Electrical Current

Definition of Current—Measuring the Flow of Electrical Charge

Electrical current is a measure of the amount of electrical charge transferred per unit of time. It represents the flow of electrons through a conductive material, such as a metal wire. It is measured in amperes.

Units and Notation for Electrical Current

The SI unit of electrical current is the ampere, defined as 1 coulomb/second. Current is a quantity, meaning it is the same number regardless of the direction of the flow, without a positive or negative number.

However, in circuit analysis, the direction of current is relevant.

The conventional symbol for current is I, which originates from the French phrase intensité de courant, meaning current intensity. Current intensity is often referred to simply as current.

The I symbol was used by André-Marie Ampère, after whom the unit of electric current is named. He used the I symbol in formulating Ampère's force law in 1820. The notation traveled from France to Great Britain, where it became standard, although at least one journal did not change from using C to I until 1896.

Ohm's Law Governing Electrical Current

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:


In this relationship, I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant and is independent of the current.

Ohm's law is used in electrical engineering for solving circuits.

AC and DC Electrical Current

The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage. These are the two main types of electrical current.

Direct current

Direct current (DC) is the unidirectional flow of electric charge. The electric charge flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was galvanic current.

Direct current is produced by sources such as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams.

Alternating current

In alternating current (AC, also ac), the movement of electric charge periodically reverses direction. In direct current, the flow of electric charge is only in one direction.

AC is the form of electric power delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave. Certain applications use different waveforms, such as triangular or square waves.

Audio and radio signals carried on electrical wires are also examples of alternating current. An important goal in these applications is ​the recovery of information encoded (or modulated) onto the AC signal.