Test: Magnetism and Matter - 1 - CUET Humanities MCQ

The torque on a magnetic dipole due to an external magnetic field is given by

where M is the magnetic moment and B is the magnitude of the external magnetic field.

These substances show permanent magnetism even when the magnetic field is removed, e.g. FeO and CrO₂. Fe, Co and Ni show ferromagnetism even at room temperature. CrO₂ is used to make magnetic tapes for use in cassette recorders. The spontaneous alignment of magnetic moments due to unpaired electrons in the same direction gives rise to ferromagnetism.

If L is length of the magnet, then magnetic moment, M = mL (where m is the pole strength).
When same magnet is bent into a semicircle of radius r, then 
Separation between the poles = 2r
New magnetic moment of the bar magnet is:

The potential at a point on the axial line due to a bar magnet is given by V = 
Hence, V ∝ M (if all other quantities are same)

Ferromagnetic substances have a very high permeability (of the order 1000 or more) and positive susceptibility ().
Now, μ_r = 1 + .
Hence, the correct choice is (3).

Magnetic susceptibility of a diamagnetic substance is not affected by temperature.
When we increase the temperature of diamagnetic substance, the susceptibility is not affected much. The paired spins remains unaffected by the increase in the inter-atomic vibration as a very large amount of energy is required to break apart paired electrons which are in their ground state when paired.

The magnetism of the magnet is due to the spin motion of electrons. Each electron in an atom is revolving in an orbit around the nucleus. The revolving electron is equivalent to a tiny loop of current. Therefore, it possesses some orbital magnetic dipole moment  = current × area of the loop.
In addition to the orbital motion, every electron is assumed to have a spin motion around its axis. Therefore, another dipole magnetic moment called spin magnet  is also associated with electron. The vector sum of  and  provides the net magnetic dipole moment , to the atom.  is much greater than , hence magnetism of magnet is due to spin motion of electrons.

When a magnetic needle is kept in a non-uniform magnetic field, the two poles of the needle experience unequal forces, which results in a net force acting on the needle. Also, since the force acting on the needle produces a turning effect, torque is experienced by the needle.

Potential energy of the dipole =  = 
= 2MB
= 2  2.0  0.25 = 1.0 J

When diamagnetic substances are placed in a magnetic field, then they are feebly repelled in the field. Frog is levitated in magnetic field produced by the current in vertical solenoid below the frog due to repulsion, so body of frog behaves as diamagnetic substance.