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Science 7–10 · Year 9

Series and parallel circuits: measuring current and voltage with one, two and three lamps

Physical sciences — Energy, content group Electrical energy (NSW Stage 5 focus area; ACARA places electric circuits in Year 6, AC9S6U03, and has no Year 9 or Year 10 content description on circuits, so only Year 9 inquiry codes are cited)

Practical, model not builtLow risk

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The idea

Adding lamps in series shares the supply voltage and lowers the current, while adding lamps in parallel gives each the full voltage and raises the total current drawn; the supply gives each unit of charge a fixed amount of energy, which series lamps share and each parallel branch receives in full.

What you need

  • 6 V DC power supply or 4 x 1.5 V cells in a holder, 1
  • 6 V 0.3 A MES lamps in holders, 3
  • digital multimeters (one as ammeter, one as voltmeter) or an ammeter 0 to 1 A and voltmeter 0 to 10 V, 2
  • switch, 1
  • 4 mm leads, 12

How to do it

  1. Build a circuit of the supply, switch and one lamp with the ammeter in series. Record the current and the voltage across the lamp. Note the brightness.
  2. Add a second lamp in series, then a third; each time record the current (it is the same everywhere in a series loop: check by moving the ammeter) and the voltage across each lamp and across the supply.
  3. Rebuild with two lamps in parallel. Record the current in each branch and the total current from the supply, and the voltage across each lamp.
  4. Add a third parallel lamp and repeat.
  5. Unscrew one lamp in the series circuit and then in the parallel circuit; record what the others do.
  6. Tabulate: for series, check that the lamp voltages add to the supply voltage; for parallel, check that the branch currents add to the supply current.

What you should see

A 6 V 0.3 A lamp has a hot resistance of 20 ohm (6.0 V / 0.3 A). Series: with a 6.0 V supply one lamp draws 0.30 A. The voltage divides equally, 3.0 V across each of two lamps and 2.0 V across each of three, and the lamps are dimmer. If the lamps kept their 20 ohm, two would draw 0.15 A and three 0.10 A, but real lamps draw noticeably more, about 0.20 A for two and 0.16 A for three: a filament running on less voltage is much cooler, and its resistance falls to about 15 ohm at 3.0 V and 13 ohm at 2.0 V (a filament model with tungsten's resistance rising by 0.45 per cent per degree and the electrical power balanced by radiation, set to draw 0.30 A at 6.0 V). Parallel: each lamp has 6.0 V and 0.30 A, and the supply current is 0.60 A for two lamps and 0.90 A for three, within the 1 A ammeter range; the lamps stay bright. Removing a series lamp turns the others off; removing a parallel lamp keeps the others lit.

What changes

What you change
number of lamps and their arrangement
What you measure
current at each point (A) and voltage across each component (V)
What you keep the same
  • same supply voltage
  • identical lamps
  • meters connected the same way each time

Common misconceptions

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

  • Current is used up as it passes through each lamp.
  • The lamp nearest the supply is brightest.
  • Adding a parallel branch reduces the current in the existing branch.

Safety card

Low riskLearners carry it out

Hazards

  • hot lamps
  • short-circuit across the supply heating leads

Controls

  • switch off while rebuilding
  • no more than 6 V
  • ammeter never connected directly across the supply

Note

Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.

Curriculum references

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

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/fab404b99e
  3. spark.iop.org/investigating-series-and-parallel-circuits
  4. spark.iop.org/voltage-parallel-circuits
  5. instructional-resources.physics.uiowa.edu/5f2050-series-and-parallel-light-bulbs

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