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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 crosssectional 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 crosssectional area is
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 Fall '08
 SPRUNGER
 Physics, Resistance

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