Physics Lectures

Electromagnetic Induction

Electric current and magnet behave alike in some ways especially when we study areas of influence around electric conductor and magnet. This close connection was first discovered by a scientist Hans Christian Oersted. To observe this behavior, connect a straight wire to a battery and allow an electric current to flow through it. Bring a compass (magnetic needle) near the wire, the compass needle will align itself along the field lines. These field lines show that the flow of electric current through wire develops a magnetic field around the wire. We observe a high magnetic field near the wire and its strength decreases upon moving away from the wire. It means that when an electric current passes through a conductor it behaves like a magnet. Now place a bar magnet on a piece of plain paper and draw magnetic lines of force with the help of a compass. The magnetic lines of force appear strong near the poles and its strength decreases upon moving away from the poles.

If we create a coil of electric wire (solenoid) and pass electric current through it, we observe magnetic field similar in shape to that of a bar magnet.

Now we know that flow of electric current through a conductor can develop a magnetic field but, is it possible to reverse this process and develop electric current from a magnet? This idea was successfully tested by Michael Faraday.

He invented first electric motor and extended his studies to show how the motor effect could work in reverse to generate electricity. In his experiment he connected a galvanometer to a solenoid and placed a bar magnet close to it, but he did not find effect of the magnet field on the solenoid as there was no deflection in galvanometer needle. It means that no current induced in solenoid.

Faraday did further research and reached a stage where he noticed that movement of bar magnet towards solenoid induces a current in the solenoid and galvanometer needle deflect away from the bar magnet (anti-clockwise direction). When he moved the bar magnet away from the solenoid current induced in the solenoid and galvanometer needle moved towards the bar magnet (clockwise direction).

He tested the same experiment with the opposite pole of a bar magnet and he noted that electricity induced in a solenoid but flow in opposite direction.  Alternatively, he kept the magnet stationary and moved the solenoid towards it, and found a similar effect. Based on this experiment he concluded that the process of generating electricity from the relative motion of magnet or solenoid is called electromagnetic induction. There are three ways to increase the e.m.f. induced in a coil:

1. Move the magnet or coil more quickly;
2. Use a greater number of turns of wire in solenoid;
3. Use a stronger magnet.
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