36.

(a) We use Eq. 25-43 with q

1

= q

2

=

−e and r = 2.00 nm:

U = k

q

1

q

2

r

= k

e

2

r

=

8.99

× 10

9 N

·m

2

C

2



(1.60

× 10

−19

C)

2

2.00

× 10

−9

m

= 1.15

× 10

−19

J .

(b) Since U > 0 and U

∝ r

−1

the potential energy U decreases as r increases.