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).