Electrical conductivity is a fundamental physical property that quantifies a material’s capacity to allow the flow of electric current. This measure describes how readily a substance permits the transport of electric charge when a voltage is applied across it. Materials with high conductivity are conductors, while those with very low conductivity function as insulators. This ability is determined by the internal atomic structure and the availability of mobile, charged particles within the material.
Defining the Movement of Charge
The flow of electric current requires the movement of charged particles, which are collectively known as charge carriers. Without a supply of mobile charges, a material cannot sustain an electric current, regardless of the applied voltage. The nature of these charge carriers dictates the two primary forms of electrical conduction: metallic and electrolytic.
In solid conductors, particularly metals, the charge is transported almost exclusively by electrons. These electrons are not bound to individual atoms but are instead free to travel throughout the entire material structure. This electronic movement constitutes metallic conduction, where the substance itself does not undergo any chemical change or physical transfer of mass.
Conversely, in liquids, such as solutions or molten salts, the charge is carried by atoms or molecules that have gained or lost electrons, becoming electrically charged ions. This mechanism, known as electrolytic conduction, involves the physical migration of matter. The positive and negative ions physically move toward opposite poles, enabling the current flow.
The distinction between these two forms of charge movement affects how the material behaves under temperature changes and the potential for chemical reactions. The mobility of electrons in metallic conductors is significantly higher than the mobility of ions in a liquid. Therefore, electrolytic conductors are generally less conductive than common metallic conductors.
How Electrons Flow in Solid Materials
In solid metals, the atoms are bound together by a unique structure called metallic bonding. This bonding mechanism is described by the “electron sea” model, where the valence electrons—the outermost electrons of each atom—are not localized to their parent atom. Instead, these electrons are delocalized and form a shared cloud of negative charge that permeates the entire lattice structure.
The metallic structure consists of a fixed, crystalline lattice of positively charged metal ions immersed in this mobile sea of electrons. When a voltage is applied, the delocalized electrons are easily accelerated and drift in a unified direction, creating the electric current. This high density of mobile electrons accounts for the exceptional conductivity of metals.
The flow of electrons is not perfectly smooth, as they inevitably encounter the positive metal ions fixed in the lattice structure. This collision process impedes the electron movement and is the source of electrical resistance in the material.
As the temperature of the metal increases, the fixed ions vibrate more vigorously. Increased thermal vibration causes more frequent and disruptive collisions with the flowing electrons, thereby reducing their mobility and increasing the material’s resistance. This explains why the conductivity of metals decreases as they heat up.
Materials classified as electrical insulators, such as wood or glass, lack this sea of delocalized electrons because their valence electrons are tightly bound to individual atoms. These tightly bound electrons require enormous energy to be mobilized, effectively preventing any significant flow of charge. Semiconductors, such as silicon, represent a middle ground, where electrons are mostly bound but can be freed to conduct electricity under specific conditions, like exposure to heat or light.
The Role of Ions in Liquid Conductivity
In liquids, conductivity relies entirely on the presence and movement of mobile ions, a process that occurs primarily in solutions known as electrolytes. Electrolytes are substances, typically ionic compounds like salts, acids, or bases, that dissolve in a solvent, such as water, to produce free-moving ions. Dissolving involves the solvent molecules overcoming electrostatic forces, causing the compound to dissociate into its constituent positive and negative ions.
For example, when table salt dissolves in water, it separates into positively charged sodium ions and negatively charged chloride ions. These charged particles are then free to move throughout the solution, becoming the charge carriers. Pure water, in contrast, is a very poor conductor because it contains an extremely low concentration of naturally dissociated ions.
When an electric potential is applied to an electrolytic solution using two immersed electrodes, the mobile ions begin to migrate. The positively charged ions, called cations, are attracted to the negative electrode (cathode), while the negatively charged ions, called anions, move toward the positive electrode (anode). The simultaneous movement of both charge types in opposite directions constitutes the electric current within the liquid.
The conductivity of a liquid depends on several factors, including the concentration of the dissolved electrolyte and the mobility of the ions. Factors like ion size and solvent viscosity affect how easily the ions can move. Unlike metallic conduction, the electrolytic process is often accompanied by chemical changes at the electrodes as the ions reach them and exchange electrons.