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Formulas/physics/Current Electricity/Electric Current (Definition)

Electric Current (Definition)

Current is the rate of flow of charge. SI unit: ampere (A = C s⁻¹). Conventional current flows from high to low potential; electrons flow oppositely.
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Derivation

Definition

If charge dQdQ flows past a cross-section in time dtdt:

I=dQdt\boxed{I = \frac{dQ}{dt}}

For steady current, I=Q/tI = Q/t.

Conventional vs electron current

Electrons (negative charge) drift opposite to the electric field. By historical convention, current direction is taken as the direction positive charges would move — i.e., opposite to electron flow. This convention is retained universally.

Sign

Current is a scalar with sign. In circuit analysis, a chosen reference direction is assigned; negative result means actual flow is opposite.

Units

[I]=Cs=A (ampere)[I] = \frac{\text{C}}{\text{s}} = \text{A (ampere)}

The ampere is a base SI unit, defined since 2019 by fixing the elementary charge e=1.602176634×10−19e = 1.602176634 \times 10^{-19} C.

Note
Current is not a vector, but current density $\vec{J}$ is. The scalar current $I$ through a surface is $\int \vec{J} \cdot d\vec{A}$ — it depends on orientation of the surface.