Electric current electrons into the conduction band. The current-carrying electrons in the conduction band are known as "free electrons", though they are often simply called "electrons" if that is clear in context. Current density is a measure of the density of an electric current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI units, the current density is measured in amperes per square metre. where "formula_6" is current in the conductor, formula_7 is the current density, and formula_8 is the differential cross-sectional area vector. The current density (current per unit area) "formula_7" in materials
Imaginary unit "imaginary" is used because there is no real number having a negative square. There are two complex square roots of , namely and , just as there are two complex square roots of every real number other than zero, which has one double square root. In contexts where is ambiguous or problematic, or the Greek is sometimes used (see ). In the disciplines of electrical engineering and control systems engineering, the imaginary unit is normally denoted by instead of , because is commonly used to denote electric current. For the history of the imaginary unit, see . The imaginary number