The two directions of current

You connect a small lamp to the terminals of a battery, its two ends marked + and −, with two wires. The battery is the source that pushes the charges around the loop. The loop is closed, the lamp lights, current flows. Current is the charge that passes a point of the loop each second. On the schematic, the drawing of that loop, the arrow that marks the direction of current leaves the plus, goes through the lamp and comes back to the minus. In the wires, though, the free electrons go the other way. Which one is right?

Around 1747, in Philadelphia, Benjamin Franklin pictures a fluid present in all matter. A body where it abounds is "plus", a body where it is short is "minus", and the fluid flows from surplus to shortage. Rubbing glass with a cloth moves some from one to the other, but nothing really said which way. Franklin bet that the glass was plus and the cloth minus. That bet became a convention, a choice made once for everyone: the conventional direction of current runs from plus to minus.

A century and a half later, in 1897, the English physicist J. J. Thomson finds electrons, tiny grains of negative charge by Franklin’s label, since they are charged like the cloth that rubbed the glass. Later it becomes clear that they are what moves in metals.

The bet was lost. Rubbed glass, which Franklin thought had a surplus, has given up electrons. In the battery too, the minus terminal has extra electrons and pushes them away; the plus terminal is short of them and pulls them in. Outside the source, in the wires and the lamp, electrons therefore go from minus to plus. We say "outside the source" because inside, it is another story, told in Who carries the current.

Nobody changed the signs, and nobody needed to. Take a point on the wire between the lamp and the plus terminal. Each second, electrons cross it toward the terminal: the lamp side loses negative, the terminal side gains it. An equal positive charge crossing the other way would leave the same balance, since losing negative is gaining positive. Current, the charge that passes that point, is therefore the same in both stories, and the calculations you will do give the same result.

Both directions are true. Outside the source, conventional current goes from plus to minus; it is the one used on schematics, and parts you will meet later, like the diode, carry an arrow that follows it. The electrons, what moves in the wire, go from minus to plus.

Remember

Outside the source, conventional current goes from plus to minus and electrons from minus to plus: it is the same current.

Do

The exercises: this text prepares level 1. The third question needs the ampere and the coulomb, which come in 1.2, and the charge of the electron.