17. The resistance of the coil is given by R = ρL/A ,where L is the length of the wire, ρ is the resistivity of copper, and A is the cross-sectional area of the wire. Since each turn of wire has length 2 πr ,where r is the radius of the coil, then L = (250)2 πr = (250)(2 π )(0 . 12 m) = 188 . 5m. If r w is the radius of the wire itself, then its cross-sectional area is
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