Current

Electric current — the rate of flow of electric charge:

I = ΔQ / Δt

Measured in amperes. One ampere is one coulomb per second.

Charge — measured in coulombs. The charge on one electron is −1.60 × 10⁻¹⁹ C, so one coulomb corresponds to about 6.25 × 10¹⁸ electrons.

Example: a charge of 45 C passing a point in 15 s gives a current of 45 ÷ 15 = 3.0 A.

Conventional current — defined as flowing from the positive terminal to the negative terminal of the supply. In a metal the charge carriers are electrons, which are negative and therefore flow the other way. The convention predates the discovery of the electron and has been kept because every equation and diagram is built on it. Use conventional current unless a question specifically asks about electron flow.

Current is not used up — a common misconception is that current is consumed by components. In a series circuit the current is the same everywhere, including before and after a lamp. What the lamp uses is energy, transferred from the charge as it passes; the charge itself continues.

The current equation — for a conductor of cross-sectional area A containing n charge carriers per unit volume, each of charge e, drifting at mean speed v:

I = Anev

n, the number density — the number of free charge carriers per cubic metre. It is what separates conductors from semiconductors and insulators: a metal has an enormous n, around 10²⁹ m⁻³, a semiconductor far fewer, and an insulator almost none.

Drift velocity is surprisingly slow — in a typical copper wire carrying a few amperes the electrons drift at well under a millimetre per second. A lamp lights immediately because the electric field is established through the circuit at close to the speed of light, setting every electron in the wire moving at once — not because electrons travel from the switch to the lamp.