Transcript Slide 1

TOPIC
ELECTROMAGNETIC
INDUCTION
Previous Knowledge Test
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What do you mean by electric current ?
Name the instrument used to detect electric current in a circuit ?
What do you understand by magnetic effect of current ?
What do you mean by magnetic field ?
What is the difference between magnetic field of a magnet & that of
associated with current carrying conductor ?
Can electric current be produced from magnetic field ?
Is there any relation between electricity & magnetism ?
Can we get electric current without a battery ?
OBJECTIVES
At the end of this presentation, you’ll know :
1. The meaning of magnetic flux,
Electromagnetic-induction
2. What are the Faraday’s laws of EMI ?
3. Electric current can be produced from magnetic
field
4. What is the working principle of power
generators, dynamos & transformers ?
5. How electricity at large scale is produced &
transmitted?
IMPORTANCE OF THE TOPIC
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Initially, the electrical energy was
obtained from chemical energy (cells
& batteries).
The present large scale production &
use of electricity would not have
been possible if the only way of
production of electrical energy had
remained of chemical nature.
CONTINUES….
IMPORTANCE OF THE TOPIC
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
This problem was solved with the
discovery of ‘Electromagnetic
induction’, which is the basis of
working of power generators &
transformers etc.
We can’nt imagine the world without
electricity and we know; electricity is
produced using this phenomenon of
’EMI’.
CONTINUES….
MAGNETIC FLUX(Ф)
The magnetic flux through any
surface held in a magnetic field
is measured by the total No. of
magnetic field lines crossing the
Surface.
Magnetic field
lines
dA
Weber SI unit of magnetic flux .
For a uniform magnetic field (B)
crossing the plane of area A at an
angle with the normal to the plane,
Ф = B A cos(θ) …….(1)
n^
θ
B
Magnetic flux linked with a surface
CONTINUES…
n^
Minimum magnetic flux is
linked with the surface, when
θ =90° (in fig.1)
Ф =B.A.cos 90° = 0
θ
B
Maximum magnetic flux is
linked with the surface,
when θ = 0° (fig.2)
Magnetic field lines
Fig.1
Ф = BA cos 0° = BA
= max. value
If the coil has N turns, total
amount of magnetic flux
linked with the coil is,
Ф = N (B.A)= NBAcosθ
n^
B
Fig.2
Magnetic flux linked with a surface in two positions
From previous slide, it is clear that, when
the orientation of a surface /coil in the
magnetic field changes. Then, magnetic
flux linked with the surface also changes.
Using this fact ,Faraday explained that emf
is induced in a coil, whenever Magnetic flux
linked with a coil / circuit changes.
In the next slides, Faraday’s experiments
are discussed.
FARADAY’S EXPERIMENTS
1. Whenever there is relative motion between the coil & the
magnet, galvanometer shows a sudden deflection.
2. This deflection indicates that current is induced in the coil.
3. The deflection is temporary.
4. It lasts so long as relative
motion between the coil &
magnet continues.
N
S
G
Continues …
FARADAY’S EXPERIMENTS
5. The deflection is more, when
the magnet is moved faster.
6. The direction of deflection is
reversed, when same pole of
magnet is moved in the
opposite direction or opposite
pole of magnet is moved in
the same direction.
NOTE :
The motion of the magnet
implies that the no. of
magnetic field lines passing
through the coil is changing.
N
G
S
EXPERIMENT NO.2. CURRENT INDUCED BY CURRENT
K
COIL 1
COIL 2
NOTE. The cause of induced emf / current in a coil is CHANGE in
MAGNETIC FLUX linked with the coil .It should be clearly
understood that mere presence of magnetic flux is not
enough.
FARADAY’S LAWS OF
ELECTROMAGNETIC INDUCTION
First Law:
Whenever the amount
magnetic flux linked with a
coil changes, an emf is
induced in the coil.The
induced emf lasts so long as
the change in magnetic flux
continues .
FARADAY’S LAWS OF
ELECTROMAGNETIC INDUCTION
Second Law:
The magnitude of emf induced in a
coil is directly proportional to the rate of change of
magnetic flux linked with the coil.
Induced emf is given by,
e = - dф /dt
Negative sign is taken because induced emf
always opposes any change in magnetic flux
associated with the coil.
This follows from Lenz’s law.
LENZ’S LAW
Experimental Verification of Lenz’s Law
S
N
Statement The induced
current flows in
such a direction
that it opposes
the change (in
magnetic flux)
responsible for
its production.
N
3
C
1
G
2
CONTINUES…
FLEMING’S RIGHT HAND RULE
It states that if the thumb ,
fore finger & the central
finger of right hand are kept
perpendicular to each other,
so that the Fore finger
points in the direction of the
Magnetic field & the thumb in
the direction of motion of the
conductor , then the induced
current flows in the direction
of the central finger.
VARIOUS METHODES OF
PRODUCING INDUCED E.M.F.
We know, the magnetic flux linked with a coil is given :
ф = BA cos θ
So, magnetic flux ‘ф’ can be changed 1. By changing the magnitude of magnetic field B,
2. By changing the area the coil linked with the
magnetic field,
3. By changing angle ‘θ’ between the direction of B
and normal to the surface area A i.e. changing the
relative orientation of the surface area & the
magnetic field.
A.C. GENERATOR
An a.c.generator is a machine which converts mechanical energy into
electrical energy.
PRINCIPLE : It is based on the phenomenon of electromagnetic induction.
C
B
N
S
A
D
B1
R2
R1
B2
A.C.GENERATOR
R
CONSTRUTION. The essential parts of an a.c. dynamo are as
follows1.Armature. ABCD is a rectangular armature coil, consists of
a large no. turns of insulated copper wire
wound over a laminated soft iron core. The coil
rotates about the central axis.
2.Field Magnet. N and S are the pole pieces of a strong
magnet.
3.Slip Rings. R1 and R2 are two hollow metallic rings, to
which two ends of armature coil are
connected.These rings rotate with the coil.
4.Brushes. B1 and B2 are two flexible metal plates or
carbon rods; fixed & kept in light contact with R1
& R2 respectively. The purpose of brushes is to
pass on current from the armature coil to
external load resistance ‘R’ .
C
B
N
S
A
D
R1
B1
R2
B2
R
Fig.1
A.C.GENERATOR
C
B
N
S
A
R2
e0
N
R1 D
B1
B
C
S
D
R1
B1
R2
B2
A
B2
e
3T/4
T
O
T/4
-e0
Y’
T/2
t
X
THEORY & WORKING
NORMAL
COIL
θ
N
S
As the armature coil is rotated in the magnetic field,
angle’θ’ between the field & normal to the coil changes
continuously. Therefore, magnetic flux linked with the
coil changes. An emf is induced in the coil.
Continues…
DIRECTION OF FLOW OF CURRENT
Initially at t = 0, the
armature ABCD is
vertical with arm AB up
and CD down. During
the motion of the
armature between t = 0
to T/2 , the arm AB
moves down and CD
moves.
The application of
Fleming’s right hand
rule tells that the
current in the
armature will flow in
the direction DCBA .
C
B
N
S
A
e
B1
R2
B
C
N
S
D
R1 D
B1
R2
B2
A
R1
B2
3T/4
O
T/4
-e0
Y’
T/2
T X
t
On the other hand ,
during the motion of
armature between t =
T/2 and T, the motion
of the two arms is just
opposite to that what
happens during t =0
to T/2 that is the arm
AB moves up and CD
moves down.
Therefore, during this
interval, the current in
the armature will flow
in the direction
ABCD.
C
B
N
S
A
N
R2
S
D
R1 D
B1
B
C
B1
R2
B2
A
R1
B2
e
3T/4
O
T/4
-e0
Y’
T/2
T
t
MAGNITUDE OF INDUCED EMF
Whenever the armature is vertical, its arms AB and CD
momentarily move parallel to the field i.e. the rate of
change of magnetic through the armature becomes zero.
Accordingly, the induced emf e corresponding to the
vertical positions of the armature is also zero i.e. at time
t = 0,T/2 and T, the induced emf produced is zero.
Whenever the armature is horizontal, the arms AB and
CD move normally to the direction of magnetic field lines
and hence they cut the magnetic field lines. Thus, at time
t = T/4 and 3T/4, the induced emf produced is maximum.
Therefore, output emf across the load resistance R during
a complete rotation of the armature will vary as shown in
previous slide.
Transformer
Electrical device which is used to convert a.c.voltages.
A transformer which increases the a.c. voltages is called a
‘step up transformer’.
A transformer which decreases the a.c. voltages is a ‘step down transformer’.
Transformer does not work on d.c. voltages.
Principle : A transformer is based on the principle of mutual
induction i.e. whenever the amount of magnetic flux
linked with a coil changes, an emf is induced
in the neighbouring coil.
Construction
A transformer consists of a rectangular soft
iron core made of laminated sheets, well
insulated from each other. Two coils - primary
& secondary are wound on the same core,
but are well insulated from each other.
The source of alternating
emf (to be changed) is connected to primary
coil and a load resistance R is connected to
the secondary.
Check your understanding
1 A magnetic field line is used to find
the direction of
(a) south-north
(b) a bar
magnet
(c) a compass needle (d) magnetic
field
2. You have a coil and a bar magnet. You
can produce an electric current by
moving
(a) the magnet, but not the coil
(b) the coil, but not the magnet
(c) either the magnet or the coil
(d) neither the magnet nor the coil
3. Faraday’s law of electromagnetic induction is
related to the law of conservation of
(a) charge
(b) energy
(c) mass
(d) angular momentum
4. The SI unit of magnetic flux is
(a) gauss
(b) oersted
(c) tesla
(d) weber
5. The direction of induced emf is given by
(a) Fleming’s left hand rule.
(b) Fleming’s right hand rule.
(c) Lenz’s law (d) Boit-Savart’s law
BIBLIOGRAPHY

NCERT TEXT BOOK OF PHYSICS FOR
CLASS XII

PRADEEP’S FUNDAMENTAL PHYSICS
FOR CLASS XII BY GOMBER & GOGIA

MODERN’S abc PHYSICS FOR CLASS XII
BY S.K.GUPTA