terry (ect328) – homework 25 – Turner – (59130)
1
This
printout
should
have
11
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
001
10.0 points
In the arrangement shown in the figure,
the resistor is
R
and a
B
magnetic field is
directed out of the paper.
The separation
between the rails is
.
Neglect the mass
m
of the bar.
Assume the bar and rails have
negligible resistance and friction.
An applied force moves the bar to the left
at a constant speed of
v .
m
v
R
B
B
a
z
What is the relationship between the elec
tric potential at the ends of the resistor (
z
or
a
) while the bar is moving on the “right” side
of the resistor (towards the resistor) and mov
ing on the “left” side of the resistor (after the
bar moves past the resistor)?
1.
V
a
> V
z
(right)
and
V
z
> V
a
(left)
2.
V
z
> V
a
(right)
and
V
z
> V
a
(left)
3.
V
a
> V
z
(right)
and
V
a
> V
z
(left)
correct
4.
V
z
> V
a
(right)
and
V
a
> V
z
(left)
5.
V
z
=
V
a
(right)
and
V
z
=
V
a
Explanation:
As the bar moves toward the resistor, the
area of the current loop decreases,
so the
induced
B
ind
is upward with
I
ind
counter
clockwire from above.
Lenz’s law dictates that before moving past
the resistor, current flows from
a
to
z
, so
a
is
at a higher potential.
After going past the resistor, Lenz’s law
dictates that the induced
B
ind
is now down
ward.
This requires the current to reverse its ro
tational direction to be clockwise from above.
However, the direction
a
to
z
(through the
resistor
R
) also reverses its rotational direc
tion. The
emf
across the bar does not change
sign;
i.e.
, the current through the resistor
R
remains in the same direction.
002
10.0 points
A rectangular coil of 50 turns, 0
.
26 m by
0
.
2 m, is rotated at 141 rad
/
s in a magnetic
field so that the axis of rotation is perpendicu
lar to the direction of the field. The maximum
emf
induced in the coil is 0
.
3 V.
What is the magnitude of the field?
Correct answer: 0
.
818331 mT.
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 Spring '10
 Turner
 Magnetic Field, Electric charge, Terry

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