Electric current is a flow of electrons minute particles far too small to see through any microscope – that are dislodged from atoms. The greater and faster the flow of electrons, the bigger the current. But it takes a flow of billions of electrons to make even a small current.

Atoms are the tiny particles that make up all matter. The much tinier electrons orbit round the atom’s nucleus, or central core, in layers known as shells. Each shell may consist of a different number of electrons. The various chemical elements that make up matter – gases and metals for example – have different numbers of electrons in their atoms. Hydrogen gas, for instance, has only one in each atom, but copper has 29 of them, distributed among four shells.

An atom’s nucleus is positively charged with electricity, and its electrons are negatively charged. So the atom is normally electrically neutral because the two charges cancel each other out. But electrons can be dislodged from the atom (usually from the outer layer) by heat, for example, or they can leave it to join another atom during a chemical reaction that involves a change in the nature of the substance, such as rust forming on metal. An atom (or group of atoms) that loses or gains electrons becomes electrically charged and is called an ion. The ion is positively charged if it loses electrons, or negatively charged if it gains them. Free electrons dislodged from an atom move about at random. But a flow is set up if a number of them are attracted towards a source of positive ions.

An electric current will not flow through a circuit unless there is an excess of electrons at one end and a deficiency at the other. This is known as the potential difference, and it is measured in volts.

Metals are good electrical conductors because their atoms have lots of loosely bound electrons to speed the electron flow, which meets little resistance. Materials that do not generally conduct electricity, such as rubber, have atoms with electrons that are not easily displaced.

An electric circuit is a pathway of wire, usually copper, from a power source and back again. The power source can be a battery or an electromagnetic generator.

Mains electricity is transmitted from generators at power stations, with the outward and return wires in the same cable. As the electrons travel along their wire pathway, they are attracted first one way, then the other, according to the changing magnetic field of the generator’s magnet. This gives alternating current, which has a wavelike motion.

Conventionally, electric current is considered to flow from positive to negative as if the generated charge is pushing current to where there is none. This convention was established before scientists knew that electrons flow to a positive charge.