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Question: Answered & Verified by Expert
Incandescent bulbs are designed by keeping in mind that the resistance of their filament increases with the increase in temperature. If at room temperature, $100 \mathrm{~W}, 60 \mathrm{~W}$ and $40 \mathrm{~W}$ bulbs have filament resistances $R_{100}, R_{60}$ and $R_{40}$, respectively, the relation between these resistances is
PhysicsCurrent ElectricityJEE Main
Options:
  • A
    $\frac{1}{R_{100}}=\frac{1}{R_{40}}+\frac{1}{R_{60}}$
  • B
    $R_{100}=R_{40}+R_{60}$
  • C
    $R_{100}>R_{60}>R_{40}$
  • D
    $\frac{1}{R_{100}}>\frac{1}{R_{60}}>\frac{1}{R_{40}}$
Solution:
2479 Upvotes Verified Answer
The correct answer is:
$\frac{1}{R_{100}}>\frac{1}{R_{60}}>\frac{1}{R_{40}}$
$R=\frac{V^2}{P}$ or $R \propto \frac{1}{P}$ $\therefore \quad \frac{1}{R_{100}}>\frac{1}{R_{60}}>\frac{1}{R_{40}}$
Hence, the correct option is (d).

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