Current

Electric current is the flow of electric charge past a point, measured in amperes, where one ampere is one coulomb of charge passing each second. The charge can be carried by electrons in a copper wire, by ions in a salt solution or a battery, or by electrons and holes in a semiconductor. Current is the quantity that actually does work in a circuit: it heats filaments, magnetises coils, lights diodes and charges the tiny capacitors inside every memory cell.

Conventional current and electron flow point in opposite directions, and the reason is an accident of history. Benjamin Franklin, in the mid-1700s, called one kind of charge positive and imagined it flowing from positive to negative. Long before anyone knew that the mobile charge in metals is the electron, the convention was entrenched, and the electron turned out to carry what Franklin had labelled negative charge. So conventional current flows from the positive terminal through the circuit to the negative one, while electrons physically drift the other way. The arrows in circuit diagrams, including the arrow on a transistor symbol, follow the conventional direction. Nothing is wrong with either description, since a flow of negative charge one way is electrically equivalent to positive charge the other, but mixing the two within one explanation is a classic source of confusion.

The electrons move astonishingly slowly, yet the effect travels almost instantly. In a copper wire carrying a few amperes, the average drift velocity of the electrons is a fraction of a millimetre per second, because there are so many mobile electrons that only a gentle net drift is needed to move a large charge per second. Their random thermal motion is far faster but cancels out on average. What races along the wire at a sizeable fraction of the speed of light is the electromagnetic disturbance: when a switch closes, the electric field is established through the whole conductor almost at once and every electron begins drifting together, like water already filling a pipe. This is why a lamp lights immediately even though no electron could have travelled from the switch to the bulb in that time.

Direct and alternating current differ in whether the flow keeps its direction, and each suits a different job. Direct current, as from a battery, flows steadily one way and powers virtually all electronic circuits, including every logic chip. Alternating current reverses direction periodically, typically 50 or 60 times a second on household mains, and won out for power distribution because transformers can raise and lower its voltage efficiently. Inside any electronic device a power supply converts mains alternating current to the low-voltage direct current the chips need. Even in a chip, however, a switching transistor produces rapid current pulses, so on the scale of billionths of a second the supply current is far from steady.

In semiconductors, current arises by two mechanisms, and the second has no counterpart in ordinary wires. Drift is the response to an electric field, as in a metal: electrons in the conduction band and holes in the valence band are pushed in opposite directions, but both add to the same conventional current. Diffusion occurs when carriers are more concentrated in one region than another and spread out statistically, like a drop of ink in water, even with no field at all. The flow across a pn junction and through the base of a bipolar transistor is largely diffusion, whereas the channel current of a field-effect transistor is mostly drift. Real devices mix both, and the balance of the two, together with doping, determines how a transistor behaves.

The most persistent misconception is that current is used up as it passes through a component. The same current that enters a resistor leaves it; what is lost is energy, converted to heat, because charge falls through a potential difference. Current is conserved at every junction of a circuit, a principle formalised by Kirchhoff. Modern relevance is direct: a processor drawing a hundred amperes at about one volt is a statement about the enormous number of charges flowing each second, and the heat and wiring problems of such currents dominate chip and data-centre power design.

Read current as the rate at which charge passes a point, a bookkeeping quantity whose sign is a historical choice, whose carriers crawl, and whose effects travel at nearly light speed.