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Question:

Incandescent bulbs are designed by keeping in mind that the resistance of their filament increases with the increase in temperature. If at room temperature, 100W, 60W and 40W bulbs have filament resistances R100, R60 and R40, respectively, the relation between these resistances is

R100=R40+R60

R100>R60>R40

1/R100=1/R40+1/R60

1/R100>1/R60>1/R40

Solution:

We know that power is given by- P=V²/R
Now, in circuit with same voltage the wattage of bulbs are written.
⇒P ∝ 1/R
Now, 100W>60W>40W
⇒P100>P60>P40
⇒1/R100>1/R60>1/R40
Answer-(D)