Thermal conduction

Thermal conduction is the diffusion of thermal energy, or heat, within a material or between materials in physical contact. It is one of three primary methods of heat transfer, alongside convection and radiation. Convection involves the macroscopic movement of fluids, while radiation uses electromagnetic waves like visible light. Conduction, however, relies on direct contact between particles. This process is vital because heat spontaneously flows along a temperature gradient. This means heat moves from a hotter body to a colder body. Without this movement, systems would not reach thermal equilibrium, which is the state where kinetic energy is distributed evenly throughout an object.

At the microscopic level, conduction is driven by molecular collisions and energy transfer. In a hotter region, molecules experience greater agitation and possess more kinetic energy. When a hot object touches a cooler surface, its high-energy molecules bump into the slower molecules of the cooler surface. These collisions transfer kinetic energy, which increases the temperature of the colder object. Mathematically, this process can be described as a form of diffusion. The rate of heat transfer, or power, depends on several specific variables. It is influenced by the thermal conductivity of the material, the cross-sectional area, the temperature difference, and the distance over which the heat travels.

The efficiency of conduction depends heavily on the state of matter and its molecular structure. Conduction is the primary mode of heat transfer for solids. In solids, strong inter-molecular forces allow particle vibrations to be transmitted easily. In contrast, liquids and gases are poor conductors. Liquids have weaker inter-molecular forces and more space between particles, making vibrations harder to pass. Gases have even more space, resulting in infrequent particle collisions. Because of these physical gaps, the transfer of energy is much slower in fluids than in solids.

Different materials utilize different mechanisms to move thermal energy. In metals, such as copper, platinum, or gold, heat moves very rapidly due to metallic bonds. These bonds create a fluid of free-moving electrons that transfer energy quickly through the solid. While phonon flux is also present in metals, the moving electrons carry most of the heat. In insulators, however, heat flux is carried almost entirely by phonon vibrations. A phonon is a type of vibration that travels through the material. Additionally, in fluids, heat transfer can occur through elastic impacts between molecules.

Engineers use a specific value to quantify how easily a medium conducts heat. This is called thermal conductivity, or the conduction coefficient, represented by the symbol k. This property is determined by the medium's phase, temperature, density, and molecular bonding. It is defined as the quantity of heat transmitted in a specific time through a certain thickness. Another related measure is thermal effusivity, which describes a material's ability to exchange thermal energy with its surroundings. Understanding these properties is essential for managing thermal resistance at the interfaces where two different materials meet.

Thermal conduction can be categorized into two distinct states: steady-state and transient. Steady-state conduction occurs when the temperature differences driving the heat flow remain constant. In this state, the temperature at any specific point in the object does not change over time. The amount of heat entering any region is exactly equal to the amount of heat leaving it. An example is a metal bar with one hot end and one cold end that has reached a stable temperature distribution. In these systems, the laws of direct current electrical conduction can be applied to model heat currents using thermal resistances.

Transient conduction, also known as non-steady-state conduction, occurs when temperatures change over time. This happens when a new heat source is introduced or an external temperature changes. For instance, an automobile engine starting causes transient conduction as the machine warms up. As the system moves toward a new equilibrium, the temperatures change until a steady state is reached. However, some transient processes do not lead to a steady state. If a hot copper ball is dropped into cool oil, the heat leaves the ball until it reaches the same temperature as the oil. In this case, the conduction eventually stops entirely once the temperature gradient disappears.