CBSE Class 12 Physics Electromagnetic Induction And Alternating Currents chapter notes and important questions

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ELECTROMAGNETIC INDUCTION AND ALTERNATING CURRENTS

GIST

1 The phenomenon in which electric current is generated by varying magnetic fields is called electromagnetic induction.

2 Magnetic flux through a surface of area A placed in a uniform magnetic field B is defined as Φ_B= B.A = BACosθ where θ is the angle between B and A. B

3 Magnetic flux is a scalar quantity and its SI unit is weber (Wb). Its dimensional formula is [*Φ] = [ML²T^-2A-¹].

4 Faraday’s laws of induction states that the magnitude of the induced e.m.f in a circuit is equal to the time rate of change of magnitude flux through the circuit.ε=-dΦ_B/dt

5 According to Lenz law, the direction of induced current or the polarity of the induced e.m.f is such that it tends to oppose the change in magnetic flux that produces it. (The negative sign in Faraday’s law indicates this fact.)

6 Lenz law obeys the principle of energy conservation.

7 The induced e.m.f can be produced by changing the (i) magnitude of B (ii) area A (iii) angle θ between the direction of B and normal to the surface area A.

8 When a metal rod of length l is placed normal to a uniform magnetic field B and moved with a velocity v perpendicular to the field, the induced e.m.f is called motional e.m.f produced across the ends of the rod which is given by ε = Blv.

9 Changing magnetic fields can setup current loops in nearby metal bodies (any conductor). Such currents are called eddy currents. They dissipate energy as heat which can be minimized by laminating the conductor.

10 Inductance is the ratio of the flux linkage to current.

11 When a current in a coil changes it induces a back e.m.f in the same coil. The selfinduced e.m.f is given by where L is the self-inductance of the coil. It is a measure of inertia of the coil against the change of current through it. Its S.I unit is henry (H).

12 A changing current in a coil can induce an e.m.f in a nearby coil. This relation, shows that Mutual inductance of coil 1 with respect to coil 2 (M₁₂ ) is due to change of current in coil 2. (M₁₂ = M₂₁ ). 

13 The self-inductance of a long solenoid is given by L = μ_o n² Al where A is the area of 0 crosssection of the solenoid, l is its length and n is the number of turns per unit length.

14 The mutual inductance of two co-axial coils is given by M₁₂ = M₂₁ = μ₀ n₁ n₂ Al where n₁ & n₂ are the number of turns per unit length of coils 1 & 2. A is the area of cross-  section and l is the length of the solenoids

15 Energy stored in an inductor in the form of magnetic field is  U_B=1/2Li² _maxand U_B=B²/ 2μ₀ Magnetic energy density.

16 In an A.C. generator, mechanical energy is converted to electrical energy by virtue of electromagnetic induction. Rotation of rectangular coil in a magnetic field causes change in flux (Φ = NBACosωt).

* Change in flux induces e.m.f in the coil which is given by ε= -dΦ/dt = NBAωSinωt ε = ε₀ Sinωt 

* Current induced in the coil I = ε/R = ε₀ Sinωt/R = I₀ Sinωt 

17 An alternating voltage ε=ε₀ Sinωt, applied to a resistor R drives a current I = I₀ Sinωt in the resistor, I₀ = ε₀ /R where ε₀ & I₀ are the peak values of voltage and current. (also represented by V_m & I_m )

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