Resistance Length ?

What equal lengths of iron wire and a lead wire, each of 1 mm diameter must be joined in parallel to give an equivalent resistance of 2 ohm ? Resistivity of iron and lead are 10 and 49 micro ohm-cm respectively.

Although this looks difficult, there is really only one unknown, "L" the length of each wire. You can find two resistances, one iron, one lead, in terms of L. You can then add these resistances (in parallel), to obtain an overall resistance, again in terms of L. This expression you then set equal to 2. You now have an equation you can solve to find L.
17 August 2012
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