Current Electricity - 1 PPT Physics Class 12

 Page 1


CURRENT  ELECTRICITY - I
1. Electric Current
2. Conventional Current
3.   Drift Velocity of electrons and current
4.   Current Density
5. Ohm’s Law
6.   Resistance, Resistivity, Conductance & 
Conductivity
7.   Temperature dependence of resistance
8.   Colour Codes for Carbon Resistors
9. Series and Parallel combination of 
resistors
10. EMF and Potential Difference of a cell
11. Internal Resistance of a cell
12. Series and Parallel combination of cells
Page 2


CURRENT  ELECTRICITY - I
1. Electric Current
2. Conventional Current
3.   Drift Velocity of electrons and current
4.   Current Density
5. Ohm’s Law
6.   Resistance, Resistivity, Conductance & 
Conductivity
7.   Temperature dependence of resistance
8.   Colour Codes for Carbon Resistors
9. Series and Parallel combination of 
resistors
10. EMF and Potential Difference of a cell
11. Internal Resistance of a cell
12. Series and Parallel combination of cells
Electric Current:
The electric current is defined as the charge flowing through 
any section of the conductor in one second.
I = q / t            (if the rate of flow of charge is steady)
I = dq / dt        (if the rate of flow of charge varies with time)
Different types of current:
I
t
0
a
b
c
d)  Alternating current whose 
magnitude varies continuously 
and direction changes 
periodically
a) Steady current which does not 
vary with time 
b) & c) Varying current whose   
magnitude varies with time
d
Page 3


CURRENT  ELECTRICITY - I
1. Electric Current
2. Conventional Current
3.   Drift Velocity of electrons and current
4.   Current Density
5. Ohm’s Law
6.   Resistance, Resistivity, Conductance & 
Conductivity
7.   Temperature dependence of resistance
8.   Colour Codes for Carbon Resistors
9. Series and Parallel combination of 
resistors
10. EMF and Potential Difference of a cell
11. Internal Resistance of a cell
12. Series and Parallel combination of cells
Electric Current:
The electric current is defined as the charge flowing through 
any section of the conductor in one second.
I = q / t            (if the rate of flow of charge is steady)
I = dq / dt        (if the rate of flow of charge varies with time)
Different types of current:
I
t
0
a
b
c
d)  Alternating current whose 
magnitude varies continuously 
and direction changes 
periodically
a) Steady current which does not 
vary with time 
b) & c) Varying current whose   
magnitude varies with time
d
Conventional Current:
Conventional current is the current 
whose direction is along the direction of 
the motion of positive charge under the 
action of electric field.
+
+
+
+
-
-
-
-
+
+
+
+
-
-
-
-
I
Drift Velocity and Current:
Drift velocity is defined as the velocity 
with which the free electrons get drifted 
towards the  positive terminal under the 
effect of the applied electric field.
I
v
d
= - (eE / m) t
- - - v
d
E
l
A
I = neA  v
d
v
d
= a t
v
d  
- drift velocity, a – acceleration, t – relaxation time,  E – electric field,        
e – electronic charge, m – mass of electron, n – number density of electrons, 
l – length of the conductor and A – Area of cross-section
Current is directly proportional 
to drift velocity.
Conventional current due to motion of 
electrons is in the direction opposite to 
that of motion of electrons.
+ + +
I
-
- -
Page 4


CURRENT  ELECTRICITY - I
1. Electric Current
2. Conventional Current
3.   Drift Velocity of electrons and current
4.   Current Density
5. Ohm’s Law
6.   Resistance, Resistivity, Conductance & 
Conductivity
7.   Temperature dependence of resistance
8.   Colour Codes for Carbon Resistors
9. Series and Parallel combination of 
resistors
10. EMF and Potential Difference of a cell
11. Internal Resistance of a cell
12. Series and Parallel combination of cells
Electric Current:
The electric current is defined as the charge flowing through 
any section of the conductor in one second.
I = q / t            (if the rate of flow of charge is steady)
I = dq / dt        (if the rate of flow of charge varies with time)
Different types of current:
I
t
0
a
b
c
d)  Alternating current whose 
magnitude varies continuously 
and direction changes 
periodically
a) Steady current which does not 
vary with time 
b) & c) Varying current whose   
magnitude varies with time
d
Conventional Current:
Conventional current is the current 
whose direction is along the direction of 
the motion of positive charge under the 
action of electric field.
+
+
+
+
-
-
-
-
+
+
+
+
-
-
-
-
I
Drift Velocity and Current:
Drift velocity is defined as the velocity 
with which the free electrons get drifted 
towards the  positive terminal under the 
effect of the applied electric field.
I
v
d
= - (eE / m) t
- - - v
d
E
l
A
I = neA  v
d
v
d
= a t
v
d  
- drift velocity, a – acceleration, t – relaxation time,  E – electric field,        
e – electronic charge, m – mass of electron, n – number density of electrons, 
l – length of the conductor and A – Area of cross-section
Current is directly proportional 
to drift velocity.
Conventional current due to motion of 
electrons is in the direction opposite to 
that of motion of electrons.
+ + +
I
-
- -
Current density:
Current density at a point, within a conductor, is the current through a unit 
area of the conductor, around that point, provided the area is perpendicular 
to the direction of flow of current at that point.
J = I / A  = nev
d
In vector form, I = J . A
Ohm’s Law:
The electric current flowing through a conductor is directly 
proportional to the potential difference across the two ends of the 
conductor when physical conditions such as temperature, mechanical 
strain, etc. remain the same.
I
V
I a V or V a I or V = R I
V
I
0
Page 5


CURRENT  ELECTRICITY - I
1. Electric Current
2. Conventional Current
3.   Drift Velocity of electrons and current
4.   Current Density
5. Ohm’s Law
6.   Resistance, Resistivity, Conductance & 
Conductivity
7.   Temperature dependence of resistance
8.   Colour Codes for Carbon Resistors
9. Series and Parallel combination of 
resistors
10. EMF and Potential Difference of a cell
11. Internal Resistance of a cell
12. Series and Parallel combination of cells
Electric Current:
The electric current is defined as the charge flowing through 
any section of the conductor in one second.
I = q / t            (if the rate of flow of charge is steady)
I = dq / dt        (if the rate of flow of charge varies with time)
Different types of current:
I
t
0
a
b
c
d)  Alternating current whose 
magnitude varies continuously 
and direction changes 
periodically
a) Steady current which does not 
vary with time 
b) & c) Varying current whose   
magnitude varies with time
d
Conventional Current:
Conventional current is the current 
whose direction is along the direction of 
the motion of positive charge under the 
action of electric field.
+
+
+
+
-
-
-
-
+
+
+
+
-
-
-
-
I
Drift Velocity and Current:
Drift velocity is defined as the velocity 
with which the free electrons get drifted 
towards the  positive terminal under the 
effect of the applied electric field.
I
v
d
= - (eE / m) t
- - - v
d
E
l
A
I = neA  v
d
v
d
= a t
v
d  
- drift velocity, a – acceleration, t – relaxation time,  E – electric field,        
e – electronic charge, m – mass of electron, n – number density of electrons, 
l – length of the conductor and A – Area of cross-section
Current is directly proportional 
to drift velocity.
Conventional current due to motion of 
electrons is in the direction opposite to 
that of motion of electrons.
+ + +
I
-
- -
Current density:
Current density at a point, within a conductor, is the current through a unit 
area of the conductor, around that point, provided the area is perpendicular 
to the direction of flow of current at that point.
J = I / A  = nev
d
In vector form, I = J . A
Ohm’s Law:
The electric current flowing through a conductor is directly 
proportional to the potential difference across the two ends of the 
conductor when physical conditions such as temperature, mechanical 
strain, etc. remain the same.
I
V
I a V or V a I or V = R I
V
I
0
Resistance:
The resistance of conductor is the opposition offered by the 
conductor to the flow of electric current through it.
R = V / I
Resistance in terms of physical features of the conductor:
I = neA | v
d
|
I = neA (e  |E| / m) t
ne
2
At
m
V
l
I =
ne
2
At
V
I
=
ml
ne
2
t A
R  =
m    l
A
R  = ?
l
where ? = 
ne
2
t
m
is resistivity or 
specific resistance
Resistance is directly proportional to 
length and inversely proportional to 
cross-sectional area of the conductor 
and depends on nature of material.
Resistivity depends upon nature of 
material and not on the geometrical 
dimensions of the conductor.