Module-I, Lecture-4
Faraday’s Laws:
1st law: Whenever magnetic flux linking with a coil changes with time an emf is induced in that coil or whenever a moving conductor cuts the magnetic flux, an emf is induced in the conductor.
2nd law: The magnitude of the induced emf is equal to the product of the number of turns of the coil and the rate of change of flux linkage.
Lenz’s law :
It states that the direction of an induced emf produced by the electromagnetic induction is such that it sets up a current which always opposes the cause that is responsible for inducing the emf.
In short the induced emf always opposes the cause producing it which is represented by
negative sign, mathematically in its expression
Consider a solenoid as shown in Fig.1. Let a bar magnet is moved towards coil such that N-pole of magnet is facing a coil which will circulate the current through the coil.
According to Lenz‟s law, the direction of current due to induced emf is so as to oppose the cause. The cause is motion of bar magnet towards coil So emf will set up a current through coil in such a way that the end of solenoid facing bar magnet will become N-pole. Hence two like poles will face each other experiencing force of repulsion which is opposite to the motion of bar magnet as shown in the above
Fleming’s rules:
1. Fleming‟s Right hand rule: This rule helps in deciding the direction of the induced emf.
Hold the right hand thumb, fore finger and the middle finger set at right angles to each other and the thumb points the direction of the motion of the conductor and the fore finger points the direction of the field and the middle finger points the direction of the induced emf.
2. Fleming’s Left hand rule: This rule helps in deciding the direction of force acting on a conductor.
Hold the left hand thumb, fore finger and the middle finger set at right angles to each other and the thumb points the direction of the force acting on the conductor and the direction of the fore finger points the direction of the magnetic field and the middle finger points the direction of the current in the conductor
The emf induced in a coil due to change of flux linked with it (change of flux is by the increase or decrease in current) is called statically induced emf.Transformer is an example of statically induced emf. Here the windings are stationary, magnetic field is moving around the conductor and produces the emf.