Electric current

Electric current describes the amount of net electric charge crossing a surface per unit time. For an average current, I = ΔQ/Δt; for a time-varying current, the instantaneous value is I = dQ/dt. The SI unit is the ampere (A), named for physicist André-Marie Ampère. One ampere corresponds to one coulomb of charge passing a point each second (1 A = 1 C/s). The 2019 SI definition fixes the elementary charge at an exact value and defines the ampere through that constant.

Current is movement of electric charge. Depending on the material, its carriers can include electrons, ions or electron deficiencies called holes. Circuit diagrams and calculations generally use conventional current: the direction in which positive charge would move. In a metal wire, mobile electrons drift in the opposite direction, but this does not change the convention or the measured current.

In direct current (DC), the net charge flow has a fixed direction; its magnitude may still vary with time. In alternating current (AC), the direction of charge motion periodically reverses. Mains electricity is commonly supplied as AC, while batteries provide DC. The current through a component depends on the circuit and the electrical conditions applied to it.

In a circuit, current is sustained when charges have a complete conducting path and an energy source, such as a battery or generator, maintains a potential difference. An electric field in the conductor drives charge motion. Current is related to voltage and resistance; for an ohmic conductor under specified conditions, Ohm's law is V = IR. A break in a simple series path prevents sustained current through that path.

Current is a central quantity in electrical engineering and physics. Component ratings, wires, fuses and circuit breakers are specified in relation to current because excessive current can heat conductors and damage equipment. In biological tissue, the effects of electric shock depend on the current through the body as well as its path and duration, so a voltage value alone does not describe the complete hazard.