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CBSE Class 12th Physics Notes for Chapter 5 Magnetism and Matter



  • When a bar magnet of dipole moment m is placed in a uniform magnetic field B
    • the force on it is zero
    • the torque on it is m x B
    • its potential energy is –m.B where we choose the zero of energy at the orientation where m is perpendicular to B.


  • The earth’s magnetic field resembles that of a (hypothetical) magnetic dipole located at the centre of the earth. The pole near the geographic north pole of the earth is called the north magnetic pole. Similarly, the pole near the geographic south pole is called the south magnetic pole. The dipole is aligned making a small angle with the rotation axis of the earth. The magnitude of the field on the earth’s surface = 4 x 10-5


  • Three quantities are needed to specify the magnetic field of the earth on its surface- the horizontal component, the magnetic declination, and the magnetic dip, these are known as the elements of the earth’s magnetic field.


  • Magnetic materials are broadly classified as: diamagnetic, paramagnetic and ferromagnetic. For diamagnetic materials the magnetic susceptibility is negative and small whereas for paramagnetic materials it is positive and small. For ferromagnetic materials, the magnetic susceptibility is large.


  • Substances which retain their ferromagnetic property for a long period of time at room temperature are called permanent magnets.


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