CBSE Class 12 Physics Notes Chapter 5 - Magnetism and Matter

What happens to the dipole moment of a magnet?

When a magnet is experiencing a dipole moment m, and it is placed in a uniform magnetic field B, then it experiences:

  • The force on it as zero
  • The torque on it as m*B
  • The potential energy can be said to be as -m*B. Where the zero of energy is determined when m is perpendicular to B

Gauss’s law for Magnetism

Gauss’s law of magnetism points out the fact that the net magnetic flux through any surface which is closed tends to be Zero.

Components of the Earth’s Magnetic Field

Three components have to be taken care of in order to specify the magnetic field of the earth:

  • The horizontal component
  • Magnetic Declination
  • Magnetic Dip

How are Magnetic Materials are classified?

Magnetic Materials are classified into three types:

  • Diamagnetic- whose χ (magnetic susceptibility of the material) is having negative value and small
  • Paramagnetic- for this the χ is positive and small
  • Ferromagnetic- for this materials, the χ is large

Permanent Magnets

When substances are exposed to room temperature, they retain their ferromagnetic properties for a longer time and are called as permanent magnets.


Important Questions

  1. Answer the following questions regarding earth’s magnetism:
    1. With the specification to the earth’s magnetic field, name the independent quantities.
    2. In southern India, If the angle of dip at a location is about 30°, do we expect a greater or smaller dip angle in Britain.
    3. If we made a map of magnetic field lines at New Delhi, would the lines come out of the ground or go into the ground
    4. In which direction would a compass be free to move in the vertical plane point if the compass is located right on the geomagnetic north or south pole?
    5. Check the order of magnitude of the earth’s field if the dipole of the magnetic moment is 20 × 1022 J T–1 and is located at its center.

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