An electric current flows when charged particles move under the influence of an electric field. An electric field is a voltage gradient—a virtual "slope" down which current can flow—and the strength of an electric field is measured in volts per unit distance, typically, volts per meter.
If the terminals of a battery are connected by a conducting wire, the voltage across the terminals creates an electric field in that wire. A more exact term for this voltage is potential difference. The strength of the electric field in the wire increases if the potential difference increases or if a shorter wire is used. The increased electriield strength encourages a stronger current to flow.
A conductor has electrically charged particles that are free to move under the influence of electric fields. Negatively charged particles may be electrons or they may be single or grouped atoms that have more electrons than protons. Such species are called anions. Positively charged particles include single or grouped atoms that have fewer electrons than protons. Under the extreme conditions of a hydrogen plasma—a hot, ionized gas—free protons can carry positive charge. Since the mass of a proton is some 1,800 times that of an electron, a proton will accelerate correspondingly less than an electron in an electric field.
If a conductor possesses both positively and negatively charged particles that are free to move, the positively charged particles will drift toward the negative electrode, or cathode, while the negatively charged particles drift toward the positively charged electrode, or anode. The total current is the sum of the individual currents of negatively and positively charged particles.
Mechanisms of conduction
Most materials, whether they are solids, liquids, or gases, consist of atoms and molecules that have no electrical charge. Each nucleus of protons and neutrons is surrounded by the correct complement of electrons to exactly balance the number of protons. Such structures would seem to preclude the conduction of electrical currents, which requires that electrically charged species are free to move under the influence of an electric field.
In fact, there are three ways in which materials can conduct electricity. Some materials exist as positive and negative ions, whose charges balance each other. In the solid state, these ions form a rigid lattice and cannot conduct. If such a solid melts or dissolves in a solvent, its charged particles can move freely and conduct a current. In a second mechanism, an electric field can split the neutral molecules of a gas or liquid into charged fragments that can conduct electricity. Finally, the electrons of certain substances—metals and graphite among them—form electron "clouds" and are easily moved by an electric field.
Classes of conductors and insulators
Most materials are classified as conductors or insulators, depending on the relative ease with which they conduct currents. In fact, all common conductors have at least some resistance to the passage of a current, and most insulators conduct if subjected to a sufficiently strong field.
Certain materials become superconductors at low temperature: they provide no resistance to the flow of current. Semiconductors, on the other hand, are poor conductors whose conductivity increases if traces of impurities are added.
