Free Physics of Solids MCQs with Answers

211. The temperature of a wire is doubled, the Young's modulus of elasticity

• A. Will also double
• B. Will become four times
• C. Will remain the same
• D. Will decrease

Explanation: Young's modulus, a measure of a material's stiffness, generally decreases with an increase in temperature. This is because higher temperatures cause atoms to vibrate more vigorously, weakening the bonds between them and thus reducing the material's ability to withstand deformation. Therefore, when the temperature of a wire is doubled, the Young's modulus of elasticity decreases. Option A is incorrect because Young's modulus does not increase with temperature. Option B is incorrect because the modulus does not quadruple; rather, it typically decreases. Option C is incorrect because it fails to consider the impact of temperature on atomic interactions and the resulting decrease in modulus.

212. A spring is stretched by applying a load to its free end. The strain produced in the spring is

• A. Volumetric
• B. Shear
• C. Longitudinal and shear
• D. Longitudinal

Explanation: When a spring is stretched by a load, it experiences two types of strain: longitudinal and shear. Longitudinal strain is due to the change in length of the spring along the direction of the applied force. Shear strain occurs because the coils of the spring also deform, changing their shape. Option C is correct because both types of strain are present. Option D is only partially correct as it only considers longitudinal strain. Option A is incorrect because volumetric strain involves a change in volume, which is not the primary deformation here. Option B is incorrect because shear strain alone does not account for the entire deformation of the spring.

213. If S is strain and Y is Young's modulus of elasticity of wire material, then energy stored in the wire per unit volume is

• A. s2 /2y
• B. 2y/s2
• C. s/2y
• D. 2s2y

Explanation: The energy stored per unit volume in a wire under elastic deformation is given by the formula U = (1/2) * Y * S2, where U is the energy per unit volume, Y is Young's modulus, and S is the strain. Rearranging this formula gives U = S2 / (2Y). The correct answer is thus s2 / 2y. Option B is incorrect because it inverts the relationship between Young's modulus and strain. Option C is incorrect because it does not square the strain, and Option D is incorrect because it incorrectly multiplies the variables.

214. Which of the following affects the elasticity of a substance?

• A. Impurity of a substance
• B. Hammering and annealing
• C. Change in temperature
• D. All of these

Explanation: The elasticity of a substance, which is its ability to return to its original shape after deformation, can be influenced by several factors:Impurity: Impurities can introduce defects in the atomic structure of a solid, affecting its elastic behaviour.Hammering and annealing: These mechanical processes can alter the internal structure of a material, changing its mechanical properties, including elasticity.Change in temperature: Temperature influences the movement of atoms and the strength of bonds, therefore impacting elasticity.Therefore, the correct answer is Option D: All of these, as each listed factor can affect the elasticity of a substance. Options A, B, and C are each individually correct but do not encompass the entire range of influences.

215. Curie temperature of iron is the temperature below which it is

• A. Superconducting
• B. Ferromagnetic
• C. Low retentivity
• D. Both (a) and (c)

Explanation: The Curie temperature is the temperature above which a ferromagnetic material loses its permanent magnetic properties and becomes paramagnetic. For iron, this temperature is about 770°C (1043 K). Below this temperature, iron is ferromagnetic, meaning it can retain its magnetic properties. Superconductivity is unrelated to the Curie temperature of iron, and low retentivity would imply a lack of ability to maintain magnetisation, which is not the case for iron below its Curie temperature.

216. A diamagnetic material in a magnetic material moves

• A. Stronger to the weaker parts of the field
• B. From weaker to the stronger parts of the field
• C. Perpendicular to the field
• D. In none of the above directions

Explanation: Diamagnetic materials are characterised by their tendency to create an induced magnetic field in the opposite direction to an applied magnetic field, causing them to be repelled by the field. This results in movement towards areas where the magnetic field is weaker. Consequently, they move from stronger parts of the field to weaker parts. This behaviour contrasts with paramagnetic and ferromagnetic materials, which are attracted to stronger fields and move towards them. Option A correctly describes the behaviour of diamagnetic materials, while Options B, C, and D are inaccurate descriptions of their behaviour in a magnetic field.

217. Electromagnets are made up of soft iron because soft iron has

• A. Low retentivity and high coercive force
• B. High retentivity and high coercive force
• C. Low retentivity and low coercive force
• D. High retentivity and low coercive forces

Explanation: Electromagnets are made of soft iron because it has low retentivity and low coercive force, meaning it can be easily magnetised and demagnetised. This property is crucial for electromagnets, which need to turn on and off easily. Soft iron quickly loses its magnetism when the electric current is removed, making it ideal for this purpose. Option A is incorrect because high coercive force would make the material difficult to demagnetise. Option B is unsuitable due to both high retentivity and high coercive force, characteristics of permanent magnets. Option D is incorrect because high retentivity means the material would not lose its magnetism easily, which is not desired for electromagnets.

218. When a magnetic substance is heated then it

• A. Remains the same
• B. Loses its magnetism
• C. Becomes a strong magnet
• D. Either (a) or (c)

Explanation: The correct answer is that when a magnetic substance is heated, it loses its magnetism. This is because the thermal energy disrupts the alignment of the magnetic domains, causing the material to lose its magnetic properties. This effect is particularly noticeable when the temperature exceeds the Curie point, the temperature at which a ferromagnetic material becomes paramagnetic. Option A is incorrect as the properties do change with temperature. Option C is incorrect because heating generally weakens magnetism. Option D is incorrect because it presents mutually exclusive outcomes that are not supported by the behaviour of magnetic substances.

219. Two wire of some material have length 1: 3 when equal force are applied upon it their extension will be?

• A. 1 : 3
• B. 1 : 9
• C. 9 : 1
• D. 3 : 1

Explanation: The extension of a wire under an applied force is directly proportional to its length when the material and force are constant. Given that the wires have lengths in the ratio 1:3, their extensions will also follow this ratio, resulting in an extension ratio of 1:3. Options B, C, and D are incorrect as they suggest disproportionate extensions that do not align with the principles of material physics.

220. Two wires of same nature and dimension have lengths 1 : 3 to produce the same amount of extension in them the required ratio of forces F1:F2 will be

• A. 1 : 3
• B. 3 : 1
• C. 9 : 1
• D. 1 : 9

Explanation: The correct answer is 3 : 1. When two wires of the same material and cross-sectional area have lengths in the ratio 1:3, the force required to produce the same extension is inversely proportional to their lengths. Therefore, the longer wire (3 times the length of the shorter one) requires 3 times more force to reach the same extension. This is why the ratio of forces F1:F2, where F1 is the force on the shorter wire, and F2 is the force on the longer wire, is 3:1. The other options incorrectly suggest either a lesser force is required for the longer wire or propose a non-linear relationship between force and length.