58.
(a) Since ρ = RA/L = πRd
2
/4L = π(1.09
× 10
−3
Ω)(5.50
× 10
−3
m)
2
/[4(1.60 m)] = 1.62
× 10
−8
Ω
·m,
the material is silver.
(b) The resistance of the round disk is
R = ρ
L
A
=
4ρL
πd
2
=
4(1.62
× 10
−8
Ω
·m)(1.00 × 10
−3
m)
π(2.00
× 10
−2
m)
2
= 5.16
× 10
−8
Ω .