Electrification and electric current

If some bodies are rubbed against each other, they acquire the ability to attract other bodies: they are electrified, i.e. charged. The ancient Greeks knew that amber (“elektron”) rubbed with silk attracts small objects; the word “electricity” comes from it. There are two kinds of charge: positive and negative (the names were introduced by Franklin). Like charges repel, unlike charges attract. In the centre of an atom is a positively charged nucleus (positive protons and uncharged neutrons) around which negative electrons move. Normally the atom has equal numbers of protons and electrons, so it is neutral. In rubbing, electrons pass from one body to the other: the body that gains electrons becomes negative, the one that loses them becomes positive; charge is not lost. Substances that can carry charge freely are conductors (metals, salt water, the human body); those that cannot are dielectrics (glass, plastic, rubber, dry air). Lightning is a huge electric discharge between clouds.