Physics and Natural Sciences
Two bulbs, one supply
A 60 W bulb is designed for a 230 V supply. What current does it draw and what is its resistance? Two of them are then connected in series to the same supply: what happens?
- Hint 1P = VI and P = V²/R.
- Hint 2In series, each bulb gets half the voltage.
- I = P/V = 60/230 ≈ 0.26 A.
- R = V²/P = 230²/60 ≈ 880 Ω.
- In series the total resistance doubles, so the current halves.
- Each bulb has 115 V across it, so each gives (115²)/880 ≈ 15 W, a quarter of its rating; together 30 W.
- In practice a filament's resistance falls when it is cooler, so each bulb takes a little more than 15 W.
Where it lands: About 0.26 A and 880 Ω; in series each glows at about 15 W, a quarter of normal.
The trap: Halving the power of each bulb instead of quartering it: power depends on voltage squared.
What a tutor might ask next: Which glows brighter in series, a 60 W bulb or a 100 W bulb?
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