taherisefat (mt23852) – hw 6 – Opyrchal – (121012)
1
This printout should have 13 questions.
Multiplechoice questions may continue on
the next column or page – Fnd all choices
before answering.
001
10.0 points
A conductor with crosssectional area 8 cm
2
carries a current of 9 A.
If the concentration of free electrons in the
conductor is 5
×
10
28
electrons
/
m
3
, what is
the drift velocity of the electrons?
Correct answer: 0
.
00140434 mm
/
s.
Explanation:
Let :
I
= 9 A
,
n
= 5
×
10
28
electrons
/
m
3
,
q
e
= 1
.
60218
×
10
−
19
C
,
and
A
= 8 cm
2
= 0
.
0008 m
2
.
The current in a conductor is given by
I
=
n q v
d
A ,
where
n
is the number of charge carriers per
unit volume,
q
is the charge per carrier,
v
d
is
the drift velocity of the carriers and
A
is the
cross section of the conduction.
Solving for
v
d
, we have
v
d
=
I
n q A
=
9 A
5
×
10
28
electrons
/
m
3
×
1
1
.
60218
×
10
−
19
C
×
1
0
.
0008 m
2
×
1000 mm
1 m
=
0
.
00140434 mm
/
s
.
002
10.0 points
A wire with a resistance
R
is lengthened to
7
.
18 times its original length by pulling it
through a small hole.
±ind the resistance of the wire after it is
stretched.
Correct answer: 51
.
5524
R
.
Explanation:
Let :
ℓ
′
= 7
.
18
ℓ .
The volume
V
of the wire is given by
V
=
ℓ A,
where
ℓ
is the length of the wire and
A
is
the crosssectional area. When the wire is
stretched to a length
ℓ
′
, the volume will re
main the same. The new crosssectional area
is then,
ℓ A
=
ℓ
′
A
′
A
′
=
ℓ A
ℓ
′
=
ℓ A
7
.
18
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 Spring '08
 moro
 Correct Answer, 1 Day, 0.106 mm, 0.212 mm

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