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 .