Thermal conductivity the list of thermal conductivities. These should be considered approximate due to the uncertainties related to material definitions. High energy generation rates within electronics or turbines require the use of materials with high thermal conductivity such as copper "(see: Copper in heat exchangers)", aluminium, and silver. On the other hand, materials with low thermal conductance, such as polystyrene foam and fire brick, are used in building construction or in furnaces in an effort to slow the flow of heat, i.e. for insulation purposes. The reciprocal of thermal conductivity is "thermal resistivity", usually expressed in kelvin-meters per watt (K⋅m⋅W). For a
Electrical resistivity and conductivity individual matrix elements are not necessarily reciprocals of one another; for example, "σ" may not be equal to 1/"ρ". This can be seen in the Hall effect, where formula_34 is nonzero. In the Hall effect, due to rotational invariance about the z-axis, formula_35 and formula_36, so the relation between resistivity and conductivity simplifies to: If the electric field is parallel to the applied current, formula_34 and formula_40 are zero. When they are zero, one number, formula_41, is enough to describe the electrical resistivity. It is then written as simply formula_9, and this reduces to the simpler expression. According to elementary