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PHYS1200
PHYSICS II
SPRING 2005
Class 18 Activity: Intensity of Waves
SOLUTION
A
.
Suppose that waves are being emitted uniformly in all directions from a source, and the
waves do not lose energy as they propagate. At a distance of 4.0 meters from the source the
intensity of the waves is 0.50 W/m².
1.
At what rate is the source radiating energy? That is, what is the power of the source,
P
s
?
P
s
= 4
π
r
²
I
= 4
(4.0 m)²(0.50 W/m²) = 100 W
2.
Find the intensity of the waves at a distance of 10.0 meters from the source.
2
1
r
I
∝
, so
16
.
0
)
m
10
(
)
m
4
(
)
m
4
(
)
m
10
(
2
2
=
=
I
I
. Then,
I
(10 m) = 0.16
I
(4 m) = 0.16(0.50 W/m²)
I
(10 m) = 0.08 W/m²
B
.
Now suppose that the radiation from the source that was used in part
A
is confined to a small
angle, so that it does not spread uniformly over the surface of a sphere. Instead, all of the
radiation passes through a spot that is 1/100 times the area of the sphere.
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 Spring '06
 Stoler
 Physics, Light, Wavelength

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