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CBSE Class 10 Science Exam 2014 Term 1 Paper

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Question : 14 of 36
Marks: +1, -0
A circuit has a line of 5 A5 \text{ A}. How many lamps of rating 40 W;220 V40 \text{ W} ; 220 \text{ V} can simultaneously run on this line safely?
Solution:  
Given, I=5 AI = 5 \text{ A},
Let, No. of lamps be nn, and
Power of each lamp =40 W= 40 \text{ W}
∴  \therefore \; Power of nn lamps =40n= 40 n watt
As,
P  =V×IP \; = V \times I
40n  =220×540 n \; = 220 \times 5
⇒    n  =220×540=552\Rightarrow \; \; n\; = \frac{220 \times 5}{40} = \frac{55}{2}
  =27.5≈27\; = 27.5 \approx 27
∴     No. of lamps   =27\therefore \; \; \text{ No. of lamps } \; = 27
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