Two charges, one atom

Rub a blown-up balloon on your hair, then set it against the wall. It stays stuck there. Now bring it close to your head: your hair stands up and follows it, each strand well apart from the next. Nothing shows to the eye. What did the rubbing give the balloon and your hair?

The Greeks already knew this with amber, a fossil resin, the gum of a very old tree hardened into yellow stone. Rubbed, it picks up bits of straw. Much later, people found that glass, wax and many other materials do the same. A body that attracts like this after rubbing is said to be charged, and what it carries would come to be called its electric charge.

In 1733, in Paris, Charles du Fay saw that this charge is not all of one kind. A piece of gold leaf, light enough to move, touches rubbed glass and takes on its charge; the glass then pushes it away, but rubbed resin pulls it in. He concluded that there are two electricities, that of glass and that of resin. Two bodies charged with the same kind push each other away; two bodies charged with opposite kinds pull together.

Around 1747, in Philadelphia, Benjamin Franklin wanted only one. To him, all matter holds the same electric fluid, and rubbing moves some of it from one body to the other. One ends up with too much, which he called "plus," or positive, and the other with too little, "minus," or negative. He put rubbed glass on the plus side and, a few years later, resin on the minus side. Du Fay’s two electricities became the two signs of a single charge.

No one ever found that fluid. What was found is the atom, the tiny grain all matter is made of, which you met in high school. At its center, the nucleus holds protons, which are positively charged, along with neutrons, which carry no charge. Around it are electrons, much lighter, which are negatively charged, and the nucleus holds them because opposites attract. An atom left to itself has as many electrons as protons, so the pluses and minuses cancel out. It is neutral, meaning it has no overall charge, and so are the wall, the table and your hair.

When two bodies are rubbed together, the protons stay in their nuclei. It is electrons, the ones their nucleus holds least tightly, that pass from one body to the other. The one that gains them has more electrons than protons and becomes negative; the one that loses them becomes positive.

Glass rubbed on silk loses electrons; resin rubbed on wool gains them. That is why the electron is called negative: it carries the charge Franklin called minus. Franklin had bet that plus was the side with too much; glass, in fact, loses electrons. His signs were kept, and that changes nothing in the rules, as The two directions of current tells.

Back to the balloon. Rubbed on your hair, it takes electrons from it and becomes negative, and your hair becomes positive. Each strand pushes its neighbors away, since they are charged like it, and all of them are pulled toward the balloon, which has the opposite charge. The rubbing created nothing; it moved electrons from your hair to the balloon. The spark at the doorknob in winter is a charge picked up while walking on the carpet, an excess or a shortage of electrons depending on your soles, jumping all at once.

The wall, for its part, is neutral. The negative balloon pushes the electrons of each atom in the wall a little toward the back, so the surface facing it ends up with slightly more protons than electrons, and the balloon holds on there.

Remember

Matter is neutral, with as many electrons as protons; when electrons pass from one body to another, the one that gains them becomes negative and the one that loses them becomes positive. Like charges repel, opposite charges attract.

Do

The exercises: first the two kinds of charge, what the proton and the electron carry, and why a table is neutral; then the balloon on the wall, two balloons rubbed on the same sweater, the spark at the doorknob, the plastic ruler that attracts neutral paper, and why rubbing creates nothing.

The word comes from the Greek ēlektron, amber. It is the fossil resin that, when rubbed, attracts straw; Gilbert drew "electric" from it around 1600, and from there "electricity".

In 1891, the Irishman George Johnstone Stoney proposes "electron" for the smallest grain of charge, which he had calculated without ever seeing it. Thomson, who finds that grain six years later, calls it "corpuscle". Stoney’s word is the one that won.

What moves in our wires is thus named after a resin. In French, électron, the same word with an accent.