Question

Humberto builds two circuits using identical components.

Circuit 1: A series circuit with three lightbulbs
Circuit 2: A parallel circuit with three lightbulbs

He then adds two more lightbulbs in a series with the bulbs in circuit 1 and two more lightbulbs on new branches in parallel with each bulb in circuit 2.

What can he expect to see?

All bulbs in circuit 1 will shine at the same brightness as the original bulbs, while all bulbs in circuit 2 will be dimmer than the original bulbs.
All bulbs in circuit 1 will shine more dimly than the original bulbs, while all bulbs in circuit 2 will have the same brightness as the original bulbs.
The original bulbs will remain at their original brightness, but the bulbs added to circuit 1 will be dimmer than the bulbs added to circuit 2.
The original bulbs will remain at their original brightness, but the bulbs added to circuit 1 will be brighter than the bulbs added to circuit 2.

Answer 1

All bulbs in circuit 1 will shine more dimly than the original bulbs, while all bulbs in circuit 2 will have the same brightness as the original bulbs.

Explanation: In the series circuit adding bulbs increases the total resistance, so the current through every bulb falls and they all get dimmer. In the parallel circuit each branch still has the full source voltage, so each bulb keeps the same brightness (assuming an ideal voltage source; a real battery with internal resistance could cause a small overall dimming).