Assignment #4
Summer 2011
Assignment #4 
DC Circuits
Chapter 27
273
. An aluminum wire having a crosssectional area equal
to
6
2
4 10
m

×
carries a current of 5.00 A.
The (
mass
)
density of aluminum is 2.7 g/cm
3
.
Assume each
aluminum atom supplies one conduction electron per
atom.
Find the drift speed of the electrons in the wire.
Atomic mass of aluminum
(p.
A22
)
= 26.982 g / mole
Avogadro’s number, N
A
22
6.02 10
=
×
per mole
275
. Suppose the current in a conductor decreases
exponentially with time according to the equation
/
( )
,
t
o
I t
I e
τ

=
where
I
o
is the initial current (at
t
= 0)
and
τ
is a constant having dimensions of time.
Consider a fixed observation point within the
conductor.
(a) How much charge passes this point
between
t
= 0 and
t
=
τ
?
(b) How much charge passes
this point between
t
= 0 and
t
= 10
τ
? (c)
What if?
How
much charge passes this point between
t
= 0 and
t
=
∝
?
2717
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 Spring '09
 AlexandreChampagne
 Electron, Current, Magnetism, Mass, equivalent resistance, heating coil, Figure P28.14, Figure P28.27

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