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Electromagnetic Induction

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Electromagnetic Induction - Lesson Summary



Electromagnetism created a revolution by leading to the devices called motors which convert electrical energy to mechanical energy. Experiments by scientists like Oersted and Faraday made a long leap by converting mechanical energy to electrical energy.

When a straight conductor is moved in a magnetic field an electric current is induced in it and the phenomenon is electromagnetic induction. The emf caused is the induced emf and the current is induced current.
 
Oersted found the same by relative motion of a magnet with respect to a coil. Faraday's experiment proved that the strength of the induced current depends on several factors like the strength of the magnet, the speed of motion of the magnet, its orientation, the number of turns in the coil and the diameter of the coil. The induced current can be detected by a galvanometer. 

Fleming’s right hand rule gives the direction of the induced current in a conductor when it is moved in a magnetic field. Transformers are based on this principle, which consist of a primary coil and a secondary coil. The number of turns in the coils is selected based on the type of the transformer to be made, namely, step-up or step-down.
 
Electric generators work on the same principle. They have an armature which is free to rotate in a magnetic field. Its terminals are connected to two slip rings, which are further connected to two brushes and they are connected across a load resistance through which the generated electricity can be trapped. The rotation of the armature in the magnetic field changes the magnetic flux in the coil of the armature and an electric current is induced. As the direction of the induced current changes for every half rotation, it is called alternating current.

The current at the power plants is distributed through transmission lines at a high voltage and hence the lines are referred to as high tension power lines. At the substations these are stepped down to a lower voltage and supplied to houses at a low voltage.

A domestic electric circuit essentially contains mains, a fuse, live or line, neutral and earth wires. From the poles supply cables bring the current to the mains. Within the house, all the equipment is connected in parallel combination.
 
Electromagnetic induction (EMI) is the process of generating an electromotive force by moving a conductor through a magnetic field.
 
The electromotive force generated due to electromagnetic induction is called induced emf. The current due to induced emf is called induced current. 

Fleming’s right hand rule



Fleming’s right hand rule states that if the index finger points in the direction of the magnetic field and the thumb indicates the direction of the motion of the conductor, then the middle finger indicates the direction of the induced current flow in the conductor.
 
An electric generator is used to convert mechanical energy into electrical energy, using electromagnetic induction.
 
Alternating current (AC) is the current induced by an AC generator. AC current changes direction periodically. Direct current (DC) always flows in one direction, but its voltage may increase or decrease.
 
Electrical components and wires fitted in a household to supply electricity to various appliances form a domestic electric circuit. The main supply cable has two wires: Live wire and neutral wire. Domestic electric circuits have earth wires to save users from severe electric shocks. An electric fuse is a safety device used to protect an electric circuit against excessive current. 

Electricity and magnetism are inter-related and are also inter-convertible. In this setup an electric current passing through a conductor produces a magnetic field which can be observed through the deflection of a magnetic compass needle placed near the conductor. This proves that moving electric charges produce magnetic fields. Electric motors work on this principle. Electromagnetic Induction.

However, in this setup we see that an electric current is induced in a closed coil when subjected to a changing magnetic field. The phenomenon in which an electric current is generated by varying magnetic fields is called electromagnetic induction.

Electric generators work on this principle. We will now discuss and learn about three experiments
by Faraday and Henry, relating to electromagnetism. In the first experimental setup, a coil is connected to a Galvanometer.

When a bar magnet with its north pole facing the coil, is moved towards or away from the coil, the
Galvanometer needle deflects to the right and left side of zero reading respectively, showing
the presence of a current in the coil.

Let us see what happens if the south pole of the magnet faces the coil. The Galvanometer needle deflects to the left side of zero reading as the magnet approaches the coil and deflects to the right when the magnet moves away from the coil, showing the presence of a current in the coil.
The Galvanometer needle deflects only as long as the bar magnet is in motion. Once the bar magnet comes to rest the galvanometer needle settles down at “0” reading indicating that there is no current in the coil.

From all these observations we can conclude that whenever there is relative motion between a bar
magnet and a coil, an electric current is induced in the coil. In the second experimental setup, there are two coils:

Coil 1 connected to a Galvanometer, Coil 2 connected to a battery Due to the steady current in coil 2, a steady magnetic field is set up around the coil 2 and this magnetic field is also linked to the coil 1.
When coil 2 is kept stationary and coil 1 is moved towards coil 2, a current is induced in coil 1 and
the galvanometer needle deflects to the left of “0”. When coil 2 is kept stationary and coil 1 is moved away from coil 2, a current is induced in coil 1 and the galvanometer needle deflects to the
right of “0”. If we keep coil 1 stationary and move coil 2 towards coil 1, a current is induced in coil 1 and a deflection is observed in the galvanometer needle to the left of “0”.

Now if we keep coil 1 stationary and move coil 2 away from coil 1, a current is induced in coil 1 and a deflection is observed in the galvanometer needle to the right of “0”. From all these observations we can conclude that whenever there is relative motion between a current carrying coil and a closed coil in which a galvanometer is connected a current is induced in the closed coil.

In the third experiment a tap key is provided in the coil 2 circuit. Here we can observe the
deflection of the galvanometer needle even when the two coils are stationary. This deflection is observed only at those instants when the tap key is either switched on or off. This happens because of the change in magnetic field during switching on and off.

When a ferromagnetic material like an iron rod is placed co axially along the two coils, the effect
of the magnetic field linked to the coil 2 increases due to the nature of the ferromagnetic material as it allows more number of magnetic lines of force to link within the area of the coil. Hence the deflection of the galvanometer needle increases indicating an increase in the induced current.

Electricity and magnetism are inter-related and the energies linked with them are also inter-convertible. Whenever there is relative motion between the magnet and a coil, an electric current is induced in the coil.

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