Magnetism and matter class 12 notes

General
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magnetism and matter class 12 notes

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Magnetism and Matter: Class 12 Notes

Introduction to Magnetism

Magnetism is a fundamental force of nature, arising due to the movement of electric charges. In this section, we will explore the different types of magnetism, their properties, and their applications.

Types of Magnetism

  1. Diamagnetism

    • Definition: Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field and are repelled by a magnet.
    • Examples: Bismuth, copper, lead, silicon, and nitrogen.
    • Properties:
      • Weak and negative susceptibility, meaning they are repelled by magnets.
      • Occurs due to paired electrons in atomic orbitals.

  2. Paramagnetism

    • Definition: Paramagnetic materials are attracted by an external magnetic field and form internal, induced magnetic fields in the direction of the applied magnetic field.
    • Examples: Aluminum, platinum, and certain metal ions like Fe$^{3+} and Mn^{2+}$.
    • Properties:
      • Possesses unpaired electrons, which contribute to a weak and positive magnetic susceptibility.
      • Alignment with the external magnetic field is temporary and disappears when the field is removed.

  3. Ferromagnetism

    • Definition: Ferromagnetic materials become strongly magnetized when exposed to a magnetic field, and retain their magnetization even when the external field is removed.
    • Examples: Iron, cobalt, nickel, and their alloys.
    • Properties:
      • Strong magnetic susceptibility.
      • Possess a spontaneous alignment of magnetic moments due to exchange interaction.
      • Exhibits hysteresis loop when exposed to varying magnetic fields.

Magnetic Properties of Materials

  • Magnetic Moments: Result from the angular momentum of electrons around the nucleus and from the spin of the electrons.
  • Magnetic Domains: Regions within a ferromagnetic material where the magnetic fields of atoms are grouped together and aligned.
  • Curie Temperature: The temperature above which a ferromagnetic material becomes paramagnetic. At Curie temperature, thermal agitation breaks the alignment of magnetic domains.

Magnetic Field and Magnetic Lines of Force

  • Magnetic Field (B-field): A vector field surrounding magnets and electric currents, representing the force exerted by magnets.

  • Magnetic Lines of Force: Invisible lines that map the strength and direction of magnetic fields. They emerge from the north pole and enter at the south pole.

    1. Properties of Magnetic Field Lines:
      • Closed loops without any beginning or end, demonstrating no source or sink in the magnetic circuit.
      • The tangent at any point on the line gives the direction of the magnetic field vector.
      • Crowded lines indicate a strong magnetic field, while lines further apart indicate a weaker field.

Earth’s Magnetism

  • Geomagnetic Field: Earth’s magnetic field is similar to that of a bar magnet tilted at an angle to the geographical axis.
    • Magnetic Declination: The angle between the geographic meridian and the magnetic meridian at a specific location.
    • Magnetic Inclination (Dip): The angle at which the Earth’s magnetic field lines penetrate the Earth’s surface.

Bar Magnet and its Properties

  • A bar magnet exhibits north and south poles that generate attractive and repulsive forces.
  • Magnetic Moment (M): A measure of a magnet’s strength and its ability to generate a magnetic field. It is the product of the pole strength and the distance between the poles. Represented by:
    \text{Magnetic Moment} = m \times 2l

Magnetic Field Due to a Bar Magnet

The magnetic field produced by a bar magnet outside the magnet can be approximated by the field due to a magnetic dipole. The mathematical expression for the magnetic field at a distance ( r ) from the center of a bar magnet is given as:

B = \frac{\mu_0}{4\pi} \cdot \frac{2m}{r^3}

where (\mu_0) is the permeability of free space and (m) is the magnetic moment of the magnet.

Magnetization and Magnetic Intensity

  • Magnetization (M): The vector field that expresses the density of permanent or induced magnetic dipole moments in a material. Defined as magnetic moment per unit volume.
    M = \frac{\text{Magnetic Moment (m)}}{\text{Volume (V)}}
  • Magnetic Intensity (H): A measure of the magnetizing field that produces magnetization in a material, represented by:
    H = \frac{B}{\mu_0} - M

Magnetic Hysteresis

  • Hysteresis Loop: Demonstates the lag between the magnetization of the material and the external magnetic field strength ( H ). The area of the loop gives the energy loss due to internal friction.

    1. Characteristics:
      • Coercivity: The intensity of the applied magnetic field required to reduce the magnetization of a material to zero.
      • Retentivity (or Remanence): The magnetization remaining in the material when the magnetizing field is reduced to zero.

Applications of Magnetism

  • Electromagnets: Highly effective in lifting heavy iron objects and in devices such as electric bells and loudspeakers.
  • Permanent Magnets: Used in various applications, including refrigerator magnets, speakers, and motors.
  • Magnetic Storage Media: Utilized in hard disks, credit cards, and magnetic tape for recording audio and video.
  • Transformer Cores: Materials with high magnetic permeability like soft iron are essential in transformer cores to minimize power loss.

By exploring the above concepts, students will gain a comprehensive understanding of magnetism and its various applications and effects in everyday life. Keep engaging with these topics to solidify your grasp, and don’t hesitate to explore further into specialized topics such as magnetoresistance and magnetohydrodynamics for a more in-depth comprehension.

For any questions or further assistance, feel free to ask!

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