Two electric bulbs marked 25 W - 220 V and 100 W - 220 V are connected in series to a 440 V supply. Which of the bulbs will fuse? (a) Both (b) 100 W (c) 25 W (d) Neither

Answer: (c). R₂₅ = (220²)/(25) = 1936 Ω, R₁₀₀ = (220²)/(100) = 484 Ω. I = (440)/(2420) = 2/11 A. V₂₅ = 2/11(1936) = 352 V > 220 V, while V₁₀₀ = 88 V < 220 V. So the 25 W bulb fuses: answer (c).

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Electrical Power & Heating Effect

Two electric bulbs marked $25\text{ W} - 220\text{ V}$ and $100\text{ W} - 220\text{ V}$ are connected in series to a $440\text{ V}$ supply. Which of the bulbs will fuse?

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Answer: (c)

$R_{25} = \frac{220^2}{25} = 1936\ \Omega$ , $R_{100} = \frac{220^2}{100} = 484\ \Omega$ .
$I = \frac{440}{2420} = \frac{2}{11}$ A.
$V_{25} = \frac{2}{11}(1936) = 352$ V $> 220$ V, while $V_{100} = 88$ V $< 220$ V.
So the 25 W bulb fuses: answer (c).

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