Quick answer: In a series circuit the components are connected in one single loop, so there is only one path for the current and the bulbs share the supply. This grade 7 science lab on series and parallel circuits gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How much does a bulb dim when you add more bulbs? In this lab you build series and parallel circuits with identical bulbs and measure the brightness of one bulb each time with a light meter. You will see which arrangement shares the supply and which one gives every bulb its own connection.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS2-3
Aim
To find out how the brightness of a bulb changes when more identical bulbs are added in series and in parallel.
Equipment
- Two 1.5 V cells in a holder (3.0 V in total)
- Three identical 2.5 V torch bulbs in holders
- Switch and about eight connecting wires
- Light meter or a phone light-meter app that reads in lux
- Ruler, a cardboard tube to shield side light, and a dim room
Safety
The voltage is low and safe, but bulbs and wires can become warm. Never connect a bare wire straight across the cells. Open the switch between changes and do not touch hot bulbs.
Method
- Build a circuit with one bulb. Close the switch, hold the light meter 5 cm from the bulb and record the brightness in lux. Open the switch.
- Add a second bulb in series. Measure the brightness of the first bulb at the same distance and record it.
- Add a third bulb in series and measure again.
- Rebuild the circuit with two bulbs in parallel and measure one bulb, then with three bulbs in parallel.
- Repeat each reading once and record the mean.
Results
Sample mean measurements from one group:
| Arrangement | Number of bulbs | Brightness of one bulb (lux) |
|---|---|---|
| Single bulb | 1 | 480 |
| Series | 2 | 45 |
| Series | 3 | 9 |
| Parallel | 2 | 472 |
| Parallel | 3 | 466 |
Analysis questions
Question 1 Describe the pattern
Describe how the brightness of one bulb changes as bulbs are added in series and in parallel.Question 2 Series percentages
Calculate the brightness with two and with three bulbs in series as a percentage of the single bulb, to 1 decimal place.Question 3 Parallel percentages
Calculate the brightness with two and with three bulbs in parallel as a percentage of the single bulb, to 1 decimal place.Question 4 Explaining series
Explain why bulbs in series are dimmer using the idea that the supply is shared.Question 5 A small drop in parallel
Suggest why the parallel bulbs were slightly dimmer than the single bulb.Question 6 Cell life
Predict how long the cells would last in each arrangement compared with a single bulb.Question 7 Reliability
Suggest two ways to make the brightness measurements more reliable.Question 8 Unscrewing a bulb
Predict what happens to the other bulbs in each arrangement when one bulb is unscrewed.
