A large number of closely wound turns of insulated copper wire. Connect its ends to the terminals of a galvanometer. Bring the coil swiftly to one pole of a magnet. Observe the deflection in the galvanometer.
Keeping the coil at rest bring one end of a magnet swiftly to the coil and then take it back at the same speed. The needle deflects to opposite directions in the two different situations and suddenly comes back to the initial position.
When a solenoid swiftly moves in a strong magnetic field, the galvanometer needle deflects indicating the flow of a current.
The process of inducing an emf in a conductor whenever the magnetic flux linked with it changes, is known as electromagnetic induction.
The emf so developed is called induced emf and the current so obtained is called induced current.
The induced emf developed in a conductor is equal to the rate of change of flux linked with it. This is Faraday's Law of electro magnetic induction.
Alternating current (AC);
Direct current (DC)
Connect the terminals of a weak cell to a galvanometer. Observe the deflections of the needle. Remove the cell and connect a solenoid to a galvanometer. Observe the deflections of the needle by continuosly moving a magnet into and out of the solenoid. The needle deflects only in one direction when the galvanometer is connected to a cell.
Thus a current flowing only in one direction is called direct current (DC).
The direction of the electric current produced by the movement of the magnet in opposite directions continously changes. Thus the current which changes its direction at regular intervals is called alternating current (AC).
AC generator (Alternator)
The device which produces electricity on the basis of electromagnetic induction by the continuous motion of either the solenoid or the magnet is called a generator.
An armature is an arrangement of an insulated conducting wire wound around a soft iron piece. The armature is continuously rotated by means of mechanical energy.
The induced current developed in one direction during the first half rotation of the armature of the AC generator and the induced current developed in the opposite direction during the next half of the rotation constitute one cycle of an AC.
DC generator
The electric current from a DC generator flows in one direction in the external circuit.
AC induced in the armature is converted into DC in the external circuit by an arrangement called split ring commutator. The flux cut in the opposite direction during the second half rotation. Hence the direction and in the opposite direction during the second half rotation. Hence the direction of flow of current in the armature changes. When the direction of the current in the armature changes during the successive half rotations, the contact of one half of the split ring shifts from one brush to the other. So the direction of the current in the external circuit does not change.
The force experienced by a current - carrying conductor in a magnetic field
The induced current develops in a solenoid when it is moved in a magnetic field. A current is passed through a conductor placed in a magnetic field.
Connect the two ends of the conductors to the terminals of a battery through a key. The copper rode is free to move freely in between the poles of a strong magnet. Then electricity is passed through it.
- A current carrying conductor in a magnetic field experiences a force. Hence the conductor moves in the direction of force. This is the principle of electric motor.
Electric Motor
As in a generator, the electric motor also contains an armature and a field magnet. An armature can freely rotate about its axis. Already learnt the principle behind the electric motor that the armature rotates when electricity is passed through it. The rotating armature gets mechanical energy through a shaft connected to the axis of the armature.
Moving coil loudspeaker
A loudspeaker reproduces sounds in audio visual equipments such as radio, television, tape recorder etc. Observe the parts of a damaged loudspeaker.
A voice coil placed between the poles of a permanent magnet is the important part of a loud speaker. A paper cone or a diaphragm is attached to this coil. The electrical signal corresponding to the variations of sounds from the micro phone is amplified by an amplifier and is fed to the voice coil. The voice coil placed in the magnetic field vibrates in accordance with the intensities of the electrical signals. These vibrations induce vibrations on the paper cone attached to the coil causing vibrations to the air around it. As a result, the original sound is reproduced more loudly.
Mutual Induction
An insulated copper wire is wound around one end of the soft iron core and the ends of the coil are connected to a battery through a switch. Another insulated copper wire is wound around the other end of the core. Connect the ends of this coil to a galvanometer. Of these, the circuit which is connected to the battery is called secondary circuit.
When there are two nearby coils the variation of current in one of them produces a change in the magnetic flux around it. The second coil is situated in this region of varying magnetic flux. Therefore by electro magnetic induction an emf is induced in the secondary coil. This phenomenon is called mutual induction.
Self Induction
The intensity of the light by switching on the circuit. Connect a 12V AC source to the circuit in the place of the 12V DC battery.
The intensity of light decreases while using an AC in the circuit. There is a change in magnetic flux linked with the solenoid when AC flows through it. Due to this change in flux an induced emf developes in the solenoid. This emf is opposite to the emf applied in the circuit. Therfore the resultant emf in the circuit decreases the brightness of the bulb also decreases.
Self induction is the phenomenon of inducing an emf in a coil caused by the variations of magnetic flux produced by a varying current in the same coil.
Transformer
Connect one of them to 6V AC supply throug a switch. The ends of the other coil are connected to an LED. The coil which is connected to the AC supply is called primary coil and that connected to the LED is called secondary coil.
The LED glows when AC passes through a primary coil. Secondary coil is placed in a magnetic field of continuously changing direction. The variation in the magnetic flux causes an induced emf in the secondary. In a transformer electrical energy is transferred from one circuit to another by electromagnetic induction.
Step-Up Transformer
Step-Down Transformer
There are two types of transformers in use - step-up transformers and step-down transformers.
The rate of change of flux in a coil depends also on the number of turns in it. Therefore the emf induced in a coil is directly proportional to the number of turns in it. If the number of turns in the secondary is twice that of primary then the output voltage will also be doubled.
Moving coil microphone
Moving coil microphone is a device which works on the principle of electro magnetic induction. In this sound energy is converted into electrical energy.
A coil placed in a magnetic field is the main part of it. A diaphragm is attached to its coil. The coil vibrates with the vibration of the diaphragm when sound waves fall on it. At that time, electrical signals are produced in it in accordance with the sound.
The rate of change of flux in a coil depends also on the number of turns in it. Therefore the emf induced in a coil is directly proportional to the number of turns in it. If the number of turns in the secondary is twice that of primary then the output voltage will also be doubled.
Moving coil microphone
Moving coil microphone is a device which works on the principle of electro magnetic induction. In this sound energy is converted into electrical energy.
A coil placed in a magnetic field is the main part of it. A diaphragm is attached to its coil. The coil vibrates with the vibration of the diaphragm when sound waves fall on it. At that time, electrical signals are produced in it in accordance with the sound.
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