ELECTROMAGNETIC INDUCTION

ELECTROMAGNETIC INDUCTION:  ELECTRICITY FROM MAGNETISM

• Magnets can produce emf  in wire  ( conductor ). The production of electrical energy from magnetism is called electromagnetic induction.

• The current produced in conductor due to magnetism is called induce current or induction current  

• Electrical energy produced by magnetism occurs one of the following ways.

1. When a straight wire is moved up and down rapidly between the two poles of a horseshoe magnet, then an emf  is induced in the wire. 
2. if a bar magnet is moved in and out of a coil of wire,  then an emf is induced in the coil.
3. If a alternating current carrying coil is kept near the passive coil then emf is induced in passive coil.

• The process of electromagnetic induction is used  for producing electricity at power stations from Alternator. 

 

FLUX LINKAGES:

• The product of number of turns (N) of the coil and the magnetic flux  linking the coil is called flux linkages.

       i.e.  Flux linkages =  number of turns (N)   X  magnetic flux linking

• Experiments show that the magnitude of e.m.f. induced in a coil is directly proportional to the rate of change of flux linkages.

                                           

FARADAY'S LAWS OF ELECTROMAGNETIC INDUCTION :

First Law :

• When the magnetic flux linkages in  a coil changes continuously  , an e.m.f. is induced in it.

Second Law :

• The magnitude of the e.m.f. induced in a coil is directly proportional to the rate of change of flux linkages.

 Electromagnetic Induction in Straight Wire :

•  If a straight conductor cuts  magnetic field  lines rapidly , then emf is induced in it. The emf produced in conductor depend on :

1. strength of magnetic field
2. relative speed  of  conductor
3. Angle between magnetic field line and motion of conductor

• When conductor cut magnetic field lines perpendicularly then maximum emf is induced in it.

• If direction of magnetic field  or  motion of conductor is reversed then induced emf  in conductor also reversed .

• If conductor forms closed circuit then an induced current flow . A galvanometer is used to detect current. The direction of current depend on the emf induced in conductor.

Fleming's Right-Hand Rule to Find Direction of Induced Current:

• Stretch out the forefinger, middle finger and thumb of  right hand so that they are at right angles to one another. If the forefinger points in the direction of magnetic field, thumb in the direction of motion of the conductor then the middle finger will point in the direction of induced current.

 

              Fleming's Right-Hand Rule.

• Consider.a conductor AB moving upwards at right angles to a uniform magnetic field. Applying Fleming's right-hand rule, it is clear that the direction of induced current is from B to A. If the motion of the conductor is downward, keeping the direction of magnetic field unchanged, then the direction of induced current will be from A to B.

  Electromagnetic Induction in Coil Due to Moving Bar magnet:

• The production of electric energy  by moving a bar magnet near or inside a fixed coil of wire is electromagnetic induction . The concept of a fixed coil and a rotating magnet is used to produce electricity on large Scale in big generators of power houses.

• After performing a large number of experiments , Faraday and Henry made the following observations about electromagnetic induction in coil :

1. A current is induced in a coil when it is moved ( or rotated ) relative to a fixed magnet. 
2. A current is also induced in a fixed coil when a magnet is moved ( or rotated ) relative to the fixed Coil. 
3. No current is induced in a coil when the coil and magnet both are stationary relative to one another. 
4. When the direction of motion of coil (or magnet) is reversed, the direction of current induced in the coil also gets reversed.

• The magnitude of current induced in the coil can be increased:

        (a)  by winding the coil on a soft iron core

        (b)  by increasing the number of turns in the coil,

        (c)  by increasing the strength of magnet, and

        (d)  by increasing the speed of rotation of coil (or manet).

Direction of Current in Coil Due to electromagnetic Induction

Lenz's law:

• In closed loop of coil , The induced current will flow in such a direction so as to oppose the cause that produces it.

  Electromagnetic Induction in Coil Due to Stationary coil  :

• If an alternating current carrying coil is kept near the passive coil then  an  e.m.f is induced  in passive coil due to electromagnetic induction.

Working:

• The alternating current carrying coil  produce alternating magnetic flux around it .if a passive coil is kept near the active coil then some flux will pass inside the passive coil .   flux linkage in passive coil are alternating in nature. the change in linkage flux induce emf in passive coil.

• When passive coil is connected with circuit then  induced current will flow through circuit. 

 The magnitude of current induced in the passive coil can be increased:

        (a)  by winding the coil on a soft iron core

        (b)  by increasing the number of turns in the passive coil,

        (c)  by increasing the strength of electromagnet, 

Galvanometer

• A galvanometer is an instrument which can detect the presence of electric current in a circuit. It is connected in series with the circuit. 

• When no current  flow through galvanometer then its pointer show  zero mark on semi- circular scale . 

• When an electric current passes through the galvanometer, then its pointer deflect either to the left side of zero mark or to the right side of the zero mark  depending on the direction of current.

 

ELECTRIC GENERATOR :

• The device which  converts  mechanical energy into electrical energy is called electric generator . A Small generator is called a dynamo.

Electric generators are of two types : 

1 . Alternating Current generator ( A . C . generator )

2 . Direct Current generator ( or D . C . generator ) .

Principle of Electric Generator :

• The electric generator works on the Principle that when a straight conductor cut magnetic field lines , then emf (current) is induced in the Conductor . Thus  electric generator is an application of electromagnetic induction.

                                 or  

• When the coil rotates , The flux linkage in coil change continuously during rotation  due to which an emf (current ) is produced in the coil .  

 Working of an A.C. generator:

• In a simple  A.C electric generator , a rectangular coil ( having two straight sides ) is made to rotate rapidly in the magnetic field between the poles of a horseshoe - type magnet .

• Free ends of rotating  conductors   are  joined with different  half slip rings  which rotate between two carbon brushes by touching it. Each half slip ring is always in contact with same  brush during rotation.

• When a coil of conductor is made to rotate in magnetic field then its one side conductor ( AB ) moves down  and other side conductor ( CD ) moves up cutting the magnetic field. Since direction of magnetic fields are same for both conductors then different polarities (positive and negative)  are produced at the free ends of conductors and hence potential difference is produced between  brushes.

• After half revolution of coil, conductor ( AB ) moves up  and other side conductor ( CD ) moves down cutting the magnetic field. Hence polarities at  two ends of conductor  interchange. And potential difference between two brushes is reversed. Thus in one complete revolution potential difference between brushes reversed two times .

• When a external circuit is connected with brushes , current flow through the circuit.Since direction of current changes in every half cycle of rotating conductor. So this type of current is called Alternating Current .

• The alternating current (A.C. ) produced in India has a frequency of 50 Hz. That is, the coil is rotated at the rate of 50 revolutions per second. Since in 1 revolution of coil, the current reverses its direction 2 times, so in 50 revolutions of coil, the current revers its direction 2 x 50= 100 times. Thus, the A.C supply in India reverses its direction 100 times in 1 second. 

D.C. GENERATOR : 

• In a simple  D.C electric generator , a rectangular coil ( having two straight conductors AB and CD   ) is made to rotate rapidly in the magnetic field between the poles of a horseshoe - type magnet .
• due to electromagnetic induction , a varying emf is induced in each conductor  but in opposite polarities.

• Free ends of rotating  conductors of a coil   are  joined with different  half split  rings  ( Commutator )  which rotate between two carbon brushes by touching them . every half revolution each half split ring change its brush.

• Due to this,  polarity of two brushes are remains unchanged.

• when a circuit is connected across two brushes , a direct current flow through circuit.

• only difference between a D.C. generator and an A.C. generator is in the way the two ends of the generator coil are linked to the outer circuit. In a D.C. generator we connect the two ends of the coil to a commutator consisting of two half rings of copper. On the other hand, in an A.C. generator, we connect the two ends of the coil to two full rings of copper called slip rings. There is no commutator in an A.C. generator