• One cell gives limited emf, which may be too small.
• A group of connected cells is a battery.
• Identical cells can be connected in two ways.
• In series. The positive pole of one cell is connected to the negative pole of the next. The emf of the battery = the sum of the emfs of the cells:
• ε(total) = n × ε(cell).
• In a circuit of three 1.5 V cells in series, a voltmeter reads 4.5 V and the lamp is brighter than with one cell.
• In parallel. All positive poles are joined together and all negative poles are joined together. The emf of the battery = the emf of one cell (the cells share the load; the battery lasts longer, but voltage does not increase).

Worked example 4

• Three identical cells of emf 3 V are connected in series to a lamp.
• Find the potential difference across the lamp (negligible internal resistance).
• V = ε = 3 × 3 = 9 V.

Worked example 5

• Four identical 1.5 V cells connected in series give 6 V.
• What emf is obtained if the same four cells are connected in parallel?
• The emf = 1.5 V (equal to one cell).