Magnetic effect of current - 2 PPT Physics Class 12

 Page 1


MAGNETIC EFFECT OF CURRENT - II
1. Lorentz Magnetic Force
2. Fleming’s Left Hand Rule
3. Force on a moving charge in uniform Electric and Magnetic fields
4. Force on a current carrying conductor in a uniform Magnetic Field
5. Force between two infinitely long parallel current-carrying 
conductors
6. Definition of ampere
7. Representation of fields due to parallel currents
8. Torque experienced by a current-carrying coil in a uniform Magnetic 
Field
9. Moving Coil Galvanometer
10.Conversion of Galvanometer into Ammeter and Voltmeter
11.Differences between Ammeter and Voltmeter
Page 2


MAGNETIC EFFECT OF CURRENT - II
1. Lorentz Magnetic Force
2. Fleming’s Left Hand Rule
3. Force on a moving charge in uniform Electric and Magnetic fields
4. Force on a current carrying conductor in a uniform Magnetic Field
5. Force between two infinitely long parallel current-carrying 
conductors
6. Definition of ampere
7. Representation of fields due to parallel currents
8. Torque experienced by a current-carrying coil in a uniform Magnetic 
Field
9. Moving Coil Galvanometer
10.Conversion of Galvanometer into Ammeter and Voltmeter
11.Differences between Ammeter and Voltmeter
Lorentz Magnetic Force:
A current carrying conductor placed in a magnetic field experiences a force 
which means that a moving charge in a magnetic field experiences force.
F
m
= q (v x B)
+
q
B
v
F
I
?
-
q
B
v
F
?
F
m
= (q v B sin ?) n
where ? is the angle between v and B
Special Cases:
i) If the charge is at rest, i.e. v  = 0, then F
m
= 0.
So, a stationary charge in a magnetic field does 
not experience any force.
ii) If ? = 0° or 180° i.e. if the charge moves parallel 
or anti-parallel to the direction of the magnetic 
field,  then F
m
= 0.
iii) If ? = 90° i.e. if the charge moves perpendicular 
to the magnetic field, then the force is 
maximum. 
F
m (max)
= q v B 
or
I
Page 3


MAGNETIC EFFECT OF CURRENT - II
1. Lorentz Magnetic Force
2. Fleming’s Left Hand Rule
3. Force on a moving charge in uniform Electric and Magnetic fields
4. Force on a current carrying conductor in a uniform Magnetic Field
5. Force between two infinitely long parallel current-carrying 
conductors
6. Definition of ampere
7. Representation of fields due to parallel currents
8. Torque experienced by a current-carrying coil in a uniform Magnetic 
Field
9. Moving Coil Galvanometer
10.Conversion of Galvanometer into Ammeter and Voltmeter
11.Differences between Ammeter and Voltmeter
Lorentz Magnetic Force:
A current carrying conductor placed in a magnetic field experiences a force 
which means that a moving charge in a magnetic field experiences force.
F
m
= q (v x B)
+
q
B
v
F
I
?
-
q
B
v
F
?
F
m
= (q v B sin ?) n
where ? is the angle between v and B
Special Cases:
i) If the charge is at rest, i.e. v  = 0, then F
m
= 0.
So, a stationary charge in a magnetic field does 
not experience any force.
ii) If ? = 0° or 180° i.e. if the charge moves parallel 
or anti-parallel to the direction of the magnetic 
field,  then F
m
= 0.
iii) If ? = 90° i.e. if the charge moves perpendicular 
to the magnetic field, then the force is 
maximum. 
F
m (max)
= q v B 
or
I
Fleming’s Left Hand Rule:
Force
(F)
Magnetic 
Field   
(B)
Electric
Current  
(I)
If the central finger, fore finger and thumb 
of left hand are stretched mutually 
perpendicular to each other and the 
central finger points to current, fore 
finger points to magnetic field, then 
thumb points in the direction of motion
(force) on the current carrying conductor.
TIP: 
Remember the phrase ‘e m f’ to represent    electric current, magnetic 
field and force in anticlockwise direction of the fingers of left hand.
Force on a moving charge in uniform Electric and Magnetic 
Fields:
When a charge q moves with velocity v in region in which both electric 
field E and magnetic field B exist, then the Lorentz force is
F = qE + q (v x B)       or F = q (E + v x B)
Page 4


MAGNETIC EFFECT OF CURRENT - II
1. Lorentz Magnetic Force
2. Fleming’s Left Hand Rule
3. Force on a moving charge in uniform Electric and Magnetic fields
4. Force on a current carrying conductor in a uniform Magnetic Field
5. Force between two infinitely long parallel current-carrying 
conductors
6. Definition of ampere
7. Representation of fields due to parallel currents
8. Torque experienced by a current-carrying coil in a uniform Magnetic 
Field
9. Moving Coil Galvanometer
10.Conversion of Galvanometer into Ammeter and Voltmeter
11.Differences between Ammeter and Voltmeter
Lorentz Magnetic Force:
A current carrying conductor placed in a magnetic field experiences a force 
which means that a moving charge in a magnetic field experiences force.
F
m
= q (v x B)
+
q
B
v
F
I
?
-
q
B
v
F
?
F
m
= (q v B sin ?) n
where ? is the angle between v and B
Special Cases:
i) If the charge is at rest, i.e. v  = 0, then F
m
= 0.
So, a stationary charge in a magnetic field does 
not experience any force.
ii) If ? = 0° or 180° i.e. if the charge moves parallel 
or anti-parallel to the direction of the magnetic 
field,  then F
m
= 0.
iii) If ? = 90° i.e. if the charge moves perpendicular 
to the magnetic field, then the force is 
maximum. 
F
m (max)
= q v B 
or
I
Fleming’s Left Hand Rule:
Force
(F)
Magnetic 
Field   
(B)
Electric
Current  
(I)
If the central finger, fore finger and thumb 
of left hand are stretched mutually 
perpendicular to each other and the 
central finger points to current, fore 
finger points to magnetic field, then 
thumb points in the direction of motion
(force) on the current carrying conductor.
TIP: 
Remember the phrase ‘e m f’ to represent    electric current, magnetic 
field and force in anticlockwise direction of the fingers of left hand.
Force on a moving charge in uniform Electric and Magnetic 
Fields:
When a charge q moves with velocity v in region in which both electric 
field E and magnetic field B exist, then the Lorentz force is
F = qE + q (v x B)       or F = q (E + v x B)
Force on a current-carrying conductor in a uniform  
Magnetic Field:
?
v
d
dl
F
I
I
B
A
l
Force experienced by each electron in 
the conductor is 
f = - e (v
d
x B)
If n be the number density of electrons, 
A be the area of cross section of the 
conductor, then no. of electrons in the 
element dl is  n A dl.
where I = neAv
d
and -ve sign represents that 
the direction of dl is opposite to that of v
d
)
or
F = I l B sin ?
-
Force experienced by the electrons in dl is
dF = n A dl [ - e (v
d
x B)]   = - n e A v
d
(dl X B)
= I (dl x B)
F = ? dF = ? I (dl x B)
F = I (l x B)
Page 5


MAGNETIC EFFECT OF CURRENT - II
1. Lorentz Magnetic Force
2. Fleming’s Left Hand Rule
3. Force on a moving charge in uniform Electric and Magnetic fields
4. Force on a current carrying conductor in a uniform Magnetic Field
5. Force between two infinitely long parallel current-carrying 
conductors
6. Definition of ampere
7. Representation of fields due to parallel currents
8. Torque experienced by a current-carrying coil in a uniform Magnetic 
Field
9. Moving Coil Galvanometer
10.Conversion of Galvanometer into Ammeter and Voltmeter
11.Differences between Ammeter and Voltmeter
Lorentz Magnetic Force:
A current carrying conductor placed in a magnetic field experiences a force 
which means that a moving charge in a magnetic field experiences force.
F
m
= q (v x B)
+
q
B
v
F
I
?
-
q
B
v
F
?
F
m
= (q v B sin ?) n
where ? is the angle between v and B
Special Cases:
i) If the charge is at rest, i.e. v  = 0, then F
m
= 0.
So, a stationary charge in a magnetic field does 
not experience any force.
ii) If ? = 0° or 180° i.e. if the charge moves parallel 
or anti-parallel to the direction of the magnetic 
field,  then F
m
= 0.
iii) If ? = 90° i.e. if the charge moves perpendicular 
to the magnetic field, then the force is 
maximum. 
F
m (max)
= q v B 
or
I
Fleming’s Left Hand Rule:
Force
(F)
Magnetic 
Field   
(B)
Electric
Current  
(I)
If the central finger, fore finger and thumb 
of left hand are stretched mutually 
perpendicular to each other and the 
central finger points to current, fore 
finger points to magnetic field, then 
thumb points in the direction of motion
(force) on the current carrying conductor.
TIP: 
Remember the phrase ‘e m f’ to represent    electric current, magnetic 
field and force in anticlockwise direction of the fingers of left hand.
Force on a moving charge in uniform Electric and Magnetic 
Fields:
When a charge q moves with velocity v in region in which both electric 
field E and magnetic field B exist, then the Lorentz force is
F = qE + q (v x B)       or F = q (E + v x B)
Force on a current-carrying conductor in a uniform  
Magnetic Field:
?
v
d
dl
F
I
I
B
A
l
Force experienced by each electron in 
the conductor is 
f = - e (v
d
x B)
If n be the number density of electrons, 
A be the area of cross section of the 
conductor, then no. of electrons in the 
element dl is  n A dl.
where I = neAv
d
and -ve sign represents that 
the direction of dl is opposite to that of v
d
)
or
F = I l B sin ?
-
Force experienced by the electrons in dl is
dF = n A dl [ - e (v
d
x B)]   = - n e A v
d
(dl X B)
= I (dl x B)
F = ? dF = ? I (dl x B)
F = I (l x B)
Forces between two parallel infinitely long current-carrying conductors:
r
F
21
F
12
I
1
P
Q
I
2
S
R
B
1
=
µ
0  
I
1 
2p r
Magnetic Field on RS due to current in PQ is
Force acting on RS due to current I
2
through it is
F
21
=
µ
0  
I
1 
2p r
I
2
l sin 90°
B
1
acts perpendicular and into the plane of the diagram by 
Right Hand Thumb Rule.  So, the angle between l and B
1
is 90° . 
l is length of the conductor.
F
21
=
µ
0  
I
1 
I
2
l
2p r
B
2
=
µ
0  
I
2 
2p r
Magnetic Field on PQ due to current in RS is
Force acting on PQ due to current I
1
through it is
F
12
=
µ
0  
I
2 
2p r
I
1
l sin 90°
F
12
=
µ
0  
I
1 
I
2
l
2p r
(The angle between l and 
B
2
is 90° and B
2
Is 
emerging out)
F
12
= F
21
= F =
µ
0  
I
1 
I
2
l
2p r
F / l =
µ
0  
I
1 
I
2
2p r
or
or
Force per unit length of the conductor is
N / m
(in magnitude)
(in magnitude)
x
B
1
B
2