Circuits
Two bulbs (parallel)
Each bulb sees the full battery voltage, so each is as bright as one alone — but the battery supplies twice the current.
Two bulbs (parallel) — interactive Circuits simulation. Each bulb sees the full battery voltage, so each is as bright as one alone — but the battery supplies twice the current.
Bulbs in parallel
Bulbs in parallel each see the full battery voltage, so each is as bright as a single bulb alone.
Parallel branches connect to the same nodes. Each bulb draws its own current while the battery supplies the sum.
- V is the same for each bulb
- I_total = I_A + I_B
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
Two identical bulbs are wired in parallel across a 9 V battery, each seeing the full battery voltage. Compared to a single bulb alone on the same battery, is each parallel bulb brighter, dimmer, or the same?
Predictions to weigh
- Each parallel bulb is the same brightness as a single bulb alone — each sees the full 9 V independently.
- Each parallel bulb is dimmer, like the series case.
- Each parallel bulb is brighter than a single bulb alone.
Variable roles
What you set:
- Each bulb's resistance R (Ω)
What you measure:
- Each bulb's current I (A)
- Total current I_total (A)
How the investigation runs
- Open the bulbs-parallel preset and press Reset. Both bulbs start with equal resistance, each across the full 9 V.
- Enable the current readout on both bulbs and the battery.
- Set both bulbs' resistance (equal to each other) for each trial and record one bulb's current and the total current.
Governing equation
Parallel Bulbs Share Voltage — V = I·R
Parallel bulbs each see the full source voltage independently: I = E/R for each one, exactly as if it were the only bulb in the circuit. Adding more parallel bulbs increases total current drawn but doesn't dim the existing ones.
What the printable worksheet asks students to work out
- For one trial, compute I_each = E/R using E = 9 V, then I_total = 2·I_each. Compare both to the table.
- Compare each bulb's current here to the switched-lamp experiment's current for the same resistance. Explain why a parallel bulb burns exactly as bright as if it were alone.
Where this shows up beyond the lab
- The total current drawn from the battery is double a single bulb's current. Explain why adding more parallel bulbs (more appliances on the same outlet) drains a battery faster, even though each individual bulb is unaffected.
- If one bulb in this parallel pair burns out (infinite resistance), what happens to the other bulb's brightness? Compare your answer to the bulbs-series experiment's burnout scenario.
- AP Physics 2 — Unit 11: Electric Circuits
- AP Physics C: Electricity and Magnetism — Unit 11: Electric Circuits
- IB Physics — B.5 Current and circuits
- General High School Physics — Electricity & DC circuits
- NGSS High School Physics — Electric current and magnetic fields
- Welcome to Bulbs Parallel
- Select the first bulb
- Press Play
- Full brightness
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.