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Brightness and the number of batteries

Curricula: KS2 (Year 6)

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Aim

Find out how the brightness of a lamp depends on the voltage, that is, on the number of batteries connected in series.

The bench

Three circuits with identical 4.5 V / 1 W lamps: one AA 1.5 V battery at the top, two below it, three at the bottom. The positive terminal of each battery is connected to the negative terminal of the next. The type of a battery is changed on its parameter card, and a burned-out lamp is replaced by clearing its "Burned out" checkbox.

Procedure

  1. Compare the brightness of the three lamps.
  2. Swap one of the batteries in the bottom circuit for an AAA battery, then for a 9 V (PP3) battery. How does the brightness change?
  3. Put the AA battery back and replace the burned-out lamp.
Expected result

With one battery the filament barely glows, with two the lamp glows at half strength, with three at full strength: 3 × 1.5 = 4.5 V, which is its nominal voltage. The more batteries, the higher the voltage and the brighter the lamp. AAA instead of AA hardly changes the brightness, since the voltage is the same. With the 9 V (PP3) battery the circuit has 12 V, almost three times the lamp's nominal voltage: the lamp flashes brightly and burns out within hundredths of a second, as the filament breaks and the circuit opens. To continue, put the AA battery back and replace the lamp.

Questions

  1. How many 1.5 V batteries are needed for the lamp to glow at full strength?
  2. Why can't a real 4.5 V lamp be connected to 9 V?
Answers
  1. Three batteries: 3 × 1.5 = 4.5 V, which is the voltage the lamp is made for.
  2. 9 V is twice the voltage the lamp is made for. A larger current flows through the thin filament, and it gets much hotter than it can stand, so it soon breaks and the lamp burns out.