Law of Electromagnetic Induction
Faraday’s Law of Induction
Faraday's Law of Induction states that the magnitude of the induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic field lines cutting it.
The “rate” here is associated with the speed of the movement of magnet / coil. When the speed changes, the magnitude of induced emf will change accordingly.
Note that when the question does not mention about speed, we do not need to explain the question with the “rate”. An explanation with “a change in magnetic field lines cutting the coil” is sufficient.
The below illustration shows the meaning of a CHANGE in magnetic field lines cutting the coil.
From position 1 to position 2, the number of magnetic field lines cutting the coil changes from 1 to 5, hence, there is an increase (change) in the magnetic field lines cutting the coil when the magnet moves from position 1 to position 2. Thus, an emf is induced during this movement.
Note that when we know the change (either increase or decrease), we have to state so.
The magnitude of induced emf can be increased by
Lenz's law states that the direction of the induced e.m.f. (and hence induced current in a closed circuit) is always such that its magnetic effect opposes the change producing it.
Note that although the direction of induced emf opposes the change producing it, it still occurs. This means the magnet will still go into the solenoid or leave the solenoid but it takes a little bit longer due to opposing force.
The following analogy might help to understand the Lenz’s Law better.
(a) As the coil moves into the magnet, there is an increase in the magnetic field lines cutting the coil. By Faraday’s Law, this induces an e.m.f. across the coil (which drives an induced current (include this only if question asks for induced current)).
(b) By Lenz’s Law, the direction of induced current always opposes the change producing it. Thus, North pole is induced on the left side of coil when it moves towards North pole of magnet. By Right Hand Grip Rule, the induced current flows clockwise in the coil
(c) If the coil is moved more slowly, there is a decrease in the rate of change of magnetic field lines cutting the coil. Thus, the magnitude of the induced current decreases, which is shown by a smaller deflection of the galvanometer.