Electrical conductivity describes a material’s ability to allow electric current to pass through it; resistivity is the measure of its opposition to that flow. These two concepts are inversely related. Conductivity depends on two factors: the concentration of available charge carriers and their mobility through the material when an electric field is applied. The relationship between conductivity and temperature is complex, depending entirely on the material’s class. Raising the temperature does not affect all materials the same way; some become better conductors while others become poorer conductors.
Temperature’s Effect on Electrical Conductivity in Metals
In metals, increasing the temperature causes electrical conductivity to decrease, corresponding to an increase in resistivity. Metals like copper and silver are excellent conductors because they possess a vast, fixed concentration of highly mobile charge carriers (free electrons). Since these electrons are already available for current flow, adding heat does not significantly increase their number.
The primary factor affecting conductivity is electron mobility. A metal’s structure consists of a rigid, repeating lattice of positively charged atomic nuclei. As the metal absorbs thermal energy, the atoms in this lattice vibrate more intensely. These vigorous lattice vibrations act as obstacles, interfering with the directional movement of the free electrons.
The electrons collide more frequently with the vibrating atomic sites, an event known as scattering. This increased scattering significantly lowers the electron’s mobility. Because the number of charge carriers remains constant, the decrease in mobility is the dominant effect, resulting in a decline in the metal’s overall electrical conductivity as temperature rises.
Temperature’s Effect on Electrical Conductivity in Semiconductors
Semiconductors, like silicon and germanium, exhibit the opposite behavior to metals, with their conductivity increasing dramatically as temperature rises. Conductivity depends on the presence of both electrons and positive charge carriers called holes. At low temperatures, nearly all electrons are locked in covalent bonds, and the material acts almost as an insulator because few charge carriers are available.
Current flow in semiconductors involves a competition between carrier generation and carrier mobility. As thermal energy is introduced, it provides the necessary energy to break covalent bonds within the crystal structure. This process excites electrons across the energy band gap into the conduction band, simultaneously leaving a hole behind.
This thermal excitation causes an exponential increase in the concentration of mobile charge carriers. Similar to metals, increased thermal vibration of the lattice causes scattering, which reduces the mobility of these carriers.
However, the increase in carrier concentration far outweighs the decrease in mobility. Because the initial concentration of carriers is so low, the thermal generation of new electron-hole pairs provides a much greater proportional boost to the material’s total conductivity. The dominant factor is the exponential growth in carrier concentration, resulting in the material becoming a better electrical conductor as temperature increases.
Temperature’s Effect on Conductivity in Electrolytes
Electrolytes, such as ionic solutions, see an increase in conductivity with rising temperature, but through a different physical process than in solids. In these liquid systems, current is carried by ions (charged atoms or molecules), rather than by free electrons.
The movement of these ions is hindered by the solvent’s viscosity. As the temperature increases, the kinetic energy of the solvent molecules rises, causing the liquid to become less viscous. This reduction allows the charged ions to move more quickly and freely.
The greater speed and easier movement of the ions directly translates to increased mobility. Increased thermal energy can also slightly increase the extent to which the solute dissociates into ions, adding a small boost to the charge carrier concentration. The combined effect of increased ion mobility and modest concentration increase causes the electrical conductivity of the electrolyte to rise with temperature.