CUET UG Physics Booster Test 2-Classification of Magnetic Materials

According to NCERT, diamagnetic substances develop an induced magnetic moment opposite to the direction of the applied magnetic field. As a result, magnetic field lines are slightly repelled from the material and the magnetic field inside becomes weaker than the applied field. The reduction is generally very small, typically of the order of one part in 10^5. In a non-uniform magnetic field, diamagnetic substances experience a force toward regions where the magnetic field is weaker. Common examples include copper, bismuth, silver, gold and water. These properties collectively define diamagnetic behaviour. Therefore all four statements are correct and option D is the correct answer.

NCERT explains that diamagnetic substances acquire an induced magnetic moment opposite to the applied magnetic field. Because of this induced magnetisation, the resultant magnetic field inside the material decreases slightly. This effect can be visualised as a slight expulsion of magnetic field lines from the interior of the material. The effect is weak because the susceptibility of diamagnetic substances is very small and negative. The field lines are neither heavily concentrated nor completely trapped. Similarly, they do not reverse direction instantaneously. Therefore only statement 1 is correct, making option A the correct answer.

According to NCERT, diamagnetic substances consist of atoms in which the magnetic moments associated with orbital and spin motions of electrons cancel one another. Although electrons possess orbital angular momentum and orbital magnetic moments, the vector sum of these moments becomes zero. Consequently, the atom has no permanent magnetic dipole moment in the absence of an external magnetic field. When a magnetic field is applied, only a small induced magnetic moment appears. Therefore the zero resultant magnetic moment arises because the individual magnetic moments cancel each other. Hence option C is correct.

Electrons orbiting the nucleus behave like current-carrying loops and therefore possess orbital magnetic moments. When an external magnetic field is applied, induced currents are produced that generate magnetic moments opposing the applied field. This behaviour follows Lenz's law, which states that induced effects oppose the cause producing them. In diamagnetic atoms, the resultant magnetic moment is initially zero because individual magnetic moments cancel one another. Therefore the correct matching is 1-a, 2-b, 3-c and 4-d. These concepts together form the atomic theory of diamagnetism discussed in NCERT.

A superconductor cooled below its critical temperature exhibits the Meissner effect, which is the complete expulsion of magnetic field from its interior. This corresponds to perfect diamagnetism. For a perfect diamagnetic material, the magnetic susceptibility is χ=-1 Using the relation μ=μ_0(1+χ) we obtain μ=μ_0(1-1)=0 Thus the absolute permeability becomes zero and no magnetic field can exist inside the material. Therefore option C is correct.

According to NCERT, superconductors are materials that exhibit zero electrical resistance below a critical temperature. In addition to being perfect conductors, they also display perfect diamagnetism. When a material enters the superconducting state, magnetic field lines are expelled from its interior, causing the magnetic field inside to become zero. This phenomenon is known as the Meissner effect. Since magnetic field lines are expelled rather than concentrated, superconductors repel nearby magnets and are themselves repelled by magnets. This repulsive interaction forms the basis of magnetic levitation. Therefore the statement that field lines are heavily concentrated inside superconductors is incorrect. Hence option C is the correct answer.

NCERT explains that paramagnetic substances contain atoms possessing permanent magnetic dipole moments. When placed in an external magnetic field, these moments partially align with the field, causing weak magnetisation in the direction of the field. Consequently, a paramagnetic material moves from weaker magnetic field regions toward stronger magnetic field regions. The enhancement of the magnetic field inside the material is very small, generally of the order of one part in 10^5. Common examples include aluminium, sodium, platinum and oxygen. However, paramagnetic substances do not show the strong attraction characteristic of ferromagnetic materials. Therefore statements 1, 3 and 4 are correct, while statement 2 is incorrect.

According to NCERT, paramagnetic substances possess permanent magnetic dipole moments at the atomic level. In an applied magnetic field, some of these dipoles align with the field direction, producing weak magnetisation. Since the magnetic susceptibility of paramagnetic substances is small and positive, the resulting enhancement of the magnetic field inside the material is also small. Typically, the increase is only about one part in 10^5. Therefore paramagnetic materials become weakly magnetised and show only a slight concentration of magnetic field lines within the material. This behaviour contrasts with diamagnetic substances, which reduce the internal magnetic field. Hence option A is correct.

NCERT states that paramagnetic substances contain permanent atomic magnetic dipoles. When an external magnetic field is applied, these dipoles tend to align in the direction of the field, producing a net magnetisation. However, thermal motion continuously randomises the orientations of the dipoles. This randomisation opposes the aligning effect of the magnetic field. As a result, only partial alignment occurs and the magnetisation remains weak. Thus the external magnetic field promotes alignment, whereas thermal motion causes randomisation. Therefore option A correctly describes the roles of the two effects.

An atom is electrically neutral because the total positive charge equals the total negative charge. However, magnetic properties depend not on net charge but on the motion of charges. According to NCERT, orbiting electrons behave like tiny current loops and possess magnetic dipole moments. In addition, electron spin contributes to the magnetic moment of an atom. The vector sum of these magnetic moments may or may not be zero. In paramagnetic atoms, the cancellation is incomplete, producing a net magnetic dipole moment. Therefore an atom can have zero net charge while still possessing a non-zero magnetic moment. Hence option B is correct.

Paramagnetic materials contain permanent atomic magnetic dipoles that tend to align with an applied magnetic field. The magnetic susceptibility χ depends on the nature of the material and also varies with temperature. According to Curie's law, susceptibility decreases as temperature increases because thermal agitation opposes dipole alignment. Consequently, lowering the temperature improves alignment and increases magnetisation rather than decreasing it. Paramagnetic susceptibility is positive because the induced magnetisation occurs in the same direction as the applied field. However, the value is usually small compared to ferromagnetic materials. Therefore statements 1, 2 and 4 are correct, whereas statement 3 is incorrect.

In a paramagnetic substance, atomic magnetic dipoles tend to align with an external magnetic field. As the field strength increases or the temperature decreases, a greater fraction of dipoles become aligned. The limiting situation occurs when essentially all dipoles are aligned parallel to the applied field. This condition corresponds to the maximum possible magnetisation and is called magnetic saturation. The Meissner effect is associated with superconductors and not paramagnetic materials. Saturation is not restricted to high temperatures and does not occur when the external field is removed. Therefore the correct answer is saturation value.

Ferromagnetic materials such as iron, cobalt and nickel exhibit extremely strong magnetic behavior because their magnetic domains can align readily with an external field. As a result, they possess very large positive susceptibility and large relative permeability values. Ferromagnets are strongly attracted toward magnets and move from weaker magnetic field regions toward stronger field regions. The statement that μᵣ is less than 1 is incorrect because values less than 1 are associated with diamagnetic materials. Ferromagnets generally have μᵣ values much greater than 1000.

Ferromagnetic materials possess magnetic domains that can align strongly under an applied magnetic field. This domain alignment results in very large magnetisation and correspondingly large magnetic permeability. Therefore the relative permeability of a ferromagnetic substance is not merely slightly greater than one; it is often hundreds or thousands of times greater than unity. Gadolinium, below its Curie temperature, behaves as a ferromagnetic material and exhibits high magnetic permeability. Hence the most appropriate representation is μᵣ >> 1.

Ferromagnetic materials consist of domains, which are relatively large regions where the magnetic moments of atoms are spontaneously aligned in the same direction. A typical domain may have a size of about 1 mm and contain approximately 10¹¹ atoms. The origin of this spontaneous alignment cannot be explained by classical physics and requires quantum mechanical exchange interactions. Domains are present even when no external magnetic field is applied. In an unmagnetised specimen, different domains point in different directions, resulting in nearly zero net magnetisation. Therefore statements 1, 2 and 3 are correct, whereas statement 4 is incorrect.

In an unmagnetised ferromagnetic specimen, domains are oriented randomly, producing nearly zero net magnetisation. When an external magnetic field is applied, domains aligned with the field direction grow in size while unfavorably oriented domains shrink. Domain motion can be observed experimentally using powdered suspensions such as the Bitter pattern technique. As the external magnetic field becomes sufficiently strong, domains continue to grow and merge, eventually producing a nearly single giant domain corresponding to magnetic saturation. Therefore the correct matching is 1-d, 2-c, 3-a and 4-b.

Alnico is a well-known hard ferromagnetic alloy consisting mainly of iron, aluminium, nickel, cobalt and copper. Hard ferromagnetic materials possess high retentivity and coercivity, enabling them to retain a substantial fraction of their magnetisation even after the external magnetic field is removed. This property makes Alnico suitable for permanent magnets used in measuring instruments, loudspeakers and electric motors. The persistence of magnetisation is a defining characteristic of hard magnetic materials. Therefore the correct answer is that magnetisation persists when the external field is removed.

Hard ferromagnetic materials possess high retentivity and coercivity. As a result, they retain significant magnetisation even after the external magnetic field is removed. This property makes them useful for permanent magnets. Soft ferromagnetic materials such as soft iron possess low retentivity and low coercivity. Their magnetisation largely disappears once the external field is removed. This allows them to be repeatedly magnetised and demagnetised in transformers and electromagnets. Therefore the correct comparison is: Hard Ferromagnets → magnetisation persists; Soft Ferromagnets → magnetisation disappears.

Diamagnetism originates from the orbital motion of electrons and is present in every material. However, the diamagnetic response is usually very weak. In materials that also exhibit paramagnetism or ferromagnetism, the stronger magnetic effects dominate and mask the diamagnetic contribution. Consequently, diamagnetism is often difficult to observe directly in such materials. Since diamagnetism is weak and universal, statements 1 and 3 are correct. Statement 4 is also correct because paramagnetic effects can overshadow diamagnetism. Statement 2 is incorrect because diamagnetism is not easily detected in ferromagnetic substances due to the overwhelming ferromagnetic response.

Ferromagnetism arises due to the spontaneous alignment of magnetic moments within domains. As temperature increases, thermal agitation becomes stronger and tends to randomize the orientations of atomic magnetic moments. At sufficiently high temperatures, known as the Curie temperature, thermal energy overcomes the interactions responsible for maintaining domain alignment. The ferromagnetic domain structure effectively breaks down, and the material loses its spontaneous magnetisation. Beyond the Curie temperature, the material behaves as a paramagnet, exhibiting only field-induced alignment. Therefore the transition from ferromagnetism to paramagnetism occurs because the domain structure disintegrates due to thermal agitation.