A copper wire of length 1 m and radius 1 mm is joined in series with an iron wire of length Z m and radius 3 mm and a current is passed through the wires. The ratio of the current density in the copper and iron wires is
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a
18 : 1
b
9 : 1
c
6 : 1
d
2 : 3
answer is B.
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Detailed Solution
Current density J=iA = iπr2 ⇒ J1J2 = i1i2 × r22r12But the wires are in series, so they have the samecurrent, hence i1=i2. So J1J2 = r22r12=9:1