Current density Current density In electromagnetism, current density is the electric current per unit area of cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional area at a given point in space, its direction being that of the motion of the charges at this point. In SI units, the electric current density is measured in amperes per square metre. Assume that "A" (SI unit: m) is a small surface centred at a given point "M" and orthogonal to the motion of the charges at "M". If "I" (SI unit: A) is the
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