Science and Technology K–6 · Year 6

Two globes, two ways: series and parallel circuits

Science understanding: Physical sciences

Practical, model not builtLow risk

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The idea

In a series circuit the energy from the cells is shared along one path, so two globes glow dimly, while in a parallel circuit each globe has its own path to the cells and glows almost as brightly as one alone, drawing more current and emptying the cells faster.

What you need

2 fresh AA 1.5 V alkaline cells in a holder
3 identical torch globes rated 2.5 V 0.3 A, in holders
8 insulated leads with alligator clips and a push switch
a digital multimeter on the 10 A dc current range and the 20 V dc voltage range
a light meter or a tablet light-meter app held 10 cm from a globe (optional)

How to do it

One globe: close the switch, measure the current from the cells and the voltage across the globe, and rate the brightness (or read the light meter at 10 cm).
Two globes in series (one after the other in a single loop): measure the current and the voltage across each globe, and rate each globe's brightness.
Two globes in parallel (each on its own loop from the cells): measure the total current and the voltage across each globe, and rate the brightness.
Unscrew one globe in the series circuit, then one in the parallel circuit, and record what happens to the other globe each time.
Repeat with three globes in series and three in parallel.
Tabulate current and brightness against the number of globes for both arrangements, and compare your readings with the model's values.
Explain which arrangement a house uses for its lights and why.

What you should see

With fresh cells (3.0 V in total, internal resistance 0.15 ohm each, the low end of the 0.15 to 0.30 ohm the Energizer E91 datasheet gives) and globes whose working resistance is 8.3 ohm (2.5 V / 0.3 A), the model gives: one globe 0.35 A and 1.01 W; two in series 0.18 A and 0.26 W each, about a quarter of one globe's power, so both glow dimly; two in parallel 0.67 A from the cells and 0.94 W each, nearly full brightness. Measured series currents come out somewhat higher than 0.18 A because a dimly glowing filament is cooler and has a lower resistance than a bright one, but the pattern holds: series globes are clearly dimmer, parallel globes nearly as bright as one. Unscrewing one series globe puts the other out; unscrewing one parallel globe keeps the other lit. Two parallel globes draw about twice the current of one globe and nearly four times that of the series pair, so they run the cells down fastest.

What changes

What you change: arrangement (series or parallel) and number of globes
What you measure: current from the cells, voltage across each globe, brightness
What you keep the same: same cells
identical globes
same leads
readings taken quickly so the cells do not run down

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Current is used up by the first globe, so the second globe in series gets less current (the same current passes through both).
Adding more globes in parallel makes each one dimmer in the same way as in series.
The cell pushes out the same current whatever is connected to it.

Safety card

Low riskLearners carry it out

Hazards

short circuit heats the cells and leads
globes get hot after a few minutes

Controls

switch opened between readings
never connect the meter across the cells on the current range
let globes cool before unscrewing

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-8PW-ST ST3-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; Stage 3 content: 'Plan and construct simple electrical circuits to model the transfer and transformation of energy', which sits in the focus area whose knowledge outcome is this one; code read from the outcomes page and the content read from the Stage 3 content page on 23 September 2026ST3-SCI-01
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-PQU-01 ST3-DAT-01
Australian Curriculum v9AC9S6U03AC9S6I01AC9S6I03AC9S6I04AC9S6I05

Sources

The pages the author read to write this activity.

All Concept Studio activities