? t
= 4.43 mC
gilvin (jg47854)  10: Current and current density  meyers  (21235)
1
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This homework is due Tuesday, March 12,
at midnight Tucson time.
001
10.0 points
A total charge of 4.43 mC passes through a
crosssectional area of a wire in 0.882 s.
What is the current in the wire?
Correct answer: 5.02268 mA.
Explanation:
Let :
Δ Q = 4.43 mC
and
Δ t = 0.882 s .
I = Δ Q =
5.02268 mA
.
Calculate the average drift speed of electrons
traveling through a copper wire with a cross
sectional
area
of
40
mm
2
when
carrying
a
current of 50 A (values similar to those for the
electric wire to your study lamp).
Assume
one electron per atom of copper contributes
to the current.
The atomic mass of copper
is 63.5 g/mol and its density is 8.93 g/cm
3
.
Avogadro’s number N
A
is 6.02 × 10
23
.
Correct answer: 9.22817 × 10
−5
m/s.
Explanation:
Let :
N = 1 ,
M = 63.5 g/mol ,
ρ = 8.93 g/cm
3
,
A = 40 mm
2
0.882 s
002
10.0 points
The drift velocity of free electrons in a cop
per
wire
is
7
mm/s,
resistivity
is
1.63 ×
10
−8
Ω · m, and the free electron density is
8.44 × 10
28
electrons/m
3
.
Calculate the electric field in the conductor.
Correct answer: 1.54273 N/C.
Explanation:
= 4 × 10
−5
m
2
,
I = 50 A ,
and
q
e
= 1.6 × 10
−19
C/electron .
We first calculate n, the number of current
carrying electrons per unit volume in copper.
Assuming
one
free
conduction
electron
per
atom,
n
=
N
A
ρ
M
,
where
N
A
is Avogodro
’s
number and ρ and M are the
density and the atomic weight
of copper, respectively
(
)
Let :
v
d
= 7 mm/s = 0.007 m/s ,
n ≡
n = 8.44 × 10
28
electrons/m
3
,
N
A
ρ
1 electron
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 Spring '11
 doc
 Electron, Current, Magnetism, Work, R1 R2 R1, R2 R1 R2

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