A copper wire of length 1m and radius 1mm is joined in series with an iron wire of length 2m and radius 3mm and a current is passed through the wire. The ratio of current densities in the copper and iron wire is
A
18 : 1
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B
9 : 1
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C
6 : 1
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D
2 : 3
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Solution
The correct option is B9 : 1 We know J=iA when i = constant J∞1A⇒JcJi=AiAc=(rirc)2=(31)2=91