Question:

Needles $N_{1}, N_2$ and $N_3$ are made of a ferromagnetic, a paramagnetic and a diamagnetic substance respectively. A magnet when brought close to them will:

Updated On: Jul 28, 2022
  • attract $N_1$ and $N_2$ strongly but repel $N_3$
  • attract $N_1$ strongly, $N_2$ weakly and repel $N_3$ weakly
  • attract $N_1$ strongly, but repel $N_2$ and $N_3$ weakly
  • attract all three of them
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The Correct Option is B

Solution and Explanation

Ferromagnetic subsiances have strong tendency to get magnetised (induced magnetic moment) in the same direction as that of applied magnetic field, so magnet attracts $N_1$ strongly. Paramagnetic substances get weakly magnetised (magnetic moment induced is small) in the same direction as that of applied magnetic field, so magnet attracts $N_2$ weakly. Diamagnetic substances also get weakly magnetised when placed in an external magnetic field but in opposite direction and hence, $N_3$ is weakly repelled by magnet.
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Concepts Used:

Magnetic Properties of Materials

Magnetic Material:

Magnetic material is one that can repel or attract other materials. The process of attraction or repulsion of these materials depends on the arrangement of electrons, which is known as the magnetic moment of that material.

Diamagnetic - With regard to the susceptibility χ, a material is diamagnetic if χ is negative. These materials produce negative magnetization when they are placed in the magnetic field. When placed in a magnetic field, these diamagnetic materials are barely magnetized. Diamagnetic substances are the ones that are repelled by magnets. 

Paramagnetic - In the presence of an external magnetic field, the substances that get weakly magnetized are known as paramagnetic substances. The magnetic dipoles align along the direction of the applied field, reinforcing and enhancing the magnetic field. These substances get weakly attracted to a magnet as they tend to move from a weaker magnetic field to a stronger one. The magnetization (M) of paramagnetic materials, discovered by Madam Curie, depends on the temperature (T) and the external magnetic field B.

M = C x (B/T) 

Where M = Magnetization

C = Curie Constant

B = External magnetic field

T = Temperature

Ferromagnetic - The materials strongly attracted by the magnetic field are known as ferromagnetic materials.