PHYS 142 Lecture 17

1
EXAM 1 Distribution
46 49 52 55 58 61 64 67 70 73 76 79
82 85 88 91 94 97 100
Mean 75.0 Standard deviation 11.7
2
The force on the top segment of the rectangular
loop is
a
b
1)up. 2)down. 3)into screen. 4)out. 5)left. 6)rig
ht 7)zero.
s
N
3
The force on the bottom segments of the
rectangular loop is
a
b
1)up. 2)down. 3)into screen. 4)out. 5)left. 6)rig
ht 7)zero.
s
N
4
The force on the left segment of the rectangular
loop is
a
b
1)up. 2)down. 3)into screen. 4)out. 5)left. 6)rig
ht. 7)zero.
s
N
5
The force on the right segment of the rectangular
loop is
a
b
1)up. 2)down. 3)into screen. 4)out. 5)left. 6)rig
ht. 7)zero.
s
N
6
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7
A mass oscillates displaying the Simple Harmonic
Motion plotted in the position vs time graph at
right.
Position
time
1) 2) 3) 4)
Velocity
Which of the velocity vs time graphs best
represent its motion?
Velocity
Velocity
Velocity
8
Position
time
k m
Velocity
9
A positively charged sphere is set spinning as
shown at left.
1)A magnetic field is set up with a North Pole
at the spheres top. 2)A magnetic field is set
up with a South Pole at the spheres
top. 3)The magnetic field lines form closed
concentric circles centered on the spin
axis. 4)Since the charge stays in a fixed
position, no magnetic field is generated.
10
S N
11
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12
Oxygen O8
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d
Iron Fe26
13
If a N pole moves towards this loop, both flat in
the plane of the page, in what direction is the
induced current? 1) clockwise 2)
counter-clockwise 3) no induced current
S
N
14
If a conducting coil is rotated one quarter-turn
(90o) in the direction shown, while within the B
field pointing right, in what direction will
current be induced while turning? 1)
clockwise 2) counter-clockwise 3) no
induced current
15

• Wire 1 (length L) forms a 1-turn loop,
• and a bar magnet is dropped through.
• Wire 2 (length 2L) forms a 2-turn loop,
• the same magnet is dropped through.
• Compare the induced EMFs for both.
• 1) ? 1 lt ? 2 2) ? 1 ? 2 3) ? 1 gt ? 2

Compare the magnitude of the induced currents in
these two cases. 1) I 1 lt I 2 2) I 1
I 2 3) I 1 gt I 2
16
A bar magnet is held above the floor and dropped.
In A, there is nothing between the magnet and
the floor. In B, the magnet falls through a
copper loop. In which case will the magnet fall
faster? 1) A 2) B 3) same speed
for both
S
S
A
B
N
N
copper
17
Answers to Concept questions
Top loop current crosses field lines into the
page. Fingers of right hand to the right, thumb
following current across the lines (into the
page)palm faces DOWN.
2)down.
1)up.
Exactly balancing the force from above.
3)into screen. 4)out.
Thumb points up, palm faces into page.
Fingers of right hand still to the right, thumb
points down.
4)
As the slide that follows this question shows.
The angle between the B field lines and the
normal to the loop is always 90 degrees. Since
cos(90) 0 the flux is always zero and
therefore not changing.
3) no induced current
Flux will be increasing. Current will oppose
increase, so induced B field will point to the
left. The Right Hand Rule indicates a CW current.
1) clockwise
1) ? 1 lt ? 2
? N ?? / ?t. Both loops have same ?? / ?t
but loop 2 has N 2.
I V/R. Loop 2 has twice the EMF of loop 1,
but also twice the resistance since it is twice
as long.
2) I 1 I 2
1) A
When the magnet is above, an induced current must
produce an N-pole at the top of the loop,
repelling the magnet. Once below, the induced
current produces an N pole at the loops bottom,
attracting the magnets S-pole. Or, think about
energy. When an induced current flows its energy
has to come from somewhere! It must come from
the kinetic energy (½mv2) of the falling magnet,
i.e., it must fall more slowly.