Question 71:
easy16. A bar magnet is oscillating in the Earth’s magnetic field with a period $T$. What happens to its period and motion if its mass is quadrupled? (2003)
The period of oscillation is $T = 2pisqrt{frac{I}{MB}}$. Moment of inertia $I propto m$. If mass is quadrupled, $I' = 4I$, so $T' = 2pisqrt{frac{4I}{MB}} = 2T$. The motion continues to be simple harmonic.
Question 72:
easy17. Two bar magnets having same geometry with magnetic moments $M$ and $2M$, are firstly placed in such a way that their similar poles are same side then its time period of oscillation is $T_1$. Now the polarity of one of the magnet is reversed then time period of oscillation is $T_2$, then: (2002)
In sum position, effective magnetic moment is $M_{text{net}} = 2M + M = 3M$, so $T_1 propto frac{1}{sqrt{3M}}$. In difference position, $M_{text{net}} = 2M - M = M$, so $T_2 propto frac{1}{sqrt{M}}$. Since $M_{text{net}}$ decreases, the period increases, meaning $T_1 < T_2$.
Question 73:
easy18. The work done in turning a magnet of magnetic moment $M$ by an angle of $90^{circ}$ from the meridian, is $n$ times the corresponding work done to turn it through an angle of $60^{circ}$. The value of $n$ is given by (1995)
Work done for $90^{circ}$ is $W_1 = MB(1 - cos 90^{circ}) = MB$. Work done for $60^{circ}$ is $W_2 = MB(1 - cos 60^{circ}) = frac{MB}{2}$. Thus, $W_1 = 2 W_2$, which gives $n = 2$.
Question 74:
easy19. An iron rod of susceptibility $599$ is subjected to a magnetising field of $1200text{ A m}^{-1}$. The permeability of the material of the rod is: ($mu_0 = 4pi times 10^{-7}text{ T m A}^{-1}$) (2020)
Relative permeability is $mu_r = 1 + chi = 1 + 599 = 600$. The permeability of the material is $mu = mu_r mu_0 = 600 times 4pi times 10^{-7} = 2400pi times 10^{-7} = 2.4pi times 10^{-4}text{ T m A}^{-1}$.
Question 75:
easy25. If the magnetic dipole moment of an atom of diamagnetic material, paramagnetic material and ferromagnetic material are denoted by $\mu_d$, $\mu_p$ and $\mu_f$ respectively, then (2005)
Diamagnetic atoms have completely paired electron configurations, leading to a net atomic dipole moment $\mu_d = 0$. Paramagnetic atoms have unpaired electrons giving a permanent non-zero atomic dipole moment $\mu_p \neq 0$.
Question 76:
easy26. Diamagnetic material in a magnetic field moves: (2003)
Diamagnetic substances are magnetized in a direction opposite to the external magnetizing field. As a result, they experience a repulsive force from strong field regions toward weaker field regions.
Question 77:
easy27. Among which the magnetic susceptibility does not depend on the temperature: (2001)
Diamagnetism is an intrinsic property arising from the orbital motion of electrons and does not involve the alignment of permanent atomic dipoles against thermal agitation. Hence, its susceptibility is independent of temperature.
Question 78:
easy28. For protecting a magnetic needle it should be placed: (1998)
To shield a region from external magnetic fields (magnetic shielding), the object should be enclosed in a ferromagnetic enclosure such as a soft iron box, which channels magnetic field lines through its walls.
Question 79:
easy29. Electromagnets are made of soft iron because soft iron has: (2010 Pre)
An electromagnet should easily magnetize when current is switched on and readily lose its magnetism when current is turned off. Thus, the material must have low retentivity and low coercivity.
Question 80:
easy30. Curie temperature is the temperature above which: (2008)
Curie temperature is the critical temperature above which thermal agitation destroys the domain structure of a ferromagnetic material, converting it into a paramagnetic material.