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C
URRENT
,
R
ESISTANCE
,
AND
E
LECTROMOTIVE
F
ORCE
25.4.
(a)
IDENTIFY:
By definition,
J = I/A
and radius is onehalf the diameter.
SET UP:
Solve for the current:
I = JA = J
π(
D
/2)
2
EXECUTE:
I
= (1.50
10
6
A/m
2
)(
π
)[(0.00102 m)/2]
2
= 1.23 A
EVALUATE:
This is a realistic current.
(b)
IDENTIFY:
The current density is
J = nqv
d
SET UP:
Solve for the drift velocity:
v
d
= J/nq
EXECUTE:
Since most laboratory wire is copper, we use the value of
n
for copper, giving
62
d
(1.50 10 A/m )
v
/[(8.5
10
28
el/m
3
)(1.60
19
10
C) = 1.1
4
10
m/s = 0.11 mm/s
EVALUATE:
This is a typical drift velocity for ordinary currents and wires.
25.16.
IDENTIFY:
Apply
L
R
A
and solve for
L
.
SET UP:
2
/4
AD
, where
0.462 mm
D
.
EXECUTE:
32
8
(1.00
)(
4)(0.462 10
m)
9.75 m.
1.72 10
m
RA
L
EVALUATE:
The resistance is proportional to the length of the wire.
25.24.
IDENTIFY:
Apply
L
R
A
and
V
IR
.
SET UP:
2
Ar
EXECUTE:
42
7
(4.50 V) (6.54 10
m)
1.37 10
m.
(17.6 A)(2.50 m)
RA
VA
L
IL
EVALUATE:
Our result for
shows that the wire is made of a metal with resistivity greater than that of good
metallic conductors such as copper and aluminum.
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This note was uploaded on 10/25/2010 for the course PHYS 260 taught by Professor Hkmiet during the Spring '08 term at George Mason.
 Spring '08
 HKMIET
 Current, Resistance, Force

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