18.12 : Mechanisms of Heat Transfer II

In convection, thermal energy is carried by the large-scale flow of matter. Ocean currents and large-scale atmospheric circulation, which result from the buoyancy of warm air and water, transfer hot air from the tropics toward the poles and cold air from the poles toward the tropics. The Earth’s rotation interacts with those flows, causing the observed eastward flow of air in the temperate zones. Convection dominates heat transfer by air, and the amount of available space for the airflow determines whether the air transfers heat rapidly or slowly. There is little heat transfer in a space filled with air with a small amount of other material that prevents flow.

The space between the Earth and the Sun is largely empty, so the Sun warms us without any possibility of heat transfer by convection or conduction. In this example, heat is transferred by radiation. That is, the hot body emits electromagnetic waves that are absorbed by the skin. No medium is required for the electromagnetic waves to propagate.

Most of the heat transfer from a fire to the observers occurs through infrared radiation. The visible light transfers relatively little thermal energy. The energy of electromagnetic radiation varies over a wide range depending on the wavelength, with shorter wavelengths (or higher frequencies) corresponding to higher energy. As more heat is radiated at higher temperatures, higher temperatures produce more intensity at every wavelength, especially at shorter wavelengths. In visible light, the wavelength determines the color—red has the longest wavelength and violet the shortest. Therefore, temperature changes are accompanied by color changes. For example, the electric heating element on a stove glows from red to orange, while the higher-temperature steel in a blast furnace glows from yellow to white.