Electrical conductivity describes a material’s ability to allow the movement of electric charge through it. This flow of charge, known as an electric current, requires the presence of mobile charge carriers within the material. The ease with which these carriers can move dictates whether a substance is a conductor, an insulator, or something in between.
The Role of Free Electrons
The high conductivity seen in metals like copper, silver, and gold is primarily due to the presence of free electrons. In these materials, the outermost electrons of each atom are not tightly bound to a single nucleus, but are instead delocalized and shared across the entire metallic structure. This creates a vast “sea of electrons” that can move throughout the material’s lattice of positively charged metal ions.
When an electrical potential difference, or voltage, is applied across a metal, these mobile electrons are compelled to move in a coordinated direction. They act as the charge carriers, readily flowing through the material to constitute an electric current. Copper, widely used in wiring, is an excellent conductor due to its large number of loosely held valence electrons. This unimpeded movement results in the material’s low electrical resistance.
The unique structure of graphite, a form of carbon, also allows for this type of electronic conduction, even though it is not a metal. Its carbon atoms are arranged in flat layers, and one electron from each atom in these layers is free to move. These delocalized electrons enable graphite to conduct electricity along the planes of its layers, similar to how a metal conducts.
The Movement of Ions in Solution
Ionic conduction drives the flow of electricity in liquids like salt water or molten salts. The charge carriers are not electrons but entire atoms or molecules that carry a net positive or negative charge, called ions. This mechanism is also responsible for electrical signals in biological systems.
When a salt like sodium chloride dissolves in water, the strong attraction between the water molecules and the salt’s components overcomes the ionic bond, causing the salt to dissociate. This process separates the crystal into positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). These now-mobile ions are free to move throughout the solution.
When a voltage is applied to this solution, the positive ions migrate toward the negative electrode, and the negative ions move toward the positive electrode. This physical movement of charged atoms constitutes the electric current in an electrolyte solution. In a molten salt, heat energy melts the compound, freeing the ions to move and conduct charge.
Factors That Influence Electrical Flow
Several factors modulate how easily charge carriers flow through a material. Temperature is a significant factor, but its effect differs dramatically between conductors. In metals, increased temperature causes the atoms of the lattice structure to vibrate more intensely, obstructing the path of electrons and lowering conductivity.
Conversely, for ionic solutions and molten salts, rising temperature typically increases conductivity. The added thermal energy causes the ions to move faster, increasing their mobility and the frequency with which they can carry charge. For solutions, the concentration of the dissolved electrolyte is also directly influential; a higher concentration means more mobile ions are available to carry the charge, leading to greater conductivity.
The presence of impurities or defects in a material’s structure can also impede the flow of charge. Introducing a different type of atom into a pure metal, as in an alloy, disrupts the ordered crystal structure. This disruption increases the scattering of free electrons, reducing the material’s overall conductivity.
Why Some Materials Resist Flow
Materials that significantly resist the flow of electric charge are known as insulators. Substances like rubber, glass, and pure water are poor conductors because they fundamentally lack mobile charge carriers. Their electrons are tightly bound within the atoms, usually held in place by strong covalent or ionic bonds, and are not free to move when a voltage is applied.
This characteristic is quantitatively described by resistivity, which is the inverse of conductivity. Insulators exhibit extremely high resistivity. This high resistivity is why insulating materials are used to safely contain and direct electric current in applications like cable sheathing and circuit boards.