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12.What(sp. resistance 48 x 10-8ohm m)is needed to make a resistance of 4.2 ohm ?lengthofwire(Diameter 0.4 mm).[109 cm]

Answer»

Given R = 4.2 ohmrho = 48 x 10^-8 ohm mr = d/2 = 0.4/2 = 0.2 mm = 2 x 10^-4 mpi = 22/7So needed L = pi r^2 R / rhoL = 22/7 * 4 * 10^-8 * 4.2 / 48 x 10^-8So L =1.1 m=110cm



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