1

2
19.1 Electric Current
Definition
:
the current is the rate at
which charge flows through this
surface.
When a net charge
Q passes through a cross section of conductor
during time
t, the current is
The SI units of current is
the ampere (A)
.
1 A = 1 C/s
1 A of current is equivalent to 1 C of charge passing through
the area in a time interval of 1 s.

3
Example:
The amount of charge that passes through the filament
of a certain light bulb in 2.00 s is 1.67 c. Find the current in the
light bulb.
Find no. of electrons?

4
19.2 Resistance and Ohm
’
s Law
When a
voltage
(potential difference) is
applied across the ends
of
a metallic
conductor
, the
current
is found to be
proportional
to the applied
voltage
.
In situations where the proportionality is exact, one
can write.
The proportionality constant R is called resistance of the conductor
.

5
The resistance is defined as the ratio.
In
SI
, resistance is expressed in volts per ampere.
A special name is given:
ohms
Example:
if a potential difference of 10 V applied across a
conductor produces a 0.2 A current,
then one concludes
the conductors has a resistance of
10 V/0.2 A = 50
.

6
Ohm
’
s Law
•
Resistance
in a conductor arises because of
collisions
between electrons and fixed charges
within the material.
•
In many materials, including most metals, the resistance
is constant over a wide range of applied voltages.
•
This is a statement of Ohm’s law.
Ohm’s Law

7
Ohmic materials:
the
I-V curve
for is
linear
.
This device does
obey Ohm’s law.
None-Ohmic materials:
the
I-V curve
for is
nonlinear
diode.
This device does not obey Ohm’s law
.

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Resistivity
•
Electrons moving inside a conductor subject to an external
potential constantly collide with atoms of the conductor.
•
They lose energy and are repeated re-accelerated by the
electric field produced by the external potential.
•
The collision process is equivalent to an internal friction.
•
This is the origin of a material’s
resistance
.

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•
The resistance
R of an ohmic conductor is proportional
to the
its length,
l
, and
inversely proportional
to the cross section
area,
A
, of the conductor
.
The constant of proportionality
is called the resistivity
of the material.
Every material has a characteristic resistivity
that depends on its
electronic structure, and the temperature.
Good conductors have low resistivity
.
Insulators have high resistivity
.

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Resistivity - Units
•
Resistance expressed in Ohms,
•
Length in meter.
•
Area are m
2
,
•
Resistivity thus has units of
m.

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Resistivity of various materials
at room temperature

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Example
(a) Calculate the resistance per unit length of a nichrome wire
of radius 0.321 mm.
Cross section:
Resistivity (Table):
1.5 x 10
m.

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