Physics Self-Test - Page 12
84- Arrange the following in the increasing order of magnetic susceptibility (χ):
- a) Paramagnetic < Diamagnetic < Ferromagnetic
- b) Paramagnetic < Ferromagnetic < Diamagnetic
- c) Ferromagnetic < Paramagnetic < Diamagnetic
- d) Diamagnetic < Paramagnetic < Ferromagnetic
Explanation: Diamagnetic substances have negative susceptibility ($\chi < 0$), paramagnetic substances have small positive susceptibility ($\chi > 0$), and ferromagnetic substances have very large positive susceptibility ($\chi \gg 1$).
85- Pick out the material that cannot be used to make permanent magnets.
- a) Copper
- b) Aluminum
- c) Steel
- d) Zinc
Explanation: Permanent magnets require strongly ferromagnetic materials (like steel or alnico) to retain structural domain alignment. Non-ferromagnetic materials cannot hold stable residual magnetization.
86- Find the true statement.
- a) Most of the substances show ferromagnetism
- b) The magnetic susceptibility of diamagnetic substances are temperature independent
- c) Water is paramagnetic in nature
- d) Typical domain size of a ferromagnetic material is 1 nm
Explanation: Diamagnetism arises from core atomic electron orbital shielding dynamics and is remarkably temperature-independent, unlike paramagnetism or ferromagnetism which are heavily disrupted by thermal agitation (Curie's Law).
87- A magnetic needle is kept in a non-uniform magnetic field. It experiences:
- a) A torque but not a force
- b) neither a force nor a torque
- c) force and torque
- d) a force but not torque
Explanation: In a uniform field, a dipole experiences only a realigning torque. However, when placed in a *non-uniform* field, the opposing poles experience unequal field magnitudes, generating a net translational force alongside the rotational torque.
88- The magnetic dipole moment of a magnetic dipole is given by the formula:
- a) M = m × 2l
- b) M = m + 2l
- c) M = m − 2l
- d) M = m / 2l
Explanation: The magnetic dipole moment $M$ is defined as the product of the pole strength ($m$) and the effective magnetic distance vector separating the two poles ($2l$).
89- What does a superconductor exhibit?
- a) Ferromagnetism
- b) Paramagnetism
- c) Diamagnetism
- d) None of these options
Explanation: Due to the Meissner Effect, superconductors completely expel internal magnetic fields when transitioning below their critical threshold temperature, exhibiting perfect diamagnetism ($\chi = -1$).
90- Which of the following is a vector quantity?
- a) Relative permeability
- b) Magnetic field intensity
- c) Flux density
- d) Magnetic potential
91- The ratio of intensity of magnetisation (M) to the magnetisation force (H) is known as:
- a) flux density
- b) susceptibility
- c) relative permeability
- d) none of the above
Explanation: Magnetic susceptibility is mathematically defined as the ratio of the induced magnetization vector to the applied magnetizing field intensity: $\chi = \frac{M}{H}$.
92- Magnetising steel is normally difficult because:
- a) it corrodes easily
- b) it has high permeability
- c) it has high specific gravity
- d) it has low permeability
Explanation: Steel exhibits lower initial magnetic permeability compared to soft iron, meaning it demands higher external magnetizing forces to reorient its magnetic domains. However, once aligned, it retains this magnetization strongly due to high retentivity.