Bulbs in Series: Shared Current, Dimmer Bulbs
1 · Predict
Two identical bulbs are wired in series across a 9 V battery. Compared to a single bulb alone on the same battery, is each series bulb brighter, dimmer, or the same?
- Each series bulb is dimmer — they share the current and the voltage.
- Each series bulb is brighter.
- Each bulb is exactly as bright as a single bulb alone.
2 · Set Up
- Open the bulbs-series preset and press Reset. Both bulbs start with equal resistance.
- Enable the current readout on both bulbs.
- Set both bulbs' resistance (equal to each other) for each trial and record the shared current.
3 · Collect Data
| Each bulb's resistance R (Ω) | Total resistance R_total (Ω) | Shared current I (A) |
|---|---|---|
| 50 | ||
| 100 | ||
| 150 |
Plot current I (y-axis) against total resistance R_total (x-axis) for your three trials.
4 · Analyze
- For one trial, compute R_total = 2R, then I = E/R_total using E = 9 V. Compare to the table.
- Compare this current to the switched-lamp experiment's current for a single bulb of the same resistance. Explain why adding a second bulb in series roughly halves the current (and each bulb's power).
5 · Extend
- If the two bulbs had different resistances, would they glow equally bright? Use P = I²R (same I for both, different R) to predict which one would be brighter.
- If one bulb in this series pair burns out (its filament breaks, effectively infinite resistance), what happens to the other bulb? Compare to the short-a-bulb experiment's switch behavior.
The Physics Behind This Experiment
Series Bulbs Share Current
Series bulbs carry the same current I = E/(2R), but each bulb only gets half the total voltage. Since power is I²R with a smaller current than a lone bulb would draw, each series bulb runs dimmer.