TOPIC

ELECTROMAGNETIC ELECTROMAGNETIC

INDUCTIONINDUCTION

Previous Knowledge Test

• 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 ?

OBJECTIVESOBJECTIVESAt the end of this presentation, you’ll knowAt 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 IMPORTANCE OF THE TOPICTOPIC

Initially, the electrical energy was Initially, the electrical energy was obtained from chemical energy (cells obtained from chemical energy (cells & batteries).& batteries).

The present large scale production & The present large scale production & use of electricity would not have use of electricity would not have been possible if the only way of been possible if the only way of production of electrical energy had production of electrical energy had remained of chemical nature.remained of chemical nature.

CONTINUES….

IMPORTANCE OF THE IMPORTANCE OF THE TOPICTOPIC

This problem was solved with the This problem was solved with the discovery of ‘discovery of ‘Electromagnetic Electromagnetic induction’induction’, which is the basis of , which is the basis of working of power generators & working of power generators & transformers etc.transformers etc.

We can’nt imagine the world without We can’nt imagine the world without electricity and we know; electricity is electricity and we know; electricity is produced using this phenomenon of produced using this phenomenon of ’’EMIEMI’.’.

CONTINUES….

MAGNETIC FLUX(Ф)MAGNETIC FLUX(Ф)

B

n

The magnetic flux through any surface held in a magnetic field is measured by the total No. of magnetic field lines crossing the Surface.

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)

θ

dA ^

Magnetic flux linked with a surface

Magnetic field lines

CONTINUES…

Minimum magnetic flux is linked with the surface, when θ =90° (in fig.1)

Ф =B.A.cos 90° = 0

Maximum magnetic flux is linked with the surface, when θ = 0° (fig.2)

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

θ

^

B

n̂

Magnetic field lines

Magnetic flux linked with a surface in two positions

Fig.1

Fig.2

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

G

N S

4. It lasts so long as relative motion between the coil & magnet continues.

Continues …

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.

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 S

FARADAY’S EXPERIMENTS

G

EXPERIMENT NO.2.EXPERIMENT NO.2. CURRENT INDUCED BY CURRENTCURRENT INDUCED BY CURRENT

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.

COIL 1 COIL 2

K

FARADAY’S LAWS OF FARADAY’S LAWS OF ELECTROMAGNETIC ELECTROMAGNETIC INDUCTIONINDUCTION

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 FARADAY’S LAWS OF ELECTROMAGNETIC ELECTROMAGNETIC INDUCTIONINDUCTION

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 LAWLENZ’S LAW

Statement - The induced current flows in such a direction that it opposes the change (in magnetic flux) responsible for its production.

Experimental Verification of Lenz’s Law

C

N

2

3

N

S

G

1

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 VARIOUS METHODES OF PRODUCING INDUCED E.M.FPRODUCING INDUCED E.M.F..

We know, the magnetic flux linked with a coil is given : We know, the magnetic flux linked with a coil is given : ф = BA cos θф = BA cos θ

So, magnetic flux ‘ф’ can be changed -So, magnetic flux ‘ф’ can be changed -

1. By changing the magnitude of magnetic field B, 1. By changing the magnitude of magnetic field B,

2. By changing the area the coil linked with the2. By changing the area the coil linked with the magnetic field, magnetic field, 3. By changing angle ‘θ’ between the direction of B and 3. By changing angle ‘θ’ between the direction of B and

normal to the surface area A i.e. changing the relativenormal to the surface area A i.e. changing the relative orientation of the surface area & the magnetic field. orientation of the surface area & the magnetic field.

N S

B1

B2

R1

R2

R

A

B C

D

A.C.GENERATOR

A.C. GENERATORA.C. GENERATORAn a.c.generator is a machine which converts mechanical energy into

electrical energy.

PRINCIPLE : It is based on the phenomenon of electromagnetic induction.

CONSTRUTION. The essential parts of an a.c. dynamo are as follows-

1.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’ .

N S

B1

B2

R1

R2

R

Fig.1

A

B C

D

A.C.GENERATOR

N S

B1

B2

R1

R2

D

C B

A

N S

B1

B2

R1

R2

A

B C

D

O

T/4 T/2

3T/4T

t

X

Y’

e0

e

-e0

THEORY & WORKINGTHEORY & WORKING

N S

θ

COILNORMAL

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…

X

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 .

DIRECTION OF FLOW OF CURRENT

O

Y’

e

-e0

N S

B1

B2

R1

R2

D

C B

A

N S

B1

B2

R1

R2

A

B C

D

T/4 T/2

3T/4T

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.

O

Y’

e

-e0

N S

B1

B2

R1

R2

D

C B

A

N S

B1

B2

R1

R2

A

B C

D

T/4 T/2

3T/4T

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.

TransformerTransformerElectrical 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.

ConstructionConstruction

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 understandingCheck your understanding1 A magnetic field line is used to find the 1 A magnetic field line is used to find the

direction ofdirection of (a) south-north (b) a bar magnet(a) south-north (b) a bar magnet (c) a compass needle (c) a compass needle (d) magnetic field(d) magnetic field 2. You have a coil and a bar magnet. You can 2. You have a coil and a bar magnet. You can

produce an electric current by movingproduce an electric current by moving (a) the magnet, but not the coil(a) the magnet, but not the coil (b) the coil, but not the magnet(b) the coil, but not the magnet (c) either the magnet or the coil(c) either the magnet or the coil (d) neither the magnet nor the coil(d) neither the magnet nor the coil

3. Faraday’s law of electromagnetic induction is 3. Faraday’s law of electromagnetic induction is related to the law of conservation ofrelated to the law of conservation of

(a) charge (a) charge (b) energy(b) energy

(c) mass (d) angular momentum(c) mass (d) angular momentum

4. The SI unit of magnetic flux is4. The SI unit of magnetic flux is

(a) gauss (b) oersted (a) gauss (b) oersted

(c) tesla (c) tesla (d) weber(d) weber

5. The direction of induced emf is given by5. The direction of induced emf is given by

(a) Fleming’s left hand rule.(a) Fleming’s left hand rule.

(b) Fleming’s right hand rule.(b) Fleming’s right hand rule.

(c) Lenz’s law(c) Lenz’s law (d) Boit-Savart’s law (d) Boit-Savart’s law

BIBLIOGRAPHYBIBLIOGRAPHY

NCERT TEXT BOOK OF PHYSICS FOR NCERT TEXT BOOK OF PHYSICS FOR CLASS XIICLASS XII

PRADEEP’S FUNDAMENTAL PHYSICS PRADEEP’S FUNDAMENTAL PHYSICS FOR CLASS XII BY GOMBER & FOR CLASS XII BY GOMBER & GOGIAGOGIA

MODERN’S abc PHYSICS FOR CLASSMODERN’S abc PHYSICS FOR CLASS XII XII BY S.K.GUPTA BY S.K.GUPTA