67.

(a) We use m

e

g = eE = eσ/ε

0

to obtain the surface charge density.

σ =

m

e

gε

0

e

=

(9.11

× 10

−31

kg)

9.8 m/s

2

 

8.85

× 10

−12 C

2

N

·m

2



1.60

× 10

−19

C

= 4.9

× 10

−22

C/m

2

.

(b) Downward (since the electric force exerted on the electron must be upward).