Correct option is (B) 20m
1000cm2 = 2π × radius × height
Let the no. of turns the wire had made on the surface be n.
For n turns, the wire should make upto 5n mm or 0.005n m height
Since the wire should completely cover the cylinder,
∴ Height of cylinder = 0.005n m
Let the length of wire needed be l m
l = 2π × radius × n
\(\frac ln\) = 2π × radius
1000cm2 = 0.1m2 = 2π × radius × height
\(0.1 = (\frac ln) \times 0.005 n\)
\(l = \frac{0.1}{0.005} = 20 m\)