Short series

This class activity allows students to investigate circuits with up to three bulbs without having to take their circuit apart. You can use it to test their understanding of current in series circuits.

Equipment

Each group of students will need:

• Low voltage power supply (e.g. 1.5 V electrical cell)
• Three identical bulbs
• Two ammeters
• Eight 4 mm leads

Preparation

Before the start of the lesson, set up one circuit as an exemplar so that students can refer to it if required.

During the activity students will need to bypass some of the bulbs by connecting leads around them. To avoid damaging the ammeters, ensure they don’t short circuit all the bulbs. There must be at least one bulb in the circuit to avoid the current becoming too high.

Procedure

Ask students to:

1. Set up the circuit below. To start, they should connect leads around two of the bulbs so that only the first bulb is lit.

2. Record the readings on both ammeters (A1 and A2).
3. Predict what will happen when they remove lead Y.
4. Disconnect lead Y and record observations.
5. Repeat for lead Z.

Discussion prompts

• Why must the ammeters readings always be the same?
• Why do they go down when you disconnect the leads?

Teaching notes

Students may not understand how leads Y and Z allow them to change the number of bulbs in the circuit. Explain this in terms of the current taking the path of least resistance. The leads have a much lower resistance than the other components. Connecting a lead around a bulb means (almost) all the current will go through the lead, not the bulb.

There is only one loop in a series circuit and so an ammeter placed anywhere in the circuit will read the same. Disconnecting a lead adds another bulb in series, increasing the circuit’s overall resistance and so reducing the current throughout.

Learning outcome

Students predict and explain what will happen to the current when another light bulb is added in series.

This experiment was safety-checked in March 2020.