lew (dl9564) – hw 6 – Opyrchal – (121006)
1
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printout
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have
13
questions.
Multiplechoice questions may continue on
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before answering.
001
10.0 points
A conductor with crosssectional area 4 cm
2
carries a current of 19 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
.
00592943 mm
/
s.
Explanation:
Let :
I
= 19 A
,
n
= 5
×
10
28
electrons
/
m
3
,
q
e
= 1
.
60218
×
10
−
19
C
,
and
A
= 4 cm
2
= 0
.
0004 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
=
19 A
5
×
10
28
electrons
/
m
3
×
1
1
.
60218
×
10
−
19
C
×
1
0
.
0004 m
2
×
1000 mm
1 m
=
0
.
00592943 mm
/
s
.
002
10.0 points
A wire with a resistance
R
is lengthened to
6
.
62 times its original length by pulling it
through a small hole.
Find the resistance of the wire after it is
stretched.
Correct answer: 43
.
8244
R
.
Explanation:
Let :
ℓ
′
= 6
.
62
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
6
.
62
ℓ
=
A
6
.
62
.
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 Fall '08
 Opyrchal
 Correct Answer

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