Current sources
In a current source non-electrostatic forces separate positive and negative particles; they gather at the poles of the source and create an electric field. Mechanical, chemical or other energy is converted into electrical energy: mechanical in an electrophore machine, chemical in a galvanic cell. The simplest galvanic cell is zinc and copper plates in a sulfuric-acid solution (the Volta cell); in practice dry cells are used, with a zinc case and a carbon rod. Several cells joined one after another form a battery. In an accumulator, too, chemical energy becomes electrical energy, but when it runs out it can be recharged from another source. The simplest circuit consists of a current source, connecting wires, a consumer (for example a lamp) and a switch; current flows only in a closed circuit. A circuit is drawn as an electric diagram, in which each element has its own symbol.
With an adult: push a galvanised nail and a copper wire (or coin) into a lemon 2–3 cm apart — a simple lemon battery. Only a sensitive instrument shows the weak current. Never join the poles of a battery directly with a wire — the wire and battery heat up; do not open or heat a battery, and do not eat the lemon afterwards.