Chapter 26
Current and Resistance
In this chapter we will introduce the following new concepts:
Electric current
( symbol
i
)
Electric current density vector
(symbol
)
Drift speed
(symbol
v
d
)

Resistance
(symbol
R
)
and resistivity
(symbol
ρ
)
of a conductor
Ohmic and nonOhmic
conductors
We will also cover the following topics:
Ohm’s law
Power in electric circuits
J
G
(26  1)
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Consider the conductor shown in fig.a.
All the points inside the conductor and
on its surface are at the same potential.
The free electrons inside the conductor
move at random directi
Electric current
ons and thus
there is no net charge transport.
We now make a break in the conductor
and insert a battery as shown in fig.b.
Points A and B are now at potentials
and
, respectively.
(
the
A
B
A
B
V
V
V
V
V
−
=
voltage of the battery)
The situation is not static any more but
charges
move inside the conductor so that
there is a net
charge flow in a particular
direction.
This net flow of electric charge
we define as "
"
electric currrent
A
B
(26  2)