Using Gauss’s law, it is
E
0
0
1
=
=
Q
Q
A
AA
Φ
ε
=ε
E
(8 3)
Now if the charge Q on the capacitor plates changes with time, there is a
current i = (dQ/dt), so that using Eq (8 3), we have
d
d
dd
d
d
tΦE
t
Q
tQ
=



ε =
ε
0
0
1
This implies that for consistency,
ε0
d
d
tΦE


  = i
(8