Current & Resistance
Current & Resistance
(Conductors
NOT
in equilibrium;
E
≠
0)
Text sections 27.1, 27.2
Current and current density
Ohm’s Law, resistivity, and resistance
Practice problems: 275, 279, 2715, 2717
Electrical Current
Electrical Current
CURRENT
I
is the charge per unit time flowing
along a wire:
if charge
dQ
flows in time
dt
Units
:
1 ampere (A) = 1
C/s
Direction
:
by convention
, the direction of
movement of
positive
charge
dt
dQ
I
≡
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I
I
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A
L = v
d
∆
t
Charge passing through the shaded circle in time
∆
t
:
Q = (number of charges/volume) x (charge on each one) x volume
Current:
I = Q/
∆
t = nqAv
d
∆
t /
∆
t
So,
Q = n ·q ·(AL) = nqAv
d
∆
t
I = nqAv
d
v
d
= average (“drift”)
velocity of each charge
Current Density
Current Density
(a
vector
vector
):
A
I
J
≡
flow
current
area
is
A
where
⊥
Units:
Amps/m
2
(Note that the “current through a surface” is the
flux of the current density
through that surface.)
So,
J
=
nq
v
d
(a
vector
equation)
In normal conductors,
J
is caused by an
electric field
in the conductor—which is
not
in equilibrium
.
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 Spring '09
 Britz
 Equilibrium, Electron, Resistance, Electric charge, current density

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