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CBSE Class 12th Physics Notes: Moving Charges and Magnetism (Part ‒ II)

Nov 30, 2016 15:00 IST

    CBSE class 12 Physics notes on Chapter-4: Moving Charges and Magnetism of NCERT class 12 Physics textbook are available in this article.

    Chapter-4: Moving Charges and Magnetism of NCERT class 12 Physics textbook is one of the most important chapters of CBSE class 12. Conceptual questions and numerical based on this chapter are frequently asked in CBSE class 12 Physic board examination.  In this article, students will get important revision notes of this chapter.

    This article is a continuation of the revision notes on Moving Charges and Magnetism (Part ‒ I). In part I, we have studied important topics like Oersted’ experiment, magnetic field, Lorentz Force, magnetic force on a current-carrying conductor, motion of a charge particle in a magnetic Field, Cyclotron etc.

    In this part we will study other important topics which are given below:

    • Magnetic Field Due to a Current Element:  Biot-Savart Law
    • Magnetic Field on the Axis of a Circular Current Loop
    • Magnetic field at the centre of a current carrying coil
    • Ampere’s Circuital Law
    • Magnetic field due to a solenoid
    • Magnetic field due to a toroid

    The notes are as follows:

    Magnetic Field Due to a Current Element:  Biot-Savart Law

    Magnetic Field Due to a Current Element Biot-Savart Law

    Image Source: NCERT Textbooks

    Magnetic field strength at a point P due to a small length dl of the conductor carrying current I is given by,

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    Magnetic Field on the Axis of a Circular Current Loop:

    Image Source: NCERT Textbooks

    Magnetic field at a point on the axis of a coil, having radius R, distance x from the centre of the coil is given by,

    Magnetic field at the centre of a current carrying coil:

    To find the magnetic field at the centre of the coil, we can put x = 0 is above equation.

    So, B at the centre of a current carrying coil on N turn is given by,

    Image Source: NCERT Textbooks

    The magnetic field lines due to a circular wire form closed loops and are shown in the figure given above. The direction of the magnetic field is given by right-hand thumb rule which is if we curl the palm of our right hand around the circular wire with the fingers pointing in the direction of the current then; the right-hand thumb gives the direction of the magnetic field.

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    Ampere’s Circuital Law

    According to this law, the line integral of magnetic field B around any closed path in vacuum is μo times the net current (I) threading through the area enclosed by the curve.

    Magnetic field due to a solenoid

    Solenoid is a device used to generate magnetic fields. It consists of a long conducting wire wound in the form of a helix where the neighboring turns are closely spaced. When current flows in the solenoid, then each turn can be regarded as conducting circular loop. The net magnetic field is the vector sum of the fields due to all the turns. Enameled wires are used for winding so that turns are insulated from each other.

    Image Source: wikipedia.org

    With the help of Ampere’s circuital law we can calculate the magnetic field due to a solenoid.

    Magnetic field at the centre of a long solenoid having n turns per unit length and carrying a current I is given by: B = μo n I

    The direction of the field is given by the right-hand rule. The solenoid is commonly used to obtain a uniform magnetic field.

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    Magnetic field due to a toroid

    Toroid is a hollow circular ring on which a large number of turns of a wire are closely wound. It can be viewed as a solenoid which has been bent into a circular shape to close on itself.

    Image Source: NCERT Textbooks

    The magnetic field in the open space inside (point P) and exterior to the toroid (point Q) is zero. The field B inside the toroid is constant in magnitude for the ideal toroid of closely wound turns and is given by, B = μo n I.

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

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