CBSE Class 12th Physics Notes: Electromagnetic Induction (Part ‒ II)

NCERT based Chapter wise Physics Notes for Class 12 are provided by Jagranjosh.com. In this article, students will get important key notes on Chapter 6, Electromagnetic Induction of CBSE class 12 Physics. These notes are continuation of CBSE Class 12th Physics Notes: Electromagnetic Induction (Part ‒ I). In part I, we have studied important concepts like Magnetic Flux, Electromagnetic Induction, Faraday’s Law of Electromagnetic Induction, Lenz’s Law etc. Now in this part we will study the concepts given below:

The key notes are as follows

Eddy Currents

The currents induced in body of a conductor when the amount of magnetic flux linked with the conductor changes.

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Applications of Eddy Currents

Electromagnetic damping

Some galvanometers have a fixed core made of nonmagnetic metallic material. When the coil oscillates, the eddy currents are generated in the core which opposes the motion & bring the coil to rest quickly.

Induction furnace

Induction furnaces are used to produce high temperatures & generally used to prepare alloys, by melting the constituent metals. When a high frequency alternating current passes through the coil (which surrounds the metal) then eddy currents are produced in the metal which in turn produce high temperatures and melts the metal.

Magnetic braking in trains

Strong electromagnets are situated above the rails in electrically powered trains. When the electromagnets are activated, the eddy currents induced in the rails oppose the motion of the train.

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Electric Power Meters

You must have observed the rotating shiny disc in the power meter of your house. The shiny metal disc in the electric power meter rotates due to the eddy currents. Electric currents are induced in the disc by magnetic fields produced by alternating currents in a coil.

Inductance

Electric current can be induced in a coil by flux change produced by another coil in its vicinity or flux change produced by the same coil. These two situations are described separately in the next two sub-sections. In both the cases, the flux through a coil is proportional to the current, i.e., ϕ_B ∝ I.

If the geometry of the coil does not vary with time then, d(ϕ_B)/dt ∝ d(I)/dt.

For a closely wound coil of N turns, the same magnetic flux is linked with all the turns.

Therefore, a term called flux linkage is used which is equal to Nϕ_B for a closely wound coil and in such a case Nϕ_B ∝ I.

The constant of proportionality, in this relation, is called inductance. In inductance depends only on geometry of coil and intrinsic properties of a material.

Self Inductance

It is the property of a coil by virtue of which, the coil opposes any change in the strength of current flowing through it by inducing e.m.f. in itself.

Coefficient of Self Inductance

If I is the magnitude of current flowing through a coil at any time, ϕ amount of magnetic flux linked with all the turns of coil at that time, then, ϕ ∝ I or ϕ = L I

Where, L is a constant of proportionality and also known as coefficient of self inductance or coefficient of self inductance.

If I = 1 then, ϕ = L I or L = ϕ. So, self inductance of a coil is numerically equal to the magnetic flux linked with the coil when unit current flows through the coil.

With slight modification of above formula, one can also say that, self inductance of a coil is numerically equal to the e.m.f. induced in the coil when the rate of change of current through the coil is unity.

SI unit of self inductance is equal to the henry (H).

Self Inductance of a Long Solenoid

If L is the inductance of a long air-cored coil of length l, having n turns per length of cross sectional area A each then, L = μ_o n²lA

Mutual Inductance

It is the property by virtue of which two closely lying coils opposes any change in the strength of current flowing through the other by developing an induced e.m.f.

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Coefficient of Mutual Inductance

If I is the strength of the current in one coil, ϕ is the amount of magnetic flux linked with all the turns of neighbouring coil, then, ϕ ∝ I or ϕ = MI

Where, M is constant of proportionality and is called coefficient of mutual or mutual inductance.

If I = 1, ϕ = M × 1 or M = ϕ

Where, M is a constant of proportionality and also known as coefficient of mutual induction.

Coefficient of mutual induction or mutual induction of two coils is numerically equal to the amount of magnetic flux linked with one coil when unit amount of current flows through the neighbouring coil.

SI unit to measure mutual inductance is henry (H).

Factors affecting Mutual Inductance of a Pair of Coil

• Geometry of two coils such as shape, size, number of turns and nature of material

• Relative placement or orientation of two coils

AC Generator

It is a device which converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction. The main components of AC dynamo are field magnet, armature, slip rings and brushes.

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Instantaneous value of induced e.m.f, is by ε = n B A ω sin ω t, where, n is the number of turns of the coil, A is the area of the coil, B is the magnetic field strength. If we denote NBAω as ε₀, then ε = ε₀ sin ωt.

Since, ω = 2πν, so we can also write can be written as ε = ε₀ sin 2π ν t where ν is the frequency of revolution of the generator’s coil. The instantaneous value of the emf and ε varies between +ε₀ and –ε₀ periodically.

CBSE Class 12th Physics Notes: Electromagnetic Induction (Part ‒ I)