Practical: Getting electricity from fruit and vegetables
A galvanic cell is a source of current that converts chemical energy into electrical energy. It consists of two different metals (electrodes) and an electrically conducting liquid (electrolyte) between them. If we push a zinc-coated (galvanised) nail and a copper coin or wire into a lemon or potato, the juice acts as the electrolyte and a voltage appears between the electrodes: a single lemon or potato cell gives about 0.9–1 V. The electrical energy comes not from the fruit but from the slow chemical dissolving of the zinc electrode. One cell cannot light an ordinary incandescent bulb (it is too weak), but 3–4 cells joined in series can run a low-power LED, a digital clock or a calculator. The textbook uses iron and copper; iron also gives a small voltage, but zinc (a galvanised nail) works better. The first battery was made in 1800 by the Italian Alessandro Volta. Lemon holds more acid, so it conducts better than potato and can give more current. Safety: use only a few fruit cells and never experiment with a mains socket; do not eat the fruit used; the nail and wire ends are sharp; mind the LED polarity (longer leg is +).
Lemon battery: into each of 3–4 lemons push one galvanised nail and one copper coin/wire (electrodes must not touch), join the cells in series (copper end of one to the nail end of the next), then connect an LED or digital clock. Repeat with potatoes and compare. Safety: the pushed-in objects are sharp – work with an adult; keep lemon juice out of eyes; do not eat the fruit used; wash hands; use only batteries and fruit cells.