Conduction, Convection, and Radiation
Students compare the three mechanisms of thermal energy transfer and identify each mechanism in everyday situations.
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Heat Moves Between Objects
Thermal energy is the energy associated with the motion of particles in matter. Temperature measures the average kinetic energy of those particles. When objects at different temperatures interact, thermal energy moves from the warmer object to the cooler object. The transfer continues until both objects reach the same temperature, a condition called thermal equilibrium. For example, an ice cube placed in warm water gains thermal energy while the water loses thermal energy. The ice may melt, and the water becomes cooler. Energy is transferred, but it is not created or destroyed. Thermal energy can move in three main ways: conduction through direct particle contact, convection through the movement of fluids, and radiation through electromagnetic waves. The method of transfer depends on the materials and conditions involved.
Conduction
Conduction is the transfer of thermal energy through direct contact between particles. When particles in a warmer region move or vibrate faster, they collide with nearby particles and transfer some energy to them. Conduction is especially effective in solids because their particles are packed closely together. Metals are good thermal conductors, while materials such as wood, plastic, foam, and air are poor conductors called insulators. Consider a metal spoon resting in a pot of hot soup. Thermal energy moves from the soup into the submerged end of the spoon and then along the spoon toward the handle. After a while, the handle feels warm even though it never touched the soup. A wooden spoon warms much more slowly because wood does not conduct thermal energy as readily as metal.
Convection
Convection is thermal energy transfer caused by the movement of a fluid, which is a liquid or gas. When part of a fluid is heated, its particles move faster and usually spread farther apart. The heated fluid becomes less dense and rises. Cooler, denser fluid sinks and takes its place. This continuous circulation forms a convection current. In a pot of water on a stove, water near the bottom gains thermal energy from the pot. That warmer water rises while cooler water near the top sinks. The moving water spreads thermal energy throughout the pot. Convection also occurs in air. For example, air warmed by a room heater rises, while cooler air sinks toward the heater. Unlike conduction, convection transfers energy partly through the bulk movement of matter and can occur only in liquids and gases.
Radiation
Radiation is the transfer of energy by electromagnetic waves. Thermal radiation is mainly infrared radiation, although very hot objects can also give off visible light. Unlike conduction and convection, radiation does not require matter, so it can travel through empty space. The Sun transfers energy across space to Earth by radiation. When sunlight reaches the ground, water, or your skin, the energy may be absorbed and converted into thermal energy. You can also feel radiation from a campfire without touching the flames or standing directly above them. Infrared waves travel outward from the fire and warm nearby objects. All objects emit some thermal radiation, but hotter objects usually transfer more energy by radiation than cooler objects. Dark, dull surfaces often absorb radiation better than light, shiny surfaces, which reflect more of the incoming radiation.
Everyday Heat Transfer
Conduction, convection, and radiation often occur together in everyday situations. When soup is heated on a stove, radiation and hot gases from the burner transfer energy toward the pot. Conduction moves energy through the metal bottom and into the soup that touches it. Convection currents then circulate the soup, carrying warmer liquid upward and cooler liquid downward. The three mechanisms differ in how energy moves. Conduction requires direct particle contact and is most effective in many solids. Convection requires the movement of a liquid or gas. Radiation travels as electromagnetic waves and can cross empty space. To identify a mechanism, ask what is carrying the energy. Direct contact suggests conduction, circulating fluid suggests convection, and energy arriving by waves suggests radiation. A single event may include more than one mechanism, so examine each part of the energy pathway.