By Squadron Leader Zahid Mir CS&IT Department, Superior University PHY-AP -19 Faraday’s Law

Micheal Faraday ( ) English Physicist & Chemist

Electromagnetic Induction In 1816, Oersted discovered that moving charges produces magnetic field. Can a reverse effect be possible? In 1831, Michael Faraday discovered that an emf (electromotive force) is setup in a conductor when it moves across a magnetic field. This induced emf lasts only when the conductor is moving in the magnetic field. This phenomena of production of induced emf is called electromagnetic induction.

Experiments Experiment # 1 When the magnet is stationary, no current flows in the coil. When we move the magnet either towards or away from the coil, a current flows in the coil but in opposite directions. The same effect is observed if we keep the magnet stationary and move the coil towards or away from the magnet. Result Something is about the changing magnetic field through the coil is causing a current in the coil.

Experiments Experiment # 2 Primary coil is connected to a switch and a battery. The coil is wrapped around a ring, and a current in the coil produces a magnetic field when the switch is closed. A secondary coil is also wrapped around the ring and is connected to a galvanometer. At the instant the switch is closed, the galvanometer needle deflects in one direction and then returns to zero. At the moment switch is opened, the needle deflects in the opposite direction and again return to zero. Result An induced emf is produced in the secondary coil by the changing magnetic field in the primary coil.

Behavior of induced current It is observed that induced current can be increased by; – Using a stronger magnetic field. – Moving the conductor faster in the magnetic field. – By increasing the number of turns of the (conductor) loop.

Magnetic Flux Magnetic lines of forces passing through a unit area is called magnetic flux.

Faraday’s Law of Electromagnetic Induction Statement “The induced emf in a circuit is directly proportional to the time rate of change of the magnetic flux through the circuit.” If circuit is a coil consisting of ‘N’ number of loops all of same area and φ B is the flux through one loop, then an emf is induced in every loop; thus the total induced emf in the coil is given as;

Lenz’s Law

Heinrich Lenz ( ) German Physicist

Statement The current produced by an induced emf moves in a direction so that its magnetic field opposes the original change in flux.

Experiment When the magnet is moved towards the stationary conducting loop, a current is induced in the direction as shown in figure (a). This induced current produces its own magnetic flux that is directed to the left and so counteracts the increasing external magnetic flux to the right (fig b).

Experiment When the magnet is moved away the stationary conducting loop, a current is induced in the direction as shown in figure (c). This induced current produces a magnetic flux that is directed to the right and so counteracts the decreasing external magnetic flux to the left (fig d).