Cells in series and in parallel
One cell gives only a limited emf, which sometimes is not enough for a device, so cells are joined. In series, the cells form a chain, the positive pole of one touching the negative pole of the next, and the total emf is the sum: n identical cells of ε volts give n × ε. Four cells of 1.5 V give 6 V. In parallel, all the positive poles are joined together and all the negative poles are joined together, and the total emf is the emf of one cell: four 1.5 V cells in parallel still give 1.5 V. A torch uses cells in series to get a large emf. Cells in parallel do not give a higher voltage, but they can supply current for longer: a power bank is built from cells in parallel so that it can charge a phone more than once. Careful: four cells in parallel are not “four times more powerful” and the emf of a battery is not the same thing as the current.
| Joining | Total emf of n cells of ε | Use |
|---|---|---|
| series | n × ε | a large emf (torch) |
| parallel | ε | lasts longer |
Cells in a holder, with a grown-up. Use AA cells in a holder and a torch bulb. First one cell, then two cells in series (the bulb is brighter), then two cells in parallel (the brightness is the same as one cell, but it lasts longer: do not run it for long). Switch on for a few seconds only. Never connect a wire straight across a cell. Do not use phone chargers or power banks. Record what you see in a table.