19. The resistance of the coil is given by
R
=
ρ
L
/
A
, where
L
is the length of the wire,
is
the resistivity of copper, and
A
is the crosssectional area of the wire. Since each turn of
wire has length 2
π
r
, where
r
is the radius of the coil, then
L
= (250)2
π
r
= (250)(2
π
)(0.12 m) = 188.5 m.
If
r
w
is the radius of the wire itself, then its crosssectional area is
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 Spring '10
 Rebello,NobelS
 Resistance, crosssectional area

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