The Broken Circuit Mystery
A group builds a circuit with two cells in a holder, a switch, two bulbs in series, and connecting wires. When the switch is closed, both bulbs light, but they are dim. The learners want brighter bulbs. They have these materials: one extra cell, one extra bulb, and one extra wire.
They try three changes, one at a time, always returning to the original circuit:
- Change 1: add the extra cell to the holder.
- Change 2: add the extra bulb in series.
- Change 3: replace one bulb with a wire.
The teacher warns that a bulb can burn out if too much current passes through it, and that cells can run down quickly.
1. Predict the result of each change and judge which change best meets the learners' goal without causing damage.
Read the worked answer
- Change 1 (add a cell): the total voltage rises, so the current rises and both bulbs become brighter. This meets the goal. Risk: too much current can overheat and burn out the bulbs, and the cells run down faster.
- Change 2 (add a bulb in series): the total resistance rises, so the current falls. All three bulbs are dimmer than before. This does not meet the goal.
- Change 3 (replace one bulb with a wire): resistance falls, so the current rises and the remaining bulb is brighter. But the bulb may burn out, and the wire becomes part of the circuit with no bulb to share the voltage.
Judgement: Change 1 is best, but it must be checked. Adding one cell to two cells gives a moderate increase, which is usually safe for the bulbs. Change 3 gives a large increase in current and risks burning out the single bulb. Change 2 fails the goal. The best answer is Change 1, with the caution that the bulbs should be checked and the switch opened when not in use to save the cells.
2. The learners notice that after Change 1 the bulbs are brighter but one bulb is slightly dimmer than the other. Give a possible reason and say what this tells them about the circuit.
Read the worked answer
Possible reasons: the two bulbs may not be identical (different ratings or ages), so one has higher resistance and takes a larger share of the voltage; or one bulb's contact in its holder is loose, adding resistance at that point; or the wires and connections add resistance that is not shared equally.
What this tells them: in a series circuit the current is the same everywhere, so a difference in brightness cannot come from different currents. It must come from different resistances, which cause different voltage drops across the bulbs. The judgement is that brightness is not a reliable measure of current on its own; learners should check bulb ratings and connections before concluding that the circuit is faulty.