thermal conductivity
- Key People:
- Anders Jonas Ångström
thermal conductivity, the ability of a substance to conduct heat or move heat from one location to another without the movement of the material conducting the heat. Thermal conductivity is measured in watts per meter kelvin (W/mK). For example, solid aluminum has a thermal conductivity of 237 W/mK at –73 °C (–99 °F), 236 W/mK at 0 °C (32 °F), and 232 W/mK at 327 °C (621 °F). Thermal conductivity of a material changes with temperature and can be related to changes in pressure, depending on the state of the material.
In physics, thermal conductivity is given the symbol k or λ. It represents one of three methods by which heat can be transferred from one location to another, the other two being convection and radiation. Fourier’s law of heat conduction gives the rate at which heat is transferred through thermal conductivity: q = –k∇T, where q is local heat flux density, and ∇T is the temperature gradient.
The process of thermal conductivity
When heat is conducted through a solid, heat moves through molecular or atomic agitation and contact between particles. Heat transfer is not the result of atoms or molecules moving through the solid. In a solid, heat moves along a difference in temperature (called a temperature gradient) from an area of high temperature and thus high particle agitation to an area of low temperature and thus low particle agitation. This transfer of heat energy continues until all the material comprising the solid is at thermal equilibrium, meaning the temperature throughout the material is the same. The time needed for this to occur depends on several factors, including the magnitude of the temperature difference within the material and the thermal characteristics of the material itself. These characteristics include the material’s atomic or molecular composition and the distance, known as the path length, through which the heat must move.
When the conducting material is a liquid or a gas, heat moves through particle agitation as well as through movement of the atoms or molecules themselves. Thermal conduction occurs fastest in a solid and slowest in a gas. When matter is in the gas phase, particles have larger distances between them and so collide less often. These collisions transfer thermal energy, so fewer collisions lead to a lower rate of heat conduction.
Examples of thermal conductivity
When cooks prepare food, they often use metal frying pans. As a pan heats on a stovetop, thermal energy from the stovetop is transferred to the metal on the bottom of the pan. This energy is then conducted throughout the pan, ultimately cooking the food. However, over time, the heat will also conduct to the handle of the pan. Therefore, frying pan manufacturers will often carefully select a material for the handle of a pan that is a poor conductor of heat so that cooks will not be in danger of burning their hands.
Thermal touch, or how the body senses temperature when touching a hot or cold object, also follows the principles of thermal conductivity. When a person touches an object, the temperatures of the object and skin change depending on both their properties and their initial temperatures.