76. The energy density of an electromagnetic wave is given by u = u
E
+ u
B
. From the discussion in
§34-4,
u
E
= u
B
=
1
2
ε
0
E
2
, so u = 2u
E
= ε
0
E
2
. Upon averaging over time this becomes
u
avg
= ε
0
E
2
= ε
0
E
2
rms
.
Combining this equation with Eq. 34-26 in the textbook, we obtain
I =
1
cµ
0
E
2
rms
=
1
cµ
0
u
avg
ε
0
=
c
2
u
avg
c
= cu
avg
where c
2
= 1/ε
0
µ
0
is used.