Bulbs in Parallel: Shared Voltage, Full Brightness
1 · Predict
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?
- 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.
2 · Set Up
- 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.
3 · Collect Data
| Each bulb's resistance R (Ω) | Each bulb's current I (A) | Total current I_total (A) |
|---|---|---|
| 50 | ||
| 100 | ||
| 150 |
Plot each bulb's current (y-axis) against 1/R (x-axis) for your three trials.
4 · Analyze
- 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.
5 · Extend
- 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.
The Physics Behind This Experiment
Parallel Bulbs Share Voltage
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.