Current, current density and effects of current

If equal charges pass through the cross-section of a conductor in any equal times, the current is steady. The current is I = q/t, with unit 1 A = 1 C/s. The current density is j = I/S – the current divided by the cross-section perpendicular to it (unit A/m² or A/mm²); it is a vector pointing along the motion of positive charges. If the carrier concentration is n, each has charge e and the mean drift speed is υ, then I = neSυ and j = neυ. In metals υ is very small (fractions of a millimetre per second), while the disturbance in the circuit spreads at nearly the speed of light. The effects of current are: heating (the conductor warms; always present in metals), chemical (substances are released in electrolytes; absent in metals) and magnetic (a magnetic field around the current – H. Ørsted, 1820; A. Ampère discovered the interaction of current-carrying conductors). The current density shows how heavily a conductor is loaded: if it is too high the wire overheats and can cause a fire, so the wire cross-section is chosen to match the current.