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ELECTROMAGNETIC INDUCTION TOPIC ELECTROMAGNETIC INDUCTION

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 ?

OBJECTIVES 2. What are the Faraday’s laws of EMI ? 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 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 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 linked with a surface 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 . Magnetic field lines dA ^ n θ B 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) Magnetic flux linked with a surface CONTINUES…

Magnetic flux linked with a surface in two positions ^ 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) 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 Fig.1 Magnetic field lines ^ 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 S G

EXPERIMENT NO.2. CURRENT INDUCED BY CURRENT K COIL 2 COIL 1 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.

Experimental Verification of Lenz’s Law 2 3 S G 1 Statement - The induced current flows in such a direction that it opposes the change (in magnetic flux) responsible for its production. 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 R1 B1 R2 A.C.GENERATOR B2 R

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’ .

A B C D N S R1 Fig.1 B1 R2 A.C.GENERATOR B2 R

C B B C N S N S A D D R1 A R1 B1 B1 R2 R2 B2 e0 B2 e 3T/4 T X O t T/4 T/2 -e0 Y’

THEORY & WORKING N S θ COIL NORMAL 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. B C C B N S N S A D D R1 A R1 B1 B1 R2 e R2 The application of Fleming’s right hand rule tells that the current in the armature will flow in the direction DCBA . B2 B2 3T/4 T X O t T/4 T/2 -e0 Y’

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. B C C B N S N S A D D R1 A R1 B1 B1 R2 R2 B2 B2 e Therefore, during this interval, the current in the armature will flow in the direction ABCD. 3T/4 T O t T/4 T/2 -e0 Y’

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