78. In the “inside” region between the plates, the individual fields (given by Eq. 24.13) are in the same
direction (
−ˆi):
E
in
=
−
50
× 10
−9
2ε
0
+
25
× 10
−9
2ε
0
ˆi =
−4.2 × 10
3
ˆi
in SI units (N/C or V/m). And in the “outside” region where x > 0.5 m, the individual fields point in
opposite directions:
E
out
=
−
50
× 10
−9
2ε
0
ˆi+
25
× 10
−9
2ε
0
ˆi =
−1.4 × 10
3
ˆi .
Therefore, by Eq. 25-18, we have
∆V =
−
0.8
0
E
· ds = −
0.5
0
|
E
|
in
dx
−
0.8
0.5
|
E
|
out
dx
=
−
4.2
× 10
3
(0.5)
−
1.4
× 10
3
(0.3)
=
2.5
× 10
3
V .