecture note from Boston University, there are three basic ways to transfer heat.

How does a solid conduct heat, and why do metals differ from foam?

In a solid, heat travels by conduction: it moves internally through vibrations of atoms and molecules. In metals, electrons also carry heat, so metals are generally very good conductors. Metals have many free electrons moving at random, and they transfer heat from one part of the metal to another.

The Boston lecture's conduction model treats thermal conductivity as a material constant. This is an assumption of the model being used, rather than a claim that the coefficient never changes under any conditions. The notes call copper a good thermal conductor, which is why some pots and pans have copper bases, and Styrofoam a good insulator.

• Metal bar
• Foam bar with many air pockets

How does heat cross liquids, gases and insulating materials?

In liquids and gases heat is often transferred by convection. The hotter part of the fluid is less dense than the cooler part, so it rises while the cooler, denser fluid sinks, setting up convection currents. Forced convection is when the fluid does not flow of its own accord but is pushed, as in forced-air heaters or cooling fans.

Insulation makes use of this. A building-science page of the US Department of Energy says insulation prevents heat flow by trapping pockets of air. The higher the R-value, the more slowly heat flows through the material.

• Trapped air pockets
• Insulating slab

Real-world example

In the Boston University stove-and-oven example, heat is conducted from the burner through the pan base to the water; convection currents help warm the water evenly. In an oven, glowing heating elements transfer heat to food by radiation. These examples separate the mechanisms for explanation, although several mechanisms often act together. Feeling warmth without touching an object does not by itself establish radiation, because moving hot air also carries heat.

Common misunderstandings

Conduction makes the substance flow. According to the notes, conduction involves no motion of the substance; heat moves internally through vibrations of atoms and molecules.

Convection happens in solids. The notes describe convection as the usual way heat moves in liquids and gases.

Insulation stops heat dead. A higher R-value means heat flows more slowly, not that it stops.

Frequently asked questions

What is forced convection?

It is convection in which the fluid does not flow of its own accord but is pushed, as by a fan or forced-air heater.

What is thermal radiation?

It is heat transfer by absorbing or giving off electromagnetic waves.

What does a high R-value mean?

It means heat flows through the material more slowly.

Summary

• Heat moves by conduction, convection and radiation, each with its own mechanism.
• Metals conduct heat well thanks to free electrons, while foam is an insulator.
• Insulation traps pockets of air and slows heat flow; a higher R-value means slower flow.