When one object touches another, the transfer of energy that occurs is called conduction.
Thermal energy is the internal energy of a system related to the motion of its particles, and heat is the transfer of this energy between systems due to a temperature difference. Every object is composed of atoms and molecules that are constantly moving and vibrating, representing their kinetic energy. When a hotter object, with faster-moving particles, comes into contact with a cooler object, energy begins to flow from the higher-temperature region to the lower-temperature region until both objects reach thermal equilibrium.
The Transfer Method: Conduction
Conduction is the process of thermal energy diffusion that requires direct physical contact between two objects or different parts of a single object. It is the primary method of heat transfer within and between solid materials. The transfer of energy moves through the materials without any macroscopic movement of the matter itself.
For example, when a metal spoon is placed into a hot bowl of soup, heat travels up the handle through conduction. The atoms in the solid material remain in their fixed positions, only increasing their vibration.
Molecular Mechanism of Energy Flow
The mechanism of conduction operates at the atomic level, depending on the constant vibration and collision of particles. In a hotter material, atoms and molecules possess greater kinetic energy, causing them to vibrate more rapidly around their fixed positions. When this energetic material touches a cooler one, the vibrating particles at the boundary collide with the slower-moving particles of the adjacent material.
These collisions transfer kinetic energy from the faster-vibrating particles to the slower ones, much like a microscopic game of billiards. The newly energized particles then begin to vibrate faster and, in turn, collide with their neighbors, propagating the thermal energy throughout the cooler object.
In metals, heat conduction is accelerated by the presence of free electrons, which are not tightly bound to individual atoms. These electrons move freely throughout the material, carrying thermal energy much faster than vibrational collisions alone. The movement of these highly mobile electrons is why metals are efficient at transferring heat.
Why Materials Matter: Conductors and Insulators
Materials differ widely in their ability to facilitate heat transfer, which categorizes them as either thermal conductors or thermal insulators. A thermal conductor is a material that allows thermal energy to pass through it easily and quickly. Metals such as copper, silver, and aluminum are excellent examples.
These materials are valued for applications like cooking, where a fast and efficient transfer of heat is desired, such as in the base of a pot or pan. Conversely, a thermal insulator is a material that resists the transfer of thermal energy. Insulators often lack the free electrons found in metals or have a structure, like air pockets in foam or wood, that limits the frequency of particle collisions.
Common examples of insulators include wood, plastic, rubber, and dry air. These materials are used to slow down heat flow, such as the plastic handles on metal cookware or the insulation within building walls. While insulators do not completely stop conduction, they dramatically reduce the rate at which energy is transferred.
Differentiating the Three Modes of Heat Transfer
Conduction is one of three primary ways thermal energy is transferred, and it is uniquely defined by the requirement of physical contact. The other two modes, convection and radiation, transfer heat through different mechanisms.
Convection is the transfer of heat through the movement of fluids, which include liquids and gases. This process involves the bulk movement of the heated fluid itself, where warmer, less dense fluid rises and cooler, denser fluid sinks, creating a circulating current. An example is the rising of hot air above a heater or the boiling of water in a pot.
Radiation is the transfer of energy by electromagnetic waves, which requires no medium or physical contact between objects. This energy can travel through a vacuum, which is how the sun’s heat reaches the Earth. The warmth felt from a distant fire or a heat lamp is transferred entirely through thermal radiation.