What length of the wire of specific resistance $48 \times {10^{ - 8}}\,\Omega \,\,m$ is needed to make a resistance of $4.2 \, \Omega$ .............. $m$ (diameter of wire = $0.4\, mm$)
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$l = \frac{{R\pi {r^2}}}{\rho }$$ = \frac{{4.2 \times 3.14 \times {{(0.2 \times {{10}^{ - 3}})}^2}}}{{48 \times {{10}^{ - 8}}}} = 1.1\, m$
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