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Unformatted text preview: brown (twb493) – ch24h1 – chiu – (56565)
This printout should have 15 questions.
Multiplechoice questions may continue on
the next column or page – ﬁnd all choices
before answering. 1 002 (part 2 of 3) 10.0 points
Which of the following arrows best indicates
the direction of the magnetic ﬁeld at the ×? 001 (part 1 of 3) 10.0 points
N closely spaced turns of wire are wound in
the direction indicated on a hollow, plastic
ring of radius R, with circular cross section,
as in the ﬁgure below. VIII I VII II
III VI IV
V R 1. II
2. IV
3. III I
I
If the current in the wire is I , determine the
magnetic ﬁeld B at the location indicated by
the ×, at the center of the cross section of the
ring.
µ0 2 N I
correct
1. B =
4π R
µ0 N I
2. B =
π R2
µ0 N I
3. B =
4π R
µ0 N I
4. B =
πR
µ0 2 N I
5. B =
4π R 2
Explanation:
Use Ampere’s Law with a circular loop C
of radius R along the toroid’s midline.
B · d = µ0 I
C 2 π R B = µ0 N I
⇒B= µ0 2 N I
.
4π R 4. V
5. VII
6. VIII correct
7. VI
8. I
Explanation:
Knowing which way the current is moving
through the wire, we can use the right hand
rule to determine that the magnetic ﬁeld is
going clockwise through the cross section of
the ring, meaning that at the × it is directed
toward the upperleft, or arrow VIII.
003 (part 3 of 3) 10.0 points
Throughout this region there is a uniform
electric ﬁeld E into the paper. This electric
ﬁeld begins to increase at a rate dE/dt, and
there continues to be a current I in the wire.
Now what is the magnitude of the magnetic
ﬁeld at the indicated point?
1. B = µ0 2 N I
R dE
+
2
4π R
2 π c2 dt brown (twb493) – ch24h1 – chiu – (56565) 2. B = µ0 N I
R 2 dE
+2
πR
4 c dt 3. B = µ0 N I
R dE
+2
4π R
2 c dt 4. B = R dE
µ0 2 N I
+2
correct
4π R
2 c dt 5. B = µ0 2 N I
R 2 dE
+
4π R
2 π c2 dt Explanation:
Use Ampere’s law, as modiﬁed by Maxwell,
around the same loop C :
B · d = µ0 I + µ0 0 C 2 π R B = µ0 I + µ0
⇒B= d ΦE
dt 0A dE
dt µ0 0 π R 2 d E
µ0 2 N I
+
4π R
2πR
dt = µ0 2 N I
R dE
+2
4π R
2 c dt 004 10.0 points
If the magnetic ﬁeld in a particular pulse has
a magnitude of 3.5 × 10−5 T (comparable to
the Earth’s magnetic ﬁeld), what is the magnitude of the associated electric ﬁeld? Remember that c = 3 × 108 m/s.
Correct answer: 10500 N/C.
Explanation:
Knowing that 005 (part 1 of 3) 10.0 points
In the ﬁgure below, electromagnetic radiation
is moving to the right, and at this time and
place the electric ﬁeld is horizontal and points
out of the page. The magnitude of the electric
ﬁeld is E = 2550 N/C. The speed of light is
c = 3 × 108 m/s. Take +x to be toward the
right, +y to be upward, and +z to be out of
the page toward you.
Direction of
propagation
E = 2550 N/C
What is the associated magnetic ﬁeld at
this time and place? Begin by ﬁnding the x
component, Bx .
Correct answer: 0 T.
Explanation:
We can determine the nonzero component
of B by using the righthand rule. With the
wave propagating in the +x direction, and the
electric ﬁeld in the +z direction, the magnetic
ﬁeld must point in the −y direction. Therefore the x and z components of the magnetic
ﬁeld are both zero.
006 (part 2 of 3) 10.0 points
Find By .
Correct answer: −8.5 × 10−6 T.
Explanation:
We know that the magnetic ﬁeld points
along the −y axis, so we just need to ﬁnd the
magnitude, which we can do using
E E
c= 2 , . c=
B B
we can write
E
E =c B
= (3 × 108 m/s)(3.5 × 10−5 T)
= 10500 N/C . B= c
2550 N/C
=
3 × 108 m/s
= 8.5 × 10−6 T . brown (twb493) – ch24h1 – chiu – (56565)
So the y component is −8.5 × 10−6 T . Correct answer: 0 T.
Explanation:
See the explanation for part 1. 1. 0, 0, −1 correct 008 (part 1 of 2) 10.0 points
At a time t0 , an electric ﬁeld is detected in
the region shown on the left diagram in the
ﬁgure below. The electric ﬁeld is zero at
all other locations. At a later time t1 , an
electric ﬁeld is detected in the region shown
on the right diagram. The electric ﬁeld is
still zero elsewhere. You conclude that an
electromagnetic wave is passing through the
region. The +z direction points out of the
page (toward you).
t0 y x netic wave is propagating in the −y direction.
009 (part 2 of 2) 10.0 points
If you were to measure the magnetic ﬁeld in
the region where the electric ﬁeld is nonzero,
what would be the direction of the magnetic
ﬁeld? 007 (part 3 of 3) 10.0 points
Find Bz . y 3 t1 x What is the direction of propagation of the
electromagnetic wave? Choose the unit vector
corresponding to this direction. 2. 1, 0, 0
3. 0, 0, 1
4. 0, 1, 0
5. 0, −1, 0
6. −1, 0, 0
Explanation:
We know that E × B points in the direction
of propagation. Pointing your thumb along
the −y axis to indicate the direction of propagation and pointing your ﬁngers along the −x
direction to indicate the direction of the electric ﬁeld, the righthand rule indicates that
the magnetic ﬁeld must point along the −z
axis.
010 (part 1 of 2) 10.0 points
A pulse of electromagnetic radiation is propagating in the +y direction. You have two
devices that can detect electric and magnetic
ﬁelds. You place detector 1 at location 1. 0, 1, 0
2. 0, 0, −1 0, −8, 0 m
and detector 2 at location
0 , 8 , 0 m. 3. 0, 0, 1
4. −1, 0, 0
5. 1, 0, 0
6. 0, −1, 0 correct
Explanation:
Based on the change in location of the electric ﬁeld between t0 and t1 , the electromag At time t = 0, detector 1 detects an electric
ﬁeld in the −x direction. At that instant,
what is the direction of the magnetic ﬁeld at
the location of detector 1? Choose the unit
vector corresponding to this direction.
1. 0, 0, 1 correct
2. 0, 1, 0 brown (twb493) – ch24h1 – chiu – (56565) The key to this problem is to begin by
deciding the direction of the magnetic ﬁeld.
Since the wave is propagating in the −z direction, and the electric ﬁeld is in the +x direction, the magnetic ﬁeld must be in the −y
direction, according to the righthand rule.
This means the x component of the magnetic ﬁeld is zero. 3. 0, 0, −1
4. 0, −1, 0
5. 1, 0, 0
6. −1, 0, 0
Explanation:
The direction of propagation is in the direction of E × B , which is in the +y direction.
Using the righthand rule, B must be in the
+z direction.
011 (part 2 of 2) 10.0 points
At what time will detector 2 detect electric
and magnetic ﬁelds? Remember that c =
3 × 108 m/s.
Correct answer: 5.33333 × 10−8 s. 013 (part 2 of 3) 10.0 points
What is By ?
Correct answer: −0.0253333 T.
Explanation:
Since we know the magnetic ﬁeld points
along the −y direction, we know this component is nonzero and we know the sign. We
just need the magnitude, which we can ﬁnd
using
E Explanation:
The wave propagates at the speed of light.
So we can write
distance
c=
∆t
distance
⇒ ∆t =
c
16 m
8 m − (−8 m)
=
=
8 m/ s
3 × 10
3 × 108 m/s
= 5.33333 × 10−8 s .
012 (part 1 of 3) 10.0 points
A pulse of radiation propagates with velocity
0, 0, −c m/s
where c = 3 × 108 m/s. The electric ﬁeld in
the pulse is
7.6 × 106 N/C, 0, 0 N/C.
What is the magnetic ﬁeld in the pulse? First
give the x component, Bx .
Correct answer: 0 T.
Explanation: 4 c= .
B We have
E
B= c
7.6 × 106 N/C
=
3 × 108 m/s
= 0.0253333 T . Taking the sign into account, the y component is −0.0253333 T .
014 (part 3 of 3) 10.0 points
What is Bz ?
Correct answer: 0 T.
Explanation:
Following the explanation for part 1, the z
component of the magnetic ﬁeld is zero.
015 10.0 points
Consider the electromagnetic wave pattern as
shown in the ﬁgure below. brown (twb493) – ch24h1 – chiu – (56565) E B What is the direction in which the wave is
traveling?
1. fromrighttoleft.
2. undetermined, since the ﬁgure shows a
standing wave.
3. fromlefttoright. correct
Explanation:
The direction of traveling is the direction of
E × B ; i.e., fromlefttoright. 5 ...
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This note was uploaded on 12/01/2011 for the course PHY 303L taught by Professor Turner during the Fall '08 term at University of Texas at Austin.
 Fall '08
 Turner
 Physics

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