Heat Transfer: Conduction, Convection, and Radiation

Learning Objectives

Define the three mechanisms of heat transfer: conduction, convection, and radiation.

Provide a real-world example of each type of heat transfer.

Distinguish between thermal conductors and thermal insulators.

How Heat Moves

Heat is thermal energy in transit, flowing from a region of higher temperature to a region of lower temperature. There are three fundamental mechanisms by which this transfer occurs.

1. Conduction

Conduction is the transfer of heat through a substance by direct contact of its particles, without any net movement of the substance itself.

Mechanism: When one part of a substance is heated, its particles gain kinetic energy and vibrate more vigorously. They collide with their less energetic neighbors, transferring kinetic energy to them. This process continues down the substance.

Medium: Occurs primarily in solids, but also in liquids and gases.

Example: Placing a metal spoon in a cup of hot coffee. The handle of the spoon becomes hot because heat is conducted from the coffee, through the spoon, to your hand.

Conductors vs. Insulators:

Thermal Conductors (like metals) transfer heat easily because they have free-moving electrons that can rapidly transport energy.

Thermal Insulators (like wood, plastic, or air) are poor conductors of heat.

2. Convection

Convection is the transfer of heat through the bulk movement of fluids (liquids or gases).

Mechanism: When a fluid is heated, it typically expands and becomes less dense. The warmer, less dense fluid rises, and cooler, denser fluid sinks to take its place. This creates a convection current that circulates heat.

Medium: Occurs only in fluids (liquids and gases).

Example: Boiling water in a pot. The water at the bottom is heated, rises, cools at the surface, and then sinks, creating a circular flow. Weather patterns and ocean currents are large-scale examples of convection.

3. Radiation

Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation.

Mechanism: All objects with a temperature above absolute zero emit thermal radiation. This radiation can travel through a vacuum and does not require a medium. When this radiation strikes another object, it is absorbed, and its energy is converted into thermal energy.

Medium: Does not require a medium; it can travel through the vacuum of space.

Example: The heat you feel from the Sun, or the warmth you feel when standing near a campfire, is primarily from thermal radiation.

Key Terms

Conduction: The process by which heat or electricity is directly transmitted through a substance when there is a difference of temperature or of electrical potential between adjoining regions, without movement of the material.
Convection: The movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat.
Radiation: The emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles that cause ionization. In heat transfer, this specifically refers to thermal radiation (like infrared).
Thermal Insulator: A material that reduces or prevents the transfer of heat.