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Flores, Michael – Homework 11 – Due: Apr 12 2004, 6:00 pm – Inst: Sonia Paban
1
This printout should have 24 questions.
Multiplechoice questions may continue on
the next column or page – fnd all choices
be±ore making your selection. The due time is
Central time.
001
(part 1 o± 1) 10 points
Given:
c
= 2
.
99792
×
10
8
m
/
s
.
The distance
±rom the Earth to Polaris is approximately
6
.
24
×
10
18
m.
I± the North Star, Polaris, were to burn out
today, in how many years would it disappear
±rom our vision?
Correct answer: 660
.
021 years.
Explanation:
Since the light travels at a fnite velocity
c
, the last ray o± light going out ±rom Polaris
would travel
t
=
d
c
=
6
.
24
×
10
18
m
2
.
99792
×
10
8
m
/
s
= 2
.
08144
×
10
10
s
seconds be±ore arriving at the Earth. In years,
this is
t
y
=
t
×
1
60
min
/
s
×
1
60
hr
/
min
×
1
24
day
/
hr
×
1
365
yr
/
day
= 660
.
021 years
.
002
(part 1 o± 5) 10 points
Given:
c
= 2
.
99792
×
10
8
m
/
s
.
Consider a monochromatic electromagnetic
plane wave propagating in the
z
direction. At
a particular point in space, the magnitude o±
the electric feld has an instantaneous value
o± 214 V
/
m in the positive
x
direction. The
wave is traveling in the positive
z
direction.
z
x
y
E
wave propagation
Compute the instantaneous magnitude o±
the magnetic feld at the same point and
time.
Correct answer: 7
.
13827
×
10

7
T.
Explanation:
For plane, electromagnetic waves, we know
that the ratio o± the instantaneous magni
tudes o± the electric and magnetic felds is
E
B
=
c .
This means that the instantaneous magnitude
o± the magnetic feld is
B
=
E
c
=
214 V
/
m
2
.
99792
×
10
8
m
/
s
= 7
.
13827
×
10

7
T
.
003
(part 2 o± 5) 10 points
What is the instantaneous magnitude o± the
Poynting vector at the same point and time?
Correct answer: 121
.
562 W
/
m
2
.
Explanation:
The Poynting vector,
~
S
, is given by
~
S
=
1
μ
0
~
E
×
~
B .
For a plane, electromagnetic wave,
~
E
and
~
B
are always perpendicular to each other and
to the direction o± propagation o± the wave.
In this case, the Poynting vector is in the
direction o± propagation and has magnitude
S
=
E B
μ
0
=
(214 V
/
m)(7
.
13827
×
10

7
T)
1
.
25664
×
10

6
N
/
A
2
= 121
.
562 W
/
m
2
.
004
(part 3 o± 5) 10 points
What are the directions o± the instantaneous
magnetic feld and the instantaneous Poynt
ing vector respectively?
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View Full Document Flores, Michael – Homework 11 – Due: Apr 12 2004, 6:00 pm – Inst: Sonia Paban
2
1.
b
B
= ˆ
+
ˆ
k
and
b
S
=ˆ
ı
2.
b
B
= ˆ
and
b
S
=
ˆ
k
correct
3.
b
B
=ˆ
ı
and
b
S
= ˆ
4.
b
B
=ˆ
ı
and
b
S
=
ˆ
k
5.
b
B
= ˆ
and
b
S
=ˆ
ı
6.
b
B
=
ˆ
k
and
b
S
= ˆ
7.
b
B
= ˆ
and
b
S
=
ˆ
k
+ˆ
ı
8.
b
B
=ˆ
ı
+ˆ
and
b
S
=
ˆ
k
9.
b
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This note was uploaded on 02/18/2010 for the course PHYS 303l taught by Professor Panab during the Spring '04 term at North Texas.
 Spring '04
 Panab
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