Electrical conductivity is a fundamental property of matter that describes how readily a material allows the flow of electric current. It quantifies the ease with which electric charge moves through a substance when a voltage is applied. Materials with high conductivity are good conductors, while those with low conductivity act as insulators. This property is foundational to materials science and the design of modern technologies.
Quantifying Conductivity
Conductivity is an intrinsic property of a material, meaning it does not depend on the object’s shape or size. It is mathematically defined as the reciprocal of electrical resistivity (\(\rho\)), which measures how strongly a material resists current flow. Therefore, low resistivity corresponds to high conductivity.
The standard international (SI) unit for measuring conductivity is the Siemens per meter (S/m). Materials are classified based on their conductivity magnitude: metals like copper are conductors (high values), materials like glass are insulators (low values), and semiconductors fall between these two extremes.
The Role of Charge Carriers
The flow of electric current requires the presence and movement of subatomic particles known as charge carriers. The nature of these carriers depends on the material’s state and chemical composition. Charge transport is typically categorized into electron conduction and ionic conduction, which explains the diverse conductivity levels across different materials.
Conduction in Solids
In solid metallic conductors, the charge carriers are free-moving electrons, often called conduction electrons. These valence electrons are delocalized from their parent atoms and form a “sea” that can move easily throughout the material’s crystal lattice structure. The high density of these mobile electrons is the primary reason metals are known for their excellent conductivity. Insulating solids, such as ceramics or polymers, lack these easily detachable electrons, which significantly restricts the movement of charge and results in very low conductivity.
Conduction in Liquids/Solutions
In liquids, particularly in solutions known as electrolytes, the charge carriers are ions, which are atoms or molecules that carry a net electrical charge. When salts or acids dissolve in water, they dissociate into positive ions (cations) and negative ions (anions). When an electric field is applied, these charged particles move in opposite directions—cations toward the negative electrode and anions toward the positive electrode—thereby constituting the electric current. The total conductivity of the solution depends directly on the concentration and mobility of these dissolved ions.
Variables That Affect Conductivity
A material’s measured conductivity value is not constant and can be significantly modified by external factors. Temperature is one of the most prominent variables, though its effect contrasts sharply between different classes of materials.
In most pure metals, an increase in temperature causes a decrease in conductivity. Increased thermal energy causes metal atoms to vibrate more vigorously within the lattice structure. These greater vibrations increase the frequency of collisions with the flowing conduction electrons, scattering them and impeding their movement. This increased scattering raises the material’s electrical resistance.
The opposite effect is observed in electrolytic solutions and semiconductors. For electrolytes, higher temperatures decrease viscosity and increase the kinetic energy of the ions, allowing them to move more rapidly and increasing mobility. In semiconductors, a temperature increase provides energy for more electrons to break free into the conduction band, increasing the number of available charge carriers and raising conductivity.
Impurities and Concentration
The presence of foreign atoms or chemical species also strongly modifies conductivity. In solid metals, introducing impurities or alloying elements disrupts the crystal lattice structure, creating defects that scatter electrons and reduce conductivity.
In contrast, the process of “doping” in semiconductors involves intentionally introducing trace impurities to precisely control the number of free electrons or “holes.” This allows for the fine-tuning of the material’s conductivity for electronic devices.
In liquid solutions, the total concentration of dissolved ionic substances directly dictates the conductivity. Adding more salt to water, for example, increases the density of charge carriers, resulting in a higher overall conductivity.