Free Physics of Solids MCQs with Answers

261. Magnets cannot be made from which of the following substances

• A. Iron
• B. Nickel
• C. Copper
• D. All of the above

Explanation: Magnets are typically made from materials that exhibit ferromagnetism, such as iron and nickel, because they have unpaired electrons that align to create a strong magnetic field. Copper, on the other hand, is diamagnetic and does not have the necessary magnetic properties to become a magnet. Therefore, the correct answer is Copper. The option 'All of the above' is incorrect because it includes iron and nickel, both of which can indeed form magnets.

262. Demagnetization of magnets can be done by

• A. Rough handling
• B. Heating
• C. Magnetizing in the opposite direction
• D. All of the above

Explanation: Demagnetisation of a magnet can occur through various methods that disrupt the alignment of its magnetic domains. Rough handling can physically jar the magnetic domains out of alignment. Heating increases atomic vibrations, which disrupts the orderly alignment of magnetic domains. Magnetising in the opposite direction directly counteracts the existing magnetic alignment. Therefore, all the provided methods can lead to demagnetisation, making Option D: All of the above the correct answer. Each of the other options only represents one aspect of demagnetisation, but all are valid methods.

263. A ferromagnetic material is heated above its curie temperature. Which one is a correct statement

• A. Ferromagnetic domains are perfectly arranged
• B. Ferromagnetic domains become random
• C. Ferromagnetic domains are not influenced
• D. Ferromagnetic material changes itself into diamagnetic material

Explanation: When a ferromagnetic material is heated above its Curie temperature, the thermal energy overcomes the magnetic energy that keeps the domains aligned. Consequently, the domains become randomly oriented, and the material loses its ferromagnetic properties, becoming paramagnetic instead. Option B is correct because it accurately describes this transition. Option A is incorrect because the domains are not perfectly arranged; they become disordered. Option C is incorrect because the domains are affected by heating. Option D is incorrect because the material becomes paramagnetic, not diamagnetic.

264. If a diamagnetic substance is brought near north or south pole of a bar magnet, it is

• A. Attracted by the poles
• B. Repelled by the poles
• C. Repelled by the north-pole and attracted by the south pole
• D. Attracted by the north pole and repelled by the south pole

Explanation: Diamagnetic substances are characterised by their ability to create a weak magnetic field in opposition to an externally applied magnetic field. As a result, when a diamagnetic substance is brought near either pole of a magnet, it is repelled. This is because the induced magnetic moment in the diamagnetic material is oriented in the opposite direction to the magnetic field of the bar magnet. Therefore, the correct answer is that a diamagnetic substance is repelled by the poles. Options A, C, and D describe behaviours that are not typical of diamagnetic materials: Option A describes attraction, which occurs with ferromagnetic and paramagnetic substances; Option C describes a mixed response, which is incorrect; and Option D also describes a behaviour not exhibited by diamagnetic materials.

265. Magnetic permeability is maximum for

• A. Diamagnetic substance
• B. Paramagnetic substance
• C. Ferromagnetic substance
• D. All of these

Explanation: Magnetic permeability is a measure of how easily a material can become magnetised or enhance an external magnetic field. Ferromagnetic substances, such as iron, cobalt, and nickel, have a very high magnetic permeability because they have a strong tendency to align their internal magnetic domains with an external magnetic field. This results in maximum enhancement of the magnetic field within the material. Diamagnetic substances, on the other hand, are characterised by a negative susceptibility, meaning they are weakly repelled by a magnetic field and have low permeability. Paramagnetic substances have a small positive susceptibility and thus moderate permeability, but it is not as high as that of ferromagnetic substances. Therefore, the correct answer is that magnetic permeability is maximum for ferromagnetic substances.

266. Which of the following is true

• A. Diamagnetism is temperature dependent
• B. Paramagnetism is temperature dependent
• C. Paramagnetism is temperature independent
• D. None of these

Explanation: Paramagnetism is temperature-dependent, which is explained by Curie's law. This law states that the magnetic susceptibility of a paramagnetic material is inversely proportional to its temperature. As the temperature increases, the thermal energy causes more randomisation in the alignment of magnetic dipoles, resulting in a decrease in magnetic susceptibility. Option A is incorrect because diamagnetism is a quantum mechanical effect that is generally independent of temperature. It arises from the orbital motion of electrons, creating tiny magnetic fields. Option C is incorrect because it incorrectly states that paramagnetism is temperature independent, which contradicts Curie's law. Option D is incorrect because there is a true statement among the options provided, specifically about paramagnetism being temperature-dependent.

267. Diamagnetic substances are

• A. Feebly attracted by magnets
• B. Strongly attracted by magnets
• C. Feebly repelled by magnets
• D. Strongly repelled by magnets

Explanation: Diamagnetic substances are materials that create an induced magnetic field in a direction opposite to an externally applied magnetic field and are thus feebly repelled by the magnetic field. This is due to the paired nature of electrons in diamagnetic materials, which means there is no net magnetic moment. Option C is correct because it accurately describes this behaviour. Options A and B describe paramagnetic and ferromagnetic materials, respectively, which are attracted to magnets. Option D is incorrect because no substantial repulsion occurs in diamagnetic materials.

268. A superconductor exhibits perfect

• A. Ferrimagnetism
• B. Ferromagnetism
• C. Paramagnetism
• D. Diamagnetism

Explanation: Superconductors exhibit perfect diamagnetism, a phenomenon known as the Meissner effect, where they completely expel magnetic fields from their interior when cooled below their critical temperature. This is the defining characteristic of superconductivity in relation to magnetic fields. The other options, such as ferrimagnetism, ferromagnetism, and paramagnetism, are different types of magnetic ordering that do not describe the behaviour of superconductors. Ferromagnetism and ferrimagnetism involve materials that can retain magnetisation, while paramagnetism involves weak attractions to magnetic fields, none of which involve the complete expulsion of magnetic fields like diamagnetism does in superconductors.

269. If a ferromagnetic material is inserted in a current carrying solenoid, the magnetic field of solenoid

• A. Largely increases
• B. Slightly increases
• C. Largely decreases
• D. Slightly decreases

Explanation: Inserting a ferromagnetic material into a current-carrying solenoid largely increases the magnetic field inside. This is because ferromagnetic materials, like iron, have a high magnetic permeability, which means they can become highly magnetised when exposed to an external magnetic field. This alignment of magnetic domains within the ferromagnetic material greatly enhances the overall magnetic field produced by the solenoid. The incorrect options either misrepresent the magnitude of this enhancement or wrongly suggest a decrease in the magnetic field, which does not occur with ferromagnetic materials.

270. In the hysteresis cycle, the value of H needed to make the intensity of magnetization zero is called

• A. Retentivity
• B. Coercive force
• C. Lorentz force
• D. None of the above

Explanation: The correct answer is Coercive force. In the hysteresis cycle, the coercive force is the measure of the resistance of a ferromagnetic material to becoming demagnetised. It is the value of the applied magnetic field strength (H) required to bring the intensity of magnetisation to zero after the material has been magnetised to saturation. Retentivity, on the other hand, refers to the remaining magnetisation in the material when the external field is removed. The Lorentz force is unrelated to the hysteresis cycle, as it deals with the force on charged particles in magnetic and electric fields.