Reilly (mar3978) – ohw21 – turner – (56725)
1
This printout should have 21 questions.
Multiplechoice questions may continue on
the next column or page – fnd all choices
beFore answering.
001
10.0 points
A circular coil is made oF
N
turns oF copper
wire as shown in the fgure. A resistor
R
is in
serted in the copper wire. Initially, a uniForm
magnetic feld oF magnitude
B
i
points hor
izontally From leFttoright through the per
pendicular plane oF the coil.
When viewed From the right the coil is
wound counterclockwise.
R
Magnetic
±ield
B
(
t
)
During a time interval
t
the feld uniFormly
changes at a constant rate, until a reversed
feld is reached equal in magnitude to the
initial feld.
The electrons in the resistor,
R ,
shown in
the fgure
1.
do not move in a preFerred direction, since
they have thermal kinetic energy.
2.
move in a direction that is undetermi
nated From the inFormation given.
3.
move leFttoright.
4.
move righttoleFt.
correct
Explanation:
As the leFttoright magnetic feld decreases
(and eventually ²ipping sign and increasing
in magnitude) it Follows From Lenz’s law (op
position to the change in magnetic feld will
tend to keep the current constant and ²owing
in the same direction) that the induced emF
will produce an leFttoright magnetic feld
arising From induced currents in the coil.
By the right hand rule, the induced current
²ows counterclockwise when viewed From the
right. The coils are wound counterclockwise
as the wire goes From the right to leFt termi
nals. The current must enter the loop From
the right terminal and exit at the leFt termi
nal.
Since the current is continuous, the current
must ²ow through the resistor in the leFtto
right direction, which means that electrons
are ²owing
righttoleFt
in the resistor. That
is, the electrons ²ow in the opposite direction
From the current since they have a negative
charge.
002 (part 1 of 4) 10.0 points
The circular loop oF wire shown in the fgure
is placed in a spatially uniForm magnetic feld
such that the plane oF the circular loop is per
pendicular to the direction For the magnetic
feld as shown in the fgure. The magnetic
feld
v
B
(
t
) varies with time, with the time de
pendence given by
B
(
t
) =
a
+
b t,
where
a
= 0
.
36 T and
b
= 0
.
044 T
/
s.
The acceleration due to gravity is 9
.
8 m
/
s
2
.
r
= 5
.
5 cm
radius
B
(
t
)
What is the magnetic ²ux through the loop
as a Function oF time?
1.
Φ
B
(
t
) = 2 (
a
+
b t
)
π r
2.
Φ
B
(
t
) = (
a
+
b
)
π r
2
3.
Φ
B
(
t
) =
p
a
t
+
b
P
π r
2
4.
Φ
B
(
t
) = 2
a π r
5.
Φ
B
(
t
) =
a π r
2
6.
Φ
B
(
t
) = 2 (
a
+
b
)
π r
7.
Φ
B
(
t
) = (
a
+
b t
)
π r
2
correct
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8.
Φ
B
(
t
) = 2
b π r
9.
Φ
B
(
t
) = 2
p
a
t
+
b
P
π r
10.
Φ
B
(
t
) =
b π r
2
Explanation:
Faraday’s Law of Induction
E
ind
=

d
Φ
B
dt
=

ΔΦ
B
Δ
t
.
Lenz’ Law is used to ±nd direction of
E
ind
.
Magnetic ²ux is de±ned by
Φ
B
≡
i
S
v
B
·
d
v
A
=
v
B
·
v
A .
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 Spring '10
 TURNER,J
 Energy, Magnetic Field, Correct Answer, electrical energy, Reilly

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