Electric current Electric current An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionised gas (plasma). The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second. Electric current is measured using a device called an ammeter. Electric currents cause Joule heating, which creates light in incandescent light bulbs. They
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