CBSE Class 12 Physics Electromagnetic Induction Theory and Examples

CBSE Class 12 Physics Electromagnetic Induction Theory and Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

1. MAGNETIC FLUX
(a) The number of lines of flux passing through an area held perpendicular to the field is equal to the magnetic flux linked with that plane.
(b) Mathematically, magnetic flux is the product of the field and the area of the plane. i.e.

Magnetic lines of force are closed curves. So the number of lines entering a closed surface is equal to the number of lines leaving the surface. Hence net flux = 0.
(j) Flux linked with a surface depends on the following quantities :
(i) Intensity of magnetic field B.
(ii) Area of the surface A.
(iii) Orientation of surface relative to magnetic field.

Ex.1 At certain location in the northern hemisphere, the earth's magnetic field has a magnitude of 42 μT and points down ward at 57º to vertical. The flux through a horizontal surface of area 2.5 m² will be- (given cos 33º = 0.839, cos 57º = 0.545)
(A) 42 × 10^-6 Wb (B) 42 × 10^-6 Wb/m² (C) 57 × 10^-6 Wb (D) 57 × 10^-6 Wb/m²

Sol. (C) The flux through the area is
φ = BA cos 57º = 42 × 10^-6 × 2.5 × 0.545 = 57 × 10^-6 Wb.

2. FARADAY'S LAWS OF ELECTROMAGNETIC INDUCTION
(a) Whenever the number of magnetic lines of force or magnetic flux passing through a circuit changes an emf is produced in the circuit called induced emf.

(b) If the circuit is closed a current flows through it called induced current.

(c) The induced emf is given by rate of change of magnetic flux linked with the circuit i.e.

3. LENZ'S LAW

(a) This gives the direction of induced emf.
(b) According to this law, the direction of induced emf or current in such as to oppose the change that produces it.
(c) From Lenz's law and Faraday's Law, induced emf is given by e = –dφ/dt , Where minus sign is to show that emf opposes the change of flux linked with it.
(d) This law is based upon Law of conservation of energy.
(e) Mechanical energy and Magnetic energy get converted into Electrical of energy in this Phenomenon called electromagnetic induction.

Problems :

Ex.2 A coil of metal wire is stationary in a nonuniform magnetic field. Is there any induced e.m.f in coil ?
Sol. No, since magnetic flux is not changing.

4. SOME GENERAL POINTS
(a) Induced emf is given by e = – dφ/dt

Q Sign is given by Lentz's Law.
(b) If magnetic flux linked with a circuit changes from φ1 to φ2 in time 'Δt' then induced emf

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