Current, Conductors and Dielectrics

Electric current is the rate of transport of electric charge past a specified point ior across a specified surface. The symbol I is generally used for constant currents and i for time- variable currents. The unit of current is the ampere ( 1A= 1C/s; in the SI, the ampere is the basic unit and the coulomb is the derived unit).

Ohm's law relates current to voltage and resistance. For simple dc circuits, I= V/R. However, when charges are suspended in a liquid or a gas, or when both positive and negative charge carriers are present with different characteristics, the simple form of Ohm's law is insufficient. Consequently, the current density J (A/m2) receives more attention in electromagnetics than does current I.

Charges in Motion

Consider the force on a positively charged particlein an electricfield in vacuum. This force, F= +QE, is unsupposed and results in constant acceleration. Thus the charge moves in the direction of E with a velocity U that increases as long as the particle is in the E field. When the charge is in the liquid or gas, it collides repeateedly with particles in the medium, resulting in random changes in direction. But, for constant E and a homogeneous medium, the random velocity components cancel out leaving a constant average velocity, known s drift velocity U, along the direction of E. Conduction in metals takes place by movement of electrons in the outermost shells of the atoms making up the crystalline structure. according to the electron- gas theory, these electrons reach an average drift velocity in much the same way as a charged particle moving through a liquid or gas. The drift velocity isdirectly proportional to the electric field intensity,

where the mobility, has the units (m^2/V)(s). each cubic meter of a conductor contains on the order of 10^28 atoms. Good conductors have one or two electrons from each atom free to move upon application of the field. the mobility varies with temperature and the crystalline structure of the solid. The particles in the solid have a vibratory motion which increases with temperature. This makes it more difficult for the charges to move. Thus, at higher temperatures, the mobility is reduced, resulting in a smaller drift velocity (or current) for a given E. In the circuit analysis this phenomenon is accounted for by stating a resistivity for each material and specifying an increase in this resistivity with increasing temperature

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Conductivity

In a liquid or gas, there are generally present both positive and negative ions, some singly charged and others doubly charged, and possibly of different masses. A conductivity expression would include all such factors. however, if it is assumed that all the negative ions are alike and so too the positive ions, then the conductivity contains two terms. In metallic conductor, only the valence electrons are free to move. Figure below are shown in motion to the left. The conductivity then contains only one term, the product of the charge density of the electrons free to move and their mobility.

Dielectric

A dielectric is an electrical insulator that may be polarized by the action of an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization: positive charges are displaced along the field and negative charges shift in the opposite direction. This creates an internal electric field which partly compensates the external field inside the dielectric. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axis aligns to the field.

While the term "insulator" refers to a low degree of electrical conduction, the term "dielectric" is typically used to describe materials with a high polarizability. The latter is expressed by a number called the dielectric constant. A common, yet notable, example is that a dielectric is the electrically insulating material between the metallic plates of a capacitor. The polarization of the dielectric by the applied electric field increases the capacitor's capacitance.

The study of dielectric properties is concerned with the storage and dissipation of electric and magnetic energy in materials. It is important to explain various phenomena in electronics, optics, and solid-state physics.

The term "dielectric" was coined by William Whewell (from "dia-electric") in response to a request from Michael Faraday.