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

Dec 20, 2016 17:00 IST

    Chapter wise notes of CBSE class 12 Physics are available in this article. These notes based on Chapter 6, Electromagnetic Induction of NCERT textbook for class 12 Physics. These notes are important for CBSE class 12 Physics exam2017.

    The topics covered in this chapter are given below

    The Experiments of Faraday and Henry

    Magnetic Flux

    Electromagnetic Induction

    Faraday’s Law of Electromagnetic Induction

    Lenz’s Law and Conservation of Energy

    Motional Electromotive Force

    The notes are as follow

    The Experiments of Faraday and Henry

    Discovery and understanding of electromagnetic induction are based on a long series of experiments carried out by Henry and Faraday.

    Experiment 1:

    Experiment 1 by Faraday and Henry: Electromagnetic Induction

    Image Source: NCERT Textbooks

    If North-pole of a bar magnet is pushed towards the coil, the pointer in the galvanometer deflects, indicating the presence of electric current in the coil. This deflection lasts as long as the bar magnet remains in motion.
    The galvanometer doesn’t show any deflection when the magnet is held at rest. When the magnet is pulled away from the coil, the galvanometer shows deflection in the opposite direction, which indicates reversal of the current’s direction.
    This shows that the relative motion between the magnet and the coil that is responsible for generation (induction) of electric current in the coil.

    CBSE Class 12th Physics Notes: Magnetism and Matter (Part ‒ I)

    Experiment 2:

    Experiment 2 by Faraday and Henry: Electromagnetic Induction

    Image Source: NCERT Textbooks

    If the bar magnet is replaced by a second coil C2 (as shown in figure given above) connected to a battery. The steady current in the coil C2 produces a steady magnetic field.

    If coil C2 is moved towards the coil C1, then the galvanometer shows a deflection. This indicates that electric current is induced in coil C1.

    When C2 is moved away, the galvanometer shows a deflection again, but this time in the opposite direction. The deflection will be observed as long as coil C2 is in motion.

    When the coil C2 is held fixed and C1 is moved, the same effects are observed. Again, it is the relative motion between the coils that induces the electric current.

    Experiment 3:

    Experiment 3 by Faraday and Henry: Electromagnetic Induction

    Image Source: NCERT textbooks

    Faraday showed that this relative motion is not an absolute requirement. Figure given above shows two coils C1 and C2 held stationary. Coil C1 is connected to galvanometer G while the second coil C2 is connected to a battery through a tapping key K.

    The galvanometer shows a momentary deflection when the tapping key K is pressed. The pointer in the galvanometer returns to zero immediately. If the key is held pressed continuously, there is no deflection in the galvanometer. When the key is released, a momentory deflection is observed again, but in the opposite direction. It is also observed that the deflection increases dramatically when an iron rod is inserted into the coils along their axis.

    Magnetic Flux:

    It is the number of magnetic field lines crossing any surface normally is called magnetic flux (ϕ) through that surface.

    ϕ = B.A cos θ

    Where B is magnetic field, A is are of the surface, ϕ is the angle between the magnetic field and area vector. SI unit of magnetic flux is weber.

    Electromagnetic Induction:

    It is the phenomenon of production of e.m.f. in a conductor due to a change in magnetic flux linked with it. The e.m.f so produced is called induced e.m.f. and current is called induced current.

    Faraday’s Law of Electromagnetic Induction:

    The magnitude of the induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit. Mathematically, the induced emf is given by ε = ‒ (dϕ/dt).

    Negative sign shows that the current induced in a circuit always flows in such a direction that it opposes the change or the cause that it opposes the change or the cause produces it.

    In the case of a closely wound coil of N turns, change of flux associated with each turn, is the same. Therefore, the expression for the total induced emf is given by

    ε = ‒ N (dϕ/dt)

    The induced emf can be increased by increasing the number of turns N of a closed coil.

    The flux can be varied by changing any one or more of the terms B, A and θ.

    Lenz’s Law and Conservation of Energy

    The polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it.

    ε = ‒ N (dϕ/dt)

    The negative sign shown in this equation represents this effect.

    Effect due to Lenz’s Law

    Image Source: NCERT textbooks

    In the above figure (a), when the North-pole of a bar magnet is being pushed towards a closed coil the magnetic flux through the coil increases. Hence current is induced in the coil in such a direction that it opposes the increase in flux. This is possible only if the current in the coil is in a counter-clockwise direction with respect to an observer situated on the side of the magnet. Vice versa case is shown in figure (b).

    Lenz’s law complies with the principle of conservation of energy. Here when the N-pole of a bar magnet is pushed into a coil as shown, the direction of induced current in the coil will act as N-pole. So, work has to be done against the magnetic repulsive force to push the magnet into the coil. The electrical energy produced in the coil as the expense of this work done

    Motional Electromotive Force

    Motional Electromotive Force

    Image Source: NCERT textbooks

    Suppose a straight conductor moving in a uniform and time independent magnetic field. In the figure given above induced e.m.f. is given by ε = ‒ [d(Blx)/dt] = Blv {where dx/dt = –v}.

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