If you find these question easy to answer, well and good. If you don’t, please revise these portions thoroughly. These are based on just the minimum level of learning expected from a Class XII student.

- State Gauss theorem in magnetism.
- Define the terms retentivity and coercivity.
- Which direction will a magnetic compass show when it is kept horizontally at the magnetic north pole of earth?
- Determine the magnitude and direction of magnetic field to be applied in a region with a vertical electric field 1000 V/m (downwards) where a proton has to move towards east with a speed of 100 m/s without any deflection. (Disregard the presence of earth’s magnetic field.)
- Derive expression for the force on a current carrying conductor in a uniform magnetic field.
- Using Ampere’s circuital law, derive an expression for the magnetic field at a point due to a straight conductor carrying current.
- The radius of the first electron orbit of a hydrogen atom is 0.5 A. The electron moves in this orbit with a uniform speed of 2.2 x 10
^{6} m/s. What is the magnetic field produced at the centre of the nucleus due to the motion of this elecetron?
- Derive the relation m
_{r} = 1+c_{m} (relation between magnetic permeability and magnetic susceptibility)
- Mention the magnetic elements of earth and define each.
- Derive expression for the magnetic dipole moment of a revolving electron.
- Describe the principle, construction and working of a cyclotron. Why electrons cannot be accelerated using a cyclotron?

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**ANSWERS**

- Gauss theorem in magnetism states that closed surface integral of magnetic field in a closed surface is zero. That is, the net magnetic flux through a closed surface is always zero. Mathematically,
- Retentivity is the magnetization retained by a specimen when the magnetizing field is zero. Coercivity is the reverse magnetizing force required to completely demagnetize and specimen.
- The compass needle will not show any particular direction as the horizontal component is zero at the magnetic poles. It can rest in any direction in which it is placed just like a non-magnetic needle.

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