Current Electricity - 2 PPT Physics Class 12

 Page 1


CURRENT  ELECTRICITY - II
1. Kirchhoff’s Laws of electricity
2. Wheatstone Bridge
3.   Metre Bridge
4. Potentiometer
i)  Principle
ii) Comparison of emf of primary cells
Page 2


CURRENT  ELECTRICITY - II
1. Kirchhoff’s Laws of electricity
2. Wheatstone Bridge
3.   Metre Bridge
4. Potentiometer
i)  Principle
ii) Comparison of emf of primary cells
KIRCHHOFF’S LAWS:
I Law or Current Law or Junction Rule:  
The algebraic sum of electric currents at a junction in any 
electrical network is always zero.
O
I
1
I
4
I
2
I
3
I
5
I
1
- I
2
- I
3
+ I
4
- I
5
= 0
Sign Conventions:
1. The incoming currents towards the junction are taken positive.
2. The outgoing currents away from the junction are taken negative.
Note:  The charges cannot accumulate at a junction.  The number 
of charges that arrive at a junction in a given time must leave in 
the same time in accordance with conservation of charges.
Page 3


CURRENT  ELECTRICITY - II
1. Kirchhoff’s Laws of electricity
2. Wheatstone Bridge
3.   Metre Bridge
4. Potentiometer
i)  Principle
ii) Comparison of emf of primary cells
KIRCHHOFF’S LAWS:
I Law or Current Law or Junction Rule:  
The algebraic sum of electric currents at a junction in any 
electrical network is always zero.
O
I
1
I
4
I
2
I
3
I
5
I
1
- I
2
- I
3
+ I
4
- I
5
= 0
Sign Conventions:
1. The incoming currents towards the junction are taken positive.
2. The outgoing currents away from the junction are taken negative.
Note:  The charges cannot accumulate at a junction.  The number 
of charges that arrive at a junction in a given time must leave in 
the same time in accordance with conservation of charges.
II Law or Voltage Law or Loop Rule:
The algebraic sum of all the potential drops and emf’s along any 
closed path in an electrical network is always zero.
Sign Conventions:
1. The emf is taken negative when we traverse from positive to negative
terminal of the cell through the electrolyte.
2. The emf is taken positive when we traverse from negative to positive
terminal of the cell through the electrolyte.
The potential falls along the direction of current in a current path 
and it rises along the direction opposite to the current path.  
3. The potential fall is taken negative.
4. The potential rise is taken positive.
Loop ABCA:
- E
1
+ I
1
.R
1
+ (I
1
+ I
2
).R
2
= 0
E
1
R
1
E
2
R
3
R
2
I
1
I
2
I
1
I
2
I
1
I
2
I
1
+ I
2
A
B
C
D
Note: The path can be traversed 
in clockwise or anticlockwise 
direction of the loop.
Loop ACDA:
- (I
1
+ I
2
).R
2
- I
2
.R
3
+  E
2
= 0
Page 4


CURRENT  ELECTRICITY - II
1. Kirchhoff’s Laws of electricity
2. Wheatstone Bridge
3.   Metre Bridge
4. Potentiometer
i)  Principle
ii) Comparison of emf of primary cells
KIRCHHOFF’S LAWS:
I Law or Current Law or Junction Rule:  
The algebraic sum of electric currents at a junction in any 
electrical network is always zero.
O
I
1
I
4
I
2
I
3
I
5
I
1
- I
2
- I
3
+ I
4
- I
5
= 0
Sign Conventions:
1. The incoming currents towards the junction are taken positive.
2. The outgoing currents away from the junction are taken negative.
Note:  The charges cannot accumulate at a junction.  The number 
of charges that arrive at a junction in a given time must leave in 
the same time in accordance with conservation of charges.
II Law or Voltage Law or Loop Rule:
The algebraic sum of all the potential drops and emf’s along any 
closed path in an electrical network is always zero.
Sign Conventions:
1. The emf is taken negative when we traverse from positive to negative
terminal of the cell through the electrolyte.
2. The emf is taken positive when we traverse from negative to positive
terminal of the cell through the electrolyte.
The potential falls along the direction of current in a current path 
and it rises along the direction opposite to the current path.  
3. The potential fall is taken negative.
4. The potential rise is taken positive.
Loop ABCA:
- E
1
+ I
1
.R
1
+ (I
1
+ I
2
).R
2
= 0
E
1
R
1
E
2
R
3
R
2
I
1
I
2
I
1
I
2
I
1
I
2
I
1
+ I
2
A
B
C
D
Note: The path can be traversed 
in clockwise or anticlockwise 
direction of the loop.
Loop ACDA:
- (I
1
+ I
2
).R
2
- I
2
.R
3
+  E
2
= 0
Wheatstone Bridge:
I
1
I
I
g
I
1
- I
g
I - I
1
E
A
B
C
D
P
Q
R
S
G
I
I I
I - I
1 
+ I
g
Loop ABDA:
-I
1
.P - I
g
.G + (I - I
1
).R = 0
Currents through the arms are assumed by 
applying Kirchhoff’s Junction Rule.
Applying Kirchhoff’s Loop Rule for:
When I
g
= 0, the bridge is said to balanced. 
By manipulating the above equations, we get
Loop BCDB:
- (I
1
- I
g
).Q + (I - I
1
+ I
g
).S + I
g
.G = 0
P
Q
R
S
Page 5


CURRENT  ELECTRICITY - II
1. Kirchhoff’s Laws of electricity
2. Wheatstone Bridge
3.   Metre Bridge
4. Potentiometer
i)  Principle
ii) Comparison of emf of primary cells
KIRCHHOFF’S LAWS:
I Law or Current Law or Junction Rule:  
The algebraic sum of electric currents at a junction in any 
electrical network is always zero.
O
I
1
I
4
I
2
I
3
I
5
I
1
- I
2
- I
3
+ I
4
- I
5
= 0
Sign Conventions:
1. The incoming currents towards the junction are taken positive.
2. The outgoing currents away from the junction are taken negative.
Note:  The charges cannot accumulate at a junction.  The number 
of charges that arrive at a junction in a given time must leave in 
the same time in accordance with conservation of charges.
II Law or Voltage Law or Loop Rule:
The algebraic sum of all the potential drops and emf’s along any 
closed path in an electrical network is always zero.
Sign Conventions:
1. The emf is taken negative when we traverse from positive to negative
terminal of the cell through the electrolyte.
2. The emf is taken positive when we traverse from negative to positive
terminal of the cell through the electrolyte.
The potential falls along the direction of current in a current path 
and it rises along the direction opposite to the current path.  
3. The potential fall is taken negative.
4. The potential rise is taken positive.
Loop ABCA:
- E
1
+ I
1
.R
1
+ (I
1
+ I
2
).R
2
= 0
E
1
R
1
E
2
R
3
R
2
I
1
I
2
I
1
I
2
I
1
I
2
I
1
+ I
2
A
B
C
D
Note: The path can be traversed 
in clockwise or anticlockwise 
direction of the loop.
Loop ACDA:
- (I
1
+ I
2
).R
2
- I
2
.R
3
+  E
2
= 0
Wheatstone Bridge:
I
1
I
I
g
I
1
- I
g
I - I
1
E
A
B
C
D
P
Q
R
S
G
I
I I
I - I
1 
+ I
g
Loop ABDA:
-I
1
.P - I
g
.G + (I - I
1
).R = 0
Currents through the arms are assumed by 
applying Kirchhoff’s Junction Rule.
Applying Kirchhoff’s Loop Rule for:
When I
g
= 0, the bridge is said to balanced. 
By manipulating the above equations, we get
Loop BCDB:
- (I
1
- I
g
).Q + (I - I
1
+ I
g
).S + I
g
.G = 0
P
Q
R
S
Metre Bridge:
A B
R.B (R)
X
G
J
K
E
l cm 100 - l cm
Metre Bridge is based 
on the principle of 
Wheatstone Bridge.
When the galvanometer 
current is made zero by 
adjusting the jockey 
position on the metre-
bridge wire for the given 
values of known and 
unknown resistances,
R       R
AJ
X        R
JB
R       AJ
X       JB
R          l
X       100 - l
(Since, 
Resistance a
length)
Therefore,            X = R (100 – l) / l