A wire of length 5m and radius 1mm has resistance of 1Ω. What length of the same material at the same temperature, and of radius 2mm will also have a resistance of 1Ω ?
A
1.25m
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B
2.5m
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C
10m
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D
20m
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Solution
The correct option is D20m R=ρlA
R1=ρ1l1A1=ρ2l2A2=R2 Material is same, ρ1=ρ2 ⟹l1πr21=l2πr22 ⟹5(1)2=l2(2)2 ⟹l2=20m