Electrical resistivity and conductivity ions. In biological membranes, currents are carried by ionic salts. Small holes in cell membranes, called ion channels, are selective to specific ions and determine the membrane resistance. The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing in a loop of superconducting wire
Thermal conductivity Time variation due to phonon decay is described with a relaxation time ("τ") approximation which states that the more the phonon number deviates from its equilibrium value, the more its time variation increases. At steady state conditions and local thermal equilibrium are assumed we get the following equation Using the relaxation time approximation for the Boltzmann equation and assuming steady-state conditions, the phonon thermal conductivity "λ" can be determined. The temperature dependence for "λ" originates from the variety of processes, whose significance for "λ" depends on the temperature range of interest. Mean free path is one factor that determines the