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 the molar volume. More rigorously, the mean free path of a molecule in a gas is given by formula_77 where "σ" is the collision cross section. So The heat capacity per particle "c" and the cross section "σ" both are temperature independent so the temperature dependence of "k" is determined by the "T" dependence of "v". For a monatomic gas, with atomic mass "M", "v" is given by formula_79 So This expression also shows why gases with a low mass (hydrogen, helium) have a high thermal conductivity. For metals at low temperatures the heat is carried mainly by the free