JEE Main Magnetism and Matter PYQ

16. (AIEEE 2008)

Relative permittivity and permeability of a material $ε_{r}$ and $μ_{r},$ respectively. Which of the following values of these quantities are allowed for a diamagnetic material?

A. $ε_{r} = 0.5, μ_{r} = 1.5$

B. $ε_{r} = 1.5, μ_{r} = 0.5$

C. $ε_{r} = 0.5, μ_{r} = 0.5$

D. $ε_{r} = 1.5, μ_{r} = 1.5$

Correct option is (B)

For a diamagnetic material, the value of $μ_{r}$ is less than one. For any material, the value of $\in_{r}$ is always greater than $1.$

17. (AIEEE 2006)

Needles $N_{1}$ , $N_{2}$ and $N_{3}$ are made of ferromagnetic, a paramagnetic and a diamagnetic substance respectively. A magnet when brought close to them will

A. attract $N_{1}$ and $N_{2}$ strongly but repel $N_{3}$

B. attract $N_{1}$ strongly, $N_{2}$ weakly and repel $N_{3}$ weakly

C. attract $N_{1}$ strongly, but repel $N_{2}$ and $N_{3}$ weakly

D. attract all three of them

Correct option is (B)

Ferromagnetic substance has magnetic domains whereas para-magnetic substances have magnetic dipoles which get attracted to a magnetic field. Diamagnetic substances do not have magnetic dipole but in the presence of external magnetic field due to their orbital motion of electrons these substances are repelled.

18. (AIEEE 2004)

Curie temperature is the temperature above which

A. a ferromagnetic material becomes paramagnetic

B. a paramagnetic material becomes diamagnetic

C. a ferromagnetic material becomes diamagnetic

D. a paramagnetic material becomes ferromagnetic

Correct option is (A)

The Curie temperature is the temperature above which a ferromagnetic material becomes paramagnetic. Ferromagnetic materials have a high degree of magnetization in the presence of a magnetic field. Above the Curie temperature, these materials lose their ferromagnetic behavior and become paramagnetic, meaning they are weakly attracted to a magnetic field and do not retain any magnetization in the absence of an external magnetic field. So, the correct option is : Option A : a ferromagnetic material becomes paramagnetic.

19. (AIEEE 2004)

The materials suitable for making electromagnets should have

A. high retentivity and low coercivity

B. low retentivity and high coercivity

C. high retentivity and high coercivity

D. low retentivity and low coercivity

Correct option is (B)

Materials suitable for making electromagnets should have low retentivity and low coercivity. Retentivity (or remanence) is the ability of a magnetic material to retain its magnetism after the removal of the magnetizing force. For an electromagnet, we want this to be low, as we want the magnet to only be magnetic when current is flowing. Coercivity is the ability of a magnetic material to resist becoming demagnetized. Again, for an electromagnet, we want this to be low, as we want to easily turn off the magnetism when the current is removed. So, the correct option is : Option B: Low retentivity and low coercivity.