Circuits
Two bulbs (series)
Bulbs share one current and split the voltage — each is dimmer than a single bulb.
Two bulbs (series) — interactive Circuits simulation. Bulbs share one current and split the voltage — each is dimmer than a single bulb. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Bulbs in series
Two identical bulbs in series share current but split voltage, so each glows dimmer than one alone.
Each bulb is a resistive load. In series they see the same current but only a fraction of the battery voltage.
- I is the same for both bulbs
- P ∝ V·I
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 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?
Predictions to weigh
- Each series bulb is brighter.
- Each series bulb is dimmer — they share the current and the voltage.
- Each bulb is exactly as bright as a single bulb alone.
Variable roles
What you set:
- Each bulb's resistance R (Ω)
What you measure:
- Total resistance R_total (Ω)
- Shared current I (A)
How the investigation runs
- 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.
Governing equation
Series Bulbs Share Current — P = I²·R
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.
What the printable worksheet asks students to work out
- 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 exactly halves the current — and why each bulb's power therefore falls to a quarter, not a half, since P = I²R goes as the square of the current.
Where this shows up beyond the lab
- 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.
- 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 Series
- Select the first bulb
- Press Play
- Both dimmer
- 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.