Electric current

Electric current is the flow of electric charge in a conductor, typically measured in amperes (A). It represents the movement of electrons through a material, and this movement can occur in various forms, such as direct current (DC), where the flow of charge is uniform and directional, or alternating current (AC), where the flow periodically reverses direction. In a circuit, electric current is driven by a voltage difference (potential difference) created by a power source, such as a battery or generator.

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Codex  0 Created 2026-09-24 Updated 2026-10-14

S

. Choose its unit normal

n

; positive electric charge crossing along

n

counts positively, while crossing against it counts negatively. If

Q_{cross} (t)

is the accumulated signed electric charge transported through

S

, then

I_{S} (t) = \frac{d Q _{cross}}{d t} = \int_{S} J (r, t) \cdot n d A, Q_{cross} (t_{2}) - Q_{cross} (t_{1}) = \int_{t_{1}}^{t_{2}} I_{S} (t) d t .

Here

J

is the current density; the surface integral collects its local normal contributions. Reversing the unit normal reverses

I_{S}

. The electric current is a signed scalar associated with the whole chosen oriented surface, whereas current density is a vector at each point. The SI unit is the ampere, one coulomb per second. ^[ref]; ^[ref].

For a constant electric current of

2 A

over a time interval of

3 s

, the transported electric charge is

6 C

. This need not be electric charge accumulating in the wire: in steady transport, electric charge enters and leaves. For a closed oriented surface with outward unit normal, electric charge conservation instead gives

I_{out} = - d Q_{inside} / d t

. Keeping transported and enclosed electric charge distinct connects electrical circuit bookkeeping to the local continuity equation.

In the context of Maxwell's equations, it is vector field that is one of the inputs of the equation.

Section "Maxwell's equations with pointlike particles" asks if the theory would work for pointlike particles in order to predict the evolution of this field as part of the equations themselves rather than as an external element.