36.
(a) Since r
1
= 10.0 cm < r = 12.0 cm < r
2
= 15.0 cm,
E(r) =
1
4πε
0
q
1
r
2
=
(8.99
× 10
9
N
·m
2
/C
2
)(4.00
× 10
−8
C)
(0.120 m)
2
= 2.50
× 10
4
N/C .
(b) Since r
1
< r
2
< r = 20.0 cm,
E(r)
=
1
4πε
0
q
1
+ q
2
r
2
=
(8.99
× 10
9
N
·m
2
/C
2
)(4.00 + 2.00)(1
× 10
−8
C)
(0.200 m)
2
=
1.35
× 10
4
N/C .