Free Physics of Solids MCQs with Answers

311. A spring obeying Hook's law has an unstretched length of 50mm and a spring constant of 400 Nm -1. What is the tension in the spring when its overall length is 70mm?

• A. 8.0N
• B. 28N
• C. 160N
• D. 400N

Explanation: In order to find the displacement x we subtract the untstrected length from the overall length. The stretched length is equal to displacement x.70-50=20F = kxF = 20*10-3*400F = 8 N

312. The resistance of the pure semiconductor decreases in a certain ratio with the?

• A. Decrease in temperature
• B. Increase in current
• C. Increase in temperature
• D. Decrease in current

Explanation: In a pure semiconductor, the resistance decreases with an increase in temperature. This happens because as temperature increases, some electrons in the valence band gain enough energy to jump to the conduction band, leaving behind holes in the valence band. This increases the number of charge carriers in the material, leading to a decrease in resistance.

313. Metals are good conductors of electricity because they contain:

• A. Large number of freely mobile electrons
• B. A large number of bound electrons
• C. A small number of free electrons
• D. A small number of bound electrons

Explanation: Metals are good conductors of electricity because they contain a large number of freely mobile electrons.

314. The temperature at which the resistance of a conductor approaches zero is called

• A. Normal temperature
• B. Critical temperature
• C. Absolute temperature
• D. Curie temperature

Explanation: The temperature at which the resistance of a conductor approaches zero is called the critical temperature.

315. The resistance of light dependant resistance LDR:

• A. Increases with light
• B. Decreases with light
• C. Decreases with darkness
• D. None of the above

Explanation: The correct answer is that the resistance of a Light Dependent Resistor (LDR) decreases with an increase in light intensity. This is because LDRs are designed to be sensitive to light, thus reducing their resistance as they are exposed to more light. Option A is incorrect because it states the opposite of what actually happens. Option C is misleading as it incorrectly suggests that resistance decreases in darkness, whereas it actually increases. Option D is incorrect as there is a correct statement among the options provided.

316. Semi-conductor materials have:

• A. Ionic bond
• B. Covalent bond
• C. Mutual bond
• D. Metallic bond

Explanation: A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. Semiconductor materials have covalent bonding between atoms (FACT). They are only able to conduct electricity due to delocalized electrons.

317. Hook's law correlates the:

• A. Force and displacement
• B. Force and extension
• C. Force and compression
• D. Stress and strain

Explanation: Hooke's Law is a principle of physics that states the strain in a solid is proportional to the applied stress within the elastic limit of that material. Mathematically, it can be expressed as Stress/Strain = Constant. This means that the material will return to its original shape after the stress is removed, as long as the elastic limit is not surpassed. The other options mention relationships that are not the fundamental focus of Hooke's Law. While force can influence displacement, extension, and compression, Hooke's Law specifically addresses the proportional relationship between stress and strain.

318. A vertical steel wire X of circular cross-section is used to suspend a load. A second wire Y, made of the same material but having twice the length and twice the diameter is used to suspend an equal load. What is the value of the ratio?

• A. 1/1
• B. 1
• C. 2
• D. 4

Explanation: Since the wires at made of the same material, their young modulus will be the same hence they have the same length, force and area. So the ratio is going to be 1/1 = 1

319. The shear modulus of elasticity G is:

• A. Option A
• B. Option B
• C. Option C
• D. Option D

Explanation: The shear modulus of elasticity/rigidity G is a material's ability to resist deformation, it is defined as the ratio of tangential stress to the shearing strain within the elastic limit. It can be understood better by the following (where y=θ).

320. Which one of the following physical quantity does not have the dimension of force per unit area:

• A. Stress
• B. Strain
• C. Young modulus
• D. Pressure

Explanation: The strain unit is dimensionless. It is the ratio between the length shift and the initial length, so it is unit-less. Thus B, strain, is the correct option