Understanding how heat moves is as important as understanding what heat does. Whenever a temperature difference exists, heat flows. This can happen quickly, like when a hot pan warms your hand through a metal handle, or slowly, like when heat seeps through the insulation of a cooler. We manage heat transfer in everyday life—by choosing insulating materials (like winter jackets), limiting air movement (with weather stripping), or using reflective surfaces (such as white roofs to stay cool in summer).

Remarkably, all heat transfer occurs through just three basic mechanisms:

1. Conduction: Heat transfer through direct contact in stationary matter. For example, the bottom of a metal pot in contact with a hot stove transfers energy via particle collisions from hot to cold regions.
2. Convection: Heat transfer through the bulk motion of fluids (liquids or gases). This happens in boiling water, blood circulation, and weather systems.
3. Radiation: Heat transfer via electromagnetic waves (like infrared radiation or visible light). Radiation doesn’t need a medium—it can travel through the vacuum of space, as when the Sun warms the Earth.

Each of these mechanisms has distinct properties, but all transfer heat as a result of temperature differences (see Figure 103.1). In the upcoming modules, we’ll examine conduction, convection, and radiation in greater detail, including how they apply to biological systems like skin, blood flow, and metabolism.

Summary

• Heat transfer always results from a temperature difference and occurs via one or more of three methods: conduction (contact), convection (fluid motion), and radiation (electromagnetic waves).