Problem 1993 - JEE Physics

Question

Read the assertion and reason carefully to mark the correct option out of the options given below. (A) If both assertion and reason are true and the reason is the correct explanation of the assertion. (B) If both assertion and reason are true but reason is not the correct explanation of the assertion, (C) If assertion is true but reason is false. (D) If the assertion and reason both the false. (E) If assertion is false but reason is true. Assertion: Diamagnetic materials can exhibit magnetism. Reason: Diamagnetic materials have permanent magnetic dipole moment.

Step-by-Step Solution

Answer

The correct option is (C) If assertion is true but reason is false since diamagnetic materials can exhibit magnetism, but they do not have a permanent magnetic dipole moment.

Step 1: 1. Understanding Diamagnetism

Diamagnetic materials are materials that do not possess any permanent magnetic dipole moment, and they repel an external magnetic field. The magnetization of these materials is weak and opposite to the applied magnetic field direction. Diamagnetism is due to the change in the orbital motion of electrons caused by an external magnetic field, which creates an induced magnetic field.

Step 2: 2. Assertion: Diamagnetic materials can exhibit magnetism

The assertion is true. Diamagnetic materials can exhibit magnetism, but it is weak and in the opposite direction to an applied external magnetic field. When an external magnetic field is removed, the induced magnetism in the material disappears. Example: Bismuth, copper, and silver are examples of diamagnetic materials.

Step 3: 3. Reason: Diamagnetic materials have a permanent magnetic dipole moment

The reason is false. Diamagnetic materials do not have a permanent magnetic dipole moment because they have paired electrons, and their intrinsic magnetic moment cancels out. The temporary magnetism in diamagnetic materials is only created because of the change in the orbital motion of electrons in response to an external magnetic field.

Step 4: 4. Choosing the correct option

Since the assertion is true, but the reason is false, we choose the correct option as (C) If assertion is true but reason is false.

Key Concepts

Permanent Magnetic Dipole Moment

In the world of magnetism, permanent magnetic dipole moments are a fascinating concept. Let's explore what they mean and how they relate to different materials. A permanent magnetic dipole moment is a constant magnetic behavior seen in materials where the atomic or molecular magnetic moments remain aligned without an 'external magnetic field'. This alignment is typically found in ferromagnetic materials like iron, cobalt, and nickel.

However, diamagnetic materials lack this characteristic. They do not have a permanent magnetic dipole moment because their electrons are paired, and these paired electrons cause their intrinsic magnetic moments to cancel each other out. This cancellation means there is no residual or permanent magnetism in diamagnetic substances.

Understanding this concept helps explain why diamagnetic materials behave the way they do when exposed to magnetism. They only exhibit temporary magnetism due to the interaction with an 'external magnetic field', not because of any inherent magnetic properties.

External Magnetic Field

An 'external magnetic field' is an invisible field that applies magnetic force on substances within its influence. This field exerts an influence on materials, trying to align their intrinsic magnetic dipoles in the direction of the field. The response of a material to this external influence depends on its magnetic properties.

Diamagnetic materials respond uniquely to such fields. Since they lack a permanent magnetic dipole moment, the effect of an external field on these materials is quite distinct. Instead of aligning with the field, diamagnetic materials generate an induced magnetic field that is weak and opposes the 'external magnetic field'. This opposing induced field is what causes diamagnetic materials to repel magnetic fields, even if it is only slightly.

This relationship with external magnetic fields is also the reason why diamagnetic materials only show magnetism temporarily. Once the external field is removed, any induced magnetism vanishes as well.

Orbital Motion of Electrons

The 'orbital motion of electrons' within atoms can have profound effects on their magnetic properties. Electrons move around the nucleus in orbitals, and this motion creates tiny current loops, almost like miniature loops of wire. These loops generate magnetic moments, contributing to the overall magnetic behavior of an atom or molecule.

In diamagnetic materials, the 'orbital motion of electrons' is key. When an external magnetic field is applied, it influences these electron orbitals. The field induces changes in the orbital motion, which results in the generation of a small, temporary induced magnetic field. This induced magnetism always opposes the external field.

Thus, the delicate balance and interaction between electron motions and external forces explain why diamagnetic materials, without a permanent magnetism, can still show a fleeting magnetic response.