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Electro Magnetic Induction


Michael Faraday    


Michael Faraday, FRS (September 22, 1791 – August 25, 1867) was an English chemist and physicist who contributed significantly to the fields of electromagnetism and electrochemistry. Faraday studied the magnetic field around a conductor carrying a DC electric current, and established the basis for the magnetic field concept in physics. He discovered electromagnetic induction, diamagnetism and electrolysis.

He established that magnetism could affect rays of light and that there was an underlying relationship between the two phenomena. His inventions of electromagnetic rotary devices formed the foundation of electric motor technology, and it was largely due to his efforts that electricity became viable for use in technology.

Although Faraday received little formal education and knew little of higher mathematics, such as calculus, he was one of the most influential scientists in history. 

Magnetism Produces Electricity

After Oersted's discovery that an electric current produces a magnetic field, attempts were made to observe the reverse effect, namely, the production of electric current by means of a magnetic field. Joseph Henry (1830) in the USA and Michael Faraday (1931) in the UK independently observed that whenever there is a change in the magnetic lines of force associated with a circuit, a current is produced in the circuit which lasts as long as the change is taking place. This phenomenon of generating current is called electromagnetic induction.
 


Experimental Demonstration of Electromagnetic Induction
Take a coil of insulated copper wire and connect its ends to a sensitive galvanometer. A galvanometer is a sensitive device used for detecting small currents. If a bar magnet NS is moved towards the coil as shown in Figure, the galvanometer needle effects, indicating the presence of a current. If the magnet is moved away from the coil, the galvanometer again shows a deflection but in the opposite direction. But if the magnet is held stationary anywhere near or inside the coil, the galvanometer does not show any deflection. 

Experimental Demonstration of Electromagnetic Induction


Thus, a current is induced in the circuit only when the magnet is moved towards or away from the coil. The same result is obtained if the magnet is kept stationary and the coil is moved towards or away from the magnet. Hence, induced current is produced if there is a relative motion between the magnet and the coil.

Cause of Induced Current

Induced current flows in a coil only when the magnetic field near the coil is changing. A magnet has lines of force of the magnetic field associated with it. If a magnet is brought closer to the coil, the number of lines of force passing through the coil increases as shown in Figure. The same result is obtained if the coil is moved towards the magnet. During this motion, an induced current is produced in the coil. When there is no motion between the coil and the magnet, the number of lines of force passing through the coil will not change and, hence, no induced current is produced in the coil.  Thus, we conclude that an induced current is produced in the coil only when the number of lines of force passing through the coil (or the magnetic flux linked with the coil) is changing.

 

 

Fleming's Right-Hand Rule
 
The direction of the induced current can be determined from Fleming's right-hand rule stated below:
 
Stretch out the thumb, forefinger and second finger of your right hand mutually at right angles to each other as shown in Figure. If the Forefinger points in the direction of the Field, and the thumb in the direction of Motion, then the second finger will point in the direction of the induced Current.   
 




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