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56. (a) Current is the transport of charge; here it is being transported
“in bulk”
due to the
volume rate of flow of the powder. From Chapter 14, we recall that the volume rate of
flow is the product of the crosssectional area (of the stream) and the (average) stream
velocity. Thus,
i
=
ρ
Av
where
is the charge per unit volume. If the crosssection is that
of a circle, then
i
=
π
R
2
v
.
(b) Recalling that a Coulomb per second is an Ampere, we obtain
i
=×
=
×
−−
11 10
2 0
17 10
33
2
5
..
.
C/m
m
m/s
A.
ch
bg
π 0.050
(c) The motion of charge is not in the same direction as the potential difference computed
in problem 68 of Chapter 24. It might be useful to think of (by analogy) Eq. 748; there,
the scalar (dot) product in
P
F v
=
⋅
G
G
makes it clear that
P
= 0 if
G
G
F v
⊥
. This suggests that
a radial potential difference and an axial flow of charge will not together produce the
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This note was uploaded on 06/03/2011 for the course PHY 2049 taught by Professor Any during the Spring '08 term at University of Florida.
 Spring '08
 Any
 Physics, Charge, Current

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