3.3 Electric Current

What is electric current?

Electric current (I) is the rate at which charge flows past a point in a circuit: I = ΔQ/Δt, measured in amperes (A). Conventional current is defined as the direction positive charge would move, even though in metal wires it's actually negatively charged electrons moving in the opposite direction.

Current, drift velocity, and area

Current depends on the number of charge carriers, their charge, their drift velocity (v_d), and the wire's cross-sectional area (A): I = nqv_dA, where n is charge carrier density. Even though electrons drift slowly, the electric field driving them propagates through the wire nearly instantaneously.

Conservation of charge in circuits

Current is a conserved quantity — charge cannot be created or destroyed. In a series circuit, the same current flows through every element. At any junction, the total current entering equals the total current leaving, consistent with Kirchhoff's junction rule.

Key terms

Electric current (I)

Rate of charge flow past a point: I = ΔQ/Δt, measured in amperes (A).

Conventional current direction

Defined as the direction positive charge moves, opposite to actual electron flow in metals.

Drift velocity

The slow average net velocity of charge carriers (electrons) through a conductor.

I = nqv_dA

Current equals carrier density × charge × drift velocity × cross-sectional area.

Kirchhoff's junction rule

Total current entering a junction equals total current leaving it, due to conservation of charge.

Try an AP-style question

6 C of charge passes a point in 2 s. What is the current?

1. 8 A
2. 12 A
3. 3 A
4. 0.33 A