p34 076

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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

0

E

2

rms

=

1

0

u

avg

ε

0

=

c

2

u

avg

c

= cu

avg

where c

2

= 1

0

µ

0

is used.


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