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Description of  Electromagnetic Induction (EMI): Physics of Magnetic Flux (Learn Physics Book 25)

Magnetic Flux (1) The total number of magnetic lines of force passing normally through an area placed in a magnetic field is equal to the magnetic flux linked with that area. (2) Net flux through the surface ( is the angle between area vector and magnetic field vector) If = 0o then = BA, If = 90o then = 0 (3) Unit and Dimension : Magnetic flux is a scalar quantity. Its S.I. unit is weber (wb), CGS unit is Maxwell or Gauss cm2 ( ). (4) Other units : Tesla m2 = Ohm Coulomb = Henry Amp. Its dimensional formula [] = [ML2T2A1] Faraday's Laws of Electromagnetic Induction (1) First law : Whenever the number of magnetic lines of force (magnetic flux) passing through a circuit changes an emf is produced in the circuit called induced emf. The induced emf persists only as long as there is change or cutting of flux. (2) Second law : The induced emf is given by rate of change of magnetic flux linked with the circuit i.e. . For N turns Negative sign indicates that induced emf (e) opposes the change of flux. (3) Other formulae : = BA cos Hence will change if either, B, A or will change So Table 23.1 : Induced i, q and P Induced current (i)Induced charge (q)Induced power (P) Induced charge is time independent. It depends on time and resistance Lenz's Law This law gives the direction of induced emf/induced current. According to this law, the direction of induced emf or current in a circuit is such as to oppose the cause that produces it. This law is based upon law of conservation of energy. (1) When N-pole of a bar magnet moves towards the coil, the flux associated with loop increases and an emf is induced in it. Since the circuit of loop is closed, induced current also flows in it. (2) Cause of this induced current, is approach of north pole and therefore to oppose the cause, i.e., to repel the approaching north pole, the induced current in loop is in such a direction so that the front face of loop behaves as north pole. Therefore induced current as seen by observer O is in anticlockwise direction. (figure) Table 23.2 : The various positions of relative motion between the magnet and the coil Position of magnet Direction of induced currentAnticlockwise directionClockwise directionClockwise directionAnticlockwise direction Behaviour of face of the coilAs a north poleAs a south poleAs a south poleAs a north pole Type of magnetic force opposedRepulsive forceAttractive forceRepulsive forceAttractive force Magnetic field linked with the coil and its progress as viewed from leftCross (), IncreasesCross (), DecreasesDots () IncreasesDots () Decreases (3) If the loop is free to move the cause of induced emf in the coil can also be termed as relative motion. Therefore to oppose the cause, the relative motion between the approaching magnet and the loop should be opposed. For this, the loop will itself start moving in the direction of motion of the magnet. (4) It is important to remember that whenever cause of induced emf is relative motion, the new motion is always in the direction of motion of the cause.